ESSENTIALS 


Veterinary   Physiology 


D.  NOEL   PATON,  M.D.,  B.Sc,  LL.D.,  F.R.S. 

PROFESSOR   OF    PHYSIOLOGY,    DNIVKRSITY,    GLASGOW 
LATE    LECTURER    IN    PHYSIOLOGY,    ROYAL   (DICK)    VETERINARY   COLLEGE,    EDINBURGH 

AND 

JOHN  BOYD  ORR,  D.S.O.,  M.C.,  M.A.,  M.D.,  D.Sc. 

DIRECTOR,     ROWETT    INSTITUTE     FOR    RESEARCH     IN     ANIMAL    NUTRITION,     ABERDEEN 
RESEARCH   LECTURER   ON   PHYSIOLOGY   OF    NUTRITION,    UNIVERSITY,    ABERDEEN 


THIRD   EDITION 
REVISED  AND   ENLARGED 


NEW  YORK 

WILLIAM   WOOD   &   COMPANY 

1920 


'f '  '.' : '. 


.\ 


PREFACE   TO   THE   FIRST   EDITION 

Between  the  Physiology  of  Man  and  that  of  the  Domestic 
Animals  there  is  no  fundamental  difterence,  and  most  of  our 
knowledge  of  human  physiology  has  been  acquired  from 
experiments  upon  the  lower  animals.  But  while  the  tissues 
of  a  mao,  a  dog,  and  a  horse  act  much  in  the  same  manner, 
the  mode  of  nutrition  of  these  tissues  is  somewhat  different, 
and  requires  special  attention  in  the  case  of  each. 

In  this  volume  the  attempt  is  made  to  give  the  essentials 
of  general  physiology  and  of  the  special  physiology  of  the 
domestic  animals  in  a  form  suitable  to  the  requirements 
of  Students  and  Practitioners  of  Veterinary  Medicine.  The 
book  is  not  intended  to  take  the  place  of  the  demonstrations 
and  practical  work  from  which  alone  physiology  can  be 
properly  learned,  but  merely  to  supplement  these  and  to 
focus  the  information  derived  from  them. 

The  student  must  take  every  opportunity  of  acquiring  a 
really  practical  knowledge,  and,  to  facilitate  the  more  direct 
association  of  the  practical  and  systematic  study  of  physi- 
ology, throughout  these  pages  references  are  made  to  de- 
scriptions of  the  experimental  and  chemical  work  which  the 
student  should  try  to  do  for  himself  or  have  demonstrated 
to  him.  The  histological  structure  of  the  tissues  and  organs 
which  is  now  studied  practically  in  every  school  is  here 
described  only  in  so  far  as  it  is  essential  for  the  proper 
understanding  of  their  physiology. 

D.  N.  P. 


461269 


PREFACE   TO   THE   THIRD   EDITION 

The  Preface  to  the  First  Edition  sufficiently  explains  the  scope 
and  purpose  of  the  book. 

The  present  Edition  has  been  almost  entirely  rewritten  in 
order  to  bring  it  up  to  date  and  to  give  greater  prominence  to 
those  parts  of  Physiology  which  have  the  most  direct  bearing 
upon  veterinary  practice. 

Our  thanks  are  due  to  Dr.  Burns  for  the  section  on  hydrogen 
ion  concentration  in  Appendix  III.,  and  to  Mr.  William  Dunlop 
of  Dunure  for  valuable  suggestions  on  the  section  dealing  with 
the  limbs  of  the  horse. 

D.  N.  P. 

J.  B.   O. 

September  1920. 


CONTENTS 


PART    I 


The  Growth  of  Physiology  . 


PART    II 

SECTION   I 
Protoplasm 

1.  The  Nature  of  Protoplasmic  Activity 

2.  The  Structure  of  Protoplasm 

3.  The  Chemistry  of  Protoplasm 


1.  Cell  Protoplasm 

2.  Nucleus 

3.  Reproduction  of  Cells 


SECTION    II 
The  Cell 


SECTION    III 

The  Tissues 

Vegetative  Tissues — 

Epithelium 

. 

32 

1.  Protective — Squamous  Epithelium 

32 

2.   Absorbing— Colu 

mnar  Epithelium 

33 

3.  Secreting  Epithelium  and  Glands 

34 

4.  Motile— Ciliated 

Epithelium 

37 

Connective  Tissues 

37 

1.  Mucoid  Tissue 

37 

2.  Fibrous  Tissue 

38 

i.  Fibres 

. 

39 

ii.  Spaces 

40 

CONTENTS 


iii.  Cells    . 

Modifications  of  Cells — 
a.  Endothelium  , 
h.  Fat  Cells  and  Fat 
c.  Pigment  Cells . 
Cartilage     . 
i.  Hyaline  Cartilage 
ii.  Elastic  Fibro-Cartilage 
iii.  White  Fibro-Cartilage 
Bone 
i.  Development  and  Structure 
ii.  Chemistry 
iii.  Metabolism    . 


B.  The  Master  Tissues — Nerve  and  Muscle — 
General  Arrangement 
Nerve — 

1.  Development 

2.  Structure   .... 

3.  Chemistry .... 

4.  Physiology  of  Neurons 

A.  Single  Neurons  or  Neurons  in  Nerves 

a.  Nerve  Fibres — 

1.  Manifestation  of  Activity  . 

2.  Excitation  . 
.3.  Conduction 

4.  Classification  of 

5.  The  Nature  of  the  Nerve  Impulse 
h.  Nerve  Cells 

Degeneration  and  Regeneration  of  N 
Changes  in  the  Cell    . 

B.  Action  of  Neurons  in  Series  . 
The  Neural  Arcs 

A.  Arrangement    . 

B.  General  Action 

I.  The  Spinal  Arc — Reflex  Action  . 
Trophic  Action 
Peripheral  Reflexes .... 

TI.  The  Cerebral  and  Cerebellar  Arcs  . 
A.  The  Ingoing  Side  of  the  Arcs    . 
I.  Body  Receptor  Mechanisms 
i.  Visceral 
ii.  Cutaneous — 
Pain 
Touch 
Temperature 


CONTENTS 


iii.  Muscle  and  Joint      .... 
Connection  of  Body  Receptors  with  Central 
Nervous  System — 

1.  Ingoing  Nerves       .... 

2.  Upgoing  Fibres  in  Spinal  Cord 

3.  Connections  with  Brain-Stem  and  Cerebrum 

4.  Cerebellar  Synapses 
Labyrintho-Cerebellar  Mechanism 

1.  Labyrinth 

(1)  Structure 

(2)  Connection 

(3)  Physiology 

2.  The  Cerebellum 

(1)  Structure 

(2)  Connections 

(3)  Physiology 

III.  Distance  Receptors  of  the  Head — 
I.  For  Chemical  Stimuli — 

1.  Buccal  Mechanism — Taste 

2.  Nasal  Mechanism — Smell 

II.  For  Vibration  of  Ether — Vision 

A.  General  Considerations 

B.  Anatomy  of  the  Eye     . 

C.  Physiology        .... 

(I.)  Monocular  Vision  . 
Formation  of  Images  upon  the  Retina 

1.  The  Dioptric  Mechanism 

Accommodation  . 

2.  Stimulation  of  the  Retin; 

1.  Reaction  to  Varying  Illumination 

2.  Localising  the  Direction  of  Illumination 

3.  Colour  Sensation  . 
(II.)     Binocular  Vision 

Single  Vision  with  Two  Eyes 

(III.)  Connections    of    the    Eyes     with    the 
Central  Nervous  System 

1.  Optic  Nerves  and  Tracts  . 

2.  Visual  Centre 

III.  For  Vibration  of  Air — Hearing 

L  General  Considerations. 

2.  The  External  Ear 

3.  The  Middle  Ear  .  .  . 

4.  The  Internal  Ear 

5.  Connections  with  the  Central  Nervous  System 

6.  Auditory  Centre  in  Cortex 


105 


106 
107 
112 
117 
117 
117 
117 
119 
121 
125 
125 
126 
127 


131 
133 

136 
136 
139 
143 

144 
144 
144 
146 
151 
151 
155 
155 
158 
158 

162 
162 
164 


166 
167 
167 
170 
172 
173 


CONTENTS 


IV.  The  Localisation  of  Receiving  Areas  in  the  Cortex    176 

V.  The  Integration  of  Sensations  in  the  Cortex  .     179 

1.  Structural  Development  ....     181 

2.  Functional  Development  ....     182 

3.  Storing  and  Associating  Part  of  Cortex  .  .18-5 

4.  Relationship  of  Consciousness  to  Cerebral  Action         .     185 

5.  Time  of  Cerebral  Action   .  .  .  .  .186 

6.  Fatigue    .  .  .  .  .  .  .186 

7.  Sleep 187 

8.  Hypnosis.  .  .  .  .  .  188 

VI.  The  Discharging  Side  of  the  Cerebral  Arc    .  .     189 

i.  The  Basal  Ganglia       .  .  .  .  .189 

ii.  The  Cortex  Cerebri     .....     189 
iii.  Fibres  from  Discharging  Area  .  .  .     194 

iv.  Outgoing  Nerves  .  .  .  •  .195 

Cranial  Nerves     ......     200 

The  Effectors— Muscle    ......     202 

i.  Development  and  Structure      .  .  .  .     2()2 

ii.  Chemistry  .  .  .  .  .  .207 

iii.  Physical  Characters  and  Physiology     .  .  .     210 

iv.  Death  of  Muscle  .  .      '      .  .  .214 

v.  Muscle  in  Action  .....     215 

A.  Visceral  Muscle     .....     215 

B.  Skeletal  Muscle— 

1.  Direct  Stimulation  of  Muscle  .  .     217 

2.  Methods  of  Stimulating  Muscle       .  .     218 

3.  The  Changes  in  Muscle  when  Stimulated   .     219 

4.  Mode  of  Action  .  .  .  .231 

5.  Special  Mechanism  of  the  Horse      .  .     233 

6.  Work  Done  .  .  .  .  .242 

7.  Heat  Produced  ....     246 

8.  Relationship  of  Work  Production  to  Heat 

Production  .  .  .  .248 

9.  The  Efficiency  of  the  Horse  as  a  Machine  .  252 
K).  Capacity  of  the  Horse  for  Work  .  .  252 
11.  Chemical  Changes  and  Source  of  Energy  .     254 

General  Metabolism        ......     264 

A.  Exchange  of  Energy — 

I.  Basal  Metabolism        .  .  .  .  .  .264 

II.  Factors  Modifying  Metabolism  ....     266 

1.  Muscular  Work  .....     266 

2.  Rate  of  Cooling  .  .  .  .  .267 

Loss  of  Heat  .  .  .  .  .267 

Heat  Production     .  .  .  .  .268 

Heat  Regulation      .  .  .  .  .269 


CONTENTS 


3.  Taking  of  Food 

.     272 

4.  Prolonged  Fasts 

.     272 

5.  Semi-starvation 

.     274 

I  Exchange  of  Material 

.     275 

Water        .... 

.     275 

Amino  Acids 

.     275 

Salts          .... 

.     279 

Accessory  Factors 

.     281 

PART  III. 


THE  NUTRITION  OF  THE  TISSUES. 


SECTION  I. 


Food. 


The  Nature  of  Food         .... 

.     283 

A.  Food-Stuffs  Yielding  Energy 

.     283 

1.  Nitrogenous  Compounds 

.     283 

2.  Fats  and  Allied  Bodies 

.     285 

3.  Carbohydrates 

.     285 

B.  Food-Stuffs  not  Yielding  Energy 

.     288 

4.  Ash  (Inorganic  Elements)      . 

.     288 

5.  Water              .... 

.     289 

6.  Accessory  Factors 

.     289 

Classification  of  Feeding  Stuffs  for  Herbivora    . 

.     290 

SECTION  11. 

Digestion. 

.  Structure  of  the  Alimentary  Canal 

.     291 

.  Physiology         ..... 

.     301 

A.  Digestion  in  Carnivora  and  Omnivora 

.     302 

I.  Digestion  in  the  Mouth     . 

.     302 

(a)  Mastication  .... 

.     302 

(6)  Insalivation  .... 

.     302 

II.  Swallowing            .... 

.     306 

III.  Digestion  in  the  Stomach 

.     308 

A.  Stomach  during  Fasting 

.     309 

CONTENTS 


IV. 


B.  Stomach  atttr  Feeding 

1.  Vascular  Changes 

2.  Secretion 

(1.)  Characters  of  Secretion     . 
(2.)  Source  of  Constituents 
(3.)  Course  of  Gastric  Digestion 

(rt)  Amylolytic  Period  . 

(6)  Proteolytic  Period  . 
(4.)  Digestion  of  the  Wall  of  the  Stomach 
(5.)  Antiseptic  Action  of  the  Gastric  Juice 
(6.)  Influence  of  Food  on  the  Gastric  Juice 
(7.)  Nervous  Mechanism  of  Secretion 
(8.)  Chemical  Stimulation 

3.  Movements  of  Stomach 

1.  Character     . 

2.  Nervous  Mechanism 
C'.  Absorption  from  the  Stomach 
D.  Regurgitation  of  the  Gastric  Conten 

1.  Into  the  Gullet  . 

2.  Vomiting 
Intestinal  Digestion 

A.  Pancreatic  Secretion 

B.  Bile    .... 

C.  Succus  Entericus 

D.  Bacterial  Action 

E.  Fate  of  the  Digestive  Secretions 

F.  Movements  of  the  Intestine  . 

1.  Small  Intestine  . 

2.  Large  Intestine   . 

3.  Defsecation 
B.  Digestion  in  Herbivora 

1.  Digestion  in  Ruminants 

2.  Digestion  in  the  Horse   . 


SECTION  III. 
Absorption  op  Food. 

1.  State  in  which  Food  is  Absorbed 

2.  Mode  of  Absorption 

3.  Channels  of  Absorption 
Storage  of  Surplus  Food 

The  Liver  as  a  Regulator  of  the  Supply  to  the  MuscL 

1.  Regulation  of  the  Supjjly  of  Sugar 

2.  Regulation  of  the  Supply  of  Fats 

3.  Regulation  of  the  Supply  of  Proteins  . 


CONTENTS 

XV 

PAGE 

The  Fieces           ....... 

361 

Availability  of  Food       .... 

363 

Digestion  Experiments  .... 

364 

Food  Reqiiireinents         .... 

368 

1.  Method  of  Determining  Requirements 

369 

2.  Rations               .... 

371 

3.  Feeding  Standards 

375 

4.  Manurial  Values 

380 

SECTION    IV. 

Manner  in  which  Nourishing  Fluids  are 

Sent 

ro   THE 

Tissues. 
The  Circulation. 


I.  General  Considerations 

II.  The  Heart   . 

A.  Structure 

B.  Relations 

C.  Physiology 


I.  The  Cardiac  Cycle— 

(A)  Frog 

(B)  Mammal    . 
Rate    . 

Sequence  of  Events 
Duration  of  Phases 
Changes  in  the  Shape 
The  Cardiac  Impulse , 

6.  Changes  in  Intracardiac  Pressure 

7.  Action  of  the  Valves 

8.  The  Flow  of  Blood  through  the  Heart 

9.  Sounds  of  the  Heart  . 
The  Work  of  the  Heart 


of  the  Chamber; 


II. 
III. 
IV, 


The  Influence  of  the  Central  Nervous  System 
The  Maintenance  and  Control  of  Cardiac  Rhythm 

1.  The  Initiation  of  Contraction 

2.  The  Conduction  of  Contraction 
V.  The  Nature  of  Cardiac  Contraction 

III.  Circulation  in  the  Blood  and  Lymph  Vessels 

1.  Structure         ...... 

2.  Physiology       ...... 

A.  Blood  Pressure    ..... 

1.  General  Distribution  of  Pressure  . 

2.  Rhythmic  Variations  of  Pressure  . 


382 

384 
384 
393 
39,3 

393 
394 
394 
395 
396 
397 
398 
399 
402 
404 
407 
410 
418 
424 
424 
427 
427 

431 
431 
432 
432 
432 
434 


CONTENTS 


PAGE 


(a)  Changes  Synchronous  with  the  Heart 

1.  The  Arterial  Pulse       .             .             .  434 

2.  The  Capillary  Pulse     .  .  .444 

3.  The  Venous  Pulse         .             .             .  444 

(b)  Changes  Synchronous  with  Kespirations — 

1.  Arterial             ....  446 

2.  Venous               ....  447 
3.  The  Mean  Blood  Pressure  ....  447 

I.  Pressure  in  the  Arteries — 

(1.)  Methods  of  Determining           .             .  447 

(2.)  Normal  Pressure            .             .             .  449 

(3.)  Factors  Controlling       .             .             .  450 

i.  Heart's  Action          .             .             .  451 

ii.  Peripheral   Resistance  —  Condition 

of  the  Arteries    .  .  .451 

1.  Method  of  Investigating            .  452 

2.  Normal  State     .             .             .  453 

3.  Nervous  Control            .             .  454 

Vaso-Constrictor         .             .  455 

Vaso-Dilator  .  .  .458 

II.  Pressure  in  the  Capillaries      .             .             .  460 

III.  Pressure  in  the  Veins              .             .             .  463 

IV.  Pressure  in  the  Lymphatics  .             .             .  464 
B.  Flow  of  Blood  .             .             .             .             .             .464 

1.  Velocity      ......  465 

2.  Special  Characters              ....  467 
G.  Special  Characters  of  Circulation  in  Certain  Situations  468 

D.  Extra-Cardiac  Factors  Maintaining  the  Circulation  ,  470 

1.  Movements  of  Respiration              .             .             .  470 

2.  Intermittent  Muscular  Exercise    .             .             .  470 

E.  Influence  of  Posture  on  the  Circulation          .             .  471 

F.  Fainting            ......  472 

(t.  Time  taken  by  the  Circulation             .             .             .  472 

H.  Flow  of  Blood  through  Different  Organs        .            .  473 


SECTION   V. 

Fluids  Carrying  Nourishment  to  the  Tissues. 

Blood  and  Lymph. 

A.  Blood  .  .  .  .  .  .  .  .474 

I.  General  Characters  ......     474 

II.  Clotting  .  .  .  .  .  .  .475 

III.  Plasma  and  Serum  ......     479 


CONTENTS 

xvii 

PAGE 

IV.  Cells  of  the  Blood         .... 

.     483 

Leucocytes      ..... 

.     483 

Blood  Platelets           .... 

.     484 

Erythrocytes              .... 

.     485 

V.  Gases  of  the  Blood         .... 

.     492 

VI.  Source  of  the  Blood  Constituents 

.     498 

VII.  Total  Amount  of  Blood 

.     501 

VIII.  Distribution  of  the  Blood 

.     503 

IX.  Fate  of  the  Blood  Constituent.s 

.     503 

B.  Lymph           ...... 

.     507 

1.  General  Character          .... 

.     507 

2.  Formation  of  Lymph     .... 

.     508 

C.  Cerebro-Spinal  Fluid             .... 

.     511 

SECTION   VL 


Respiration. 


A.  External  Respiration  .  .  .         .  .  .  .513 

I.  The  Structure  of  the  Respiratory  Mechanism  .  .513 

II.  Physiology  — 

1.  Physical  Considerations    .  .  .  .  .515 

2.  Passage  of  Air  into  and  out  of  the  Lungs             .             .  516 

i.  Movements  of  Respiration  ....  516 

In.spiration  .  .  .  .  .516 

Expiration              .....  521 

Special  Respiratory  Movements    .             .             .  522 

ii.  Amount  of  Air  Respired      ....  522 

iii.  Interchange  of  Air  by  Diffusion       .             .             .  523 

iv.  Breath  Sounds          .....  524 

V.  Rhythm  of  Respiration        ....  525 

vi.  Nervous  Control  of  Respiration       .             .             .  526 

1.  Chemical  Regulation     ....  527 

2.  Reflex  Regulation          ....  532 

3.  Interaction  of  Circulation  and  Respiration           .             .  535 

A.  Influence  of  Respiration  on  Circulation     .             .  535 

B.  Influence  of  the  Heart  on  Respiration        .             .  536 

4.  Interchange  between  the  Air  and  the  Blood         .             .  537 

i.  Effects  of  Respiration  on  the  Air  Breathed              .  537 

ii.  Effects  of  Respiration  on  the  Blood              .             .  538 

iii.  The  Causes  of  the  Respiratory  Exchange    .             .  539 

1.  Partial  Pressure  of  Gases  in  the  Air  Vesicles  .  539 

2.  Partial  Pressure  of  Gases  in  the  Blood             .  541 

B.  Intermediate  Respiration      ......  545 

C.  Internal  Respiration              ......  547 


CONTENTS 


D.  Extent  of  Respiratory  Exchange 

E.  Ventilation    . 

F.  Asphyxia 


PAGE 

547 
548 
548 


Voice. 


A.  Structure  of  the  Larynx 

B.  Physiology  of  Voice  . 


550 
552 


SECTION   VII. 
Excretion  of  Matter  from  the  Body. 


A.  Excretion  by  the  Lungs  (see  Respiration). 

B.  Excretion  by  the  Intestine  {see  Intestine). 

C.   Excretion  by  the  Kidneys    .... 

.     554 

I.  Urine 

.     554 

1.  General  Consideration 

.     554 

2.  Physical  Characters 

.      557 

3.  Composition           .... 

.     558 

II.  Formation  of  LTrine       .... 

.     567 

1.  Structure  of  the  Kidney    , 

.     571 

2.  Physiology  of  Secretion     . 

.     572 

(a)  Malpighiau  Bodies  . 

.     573 

(fc)  Tubules        ... 

.     575 

III.  Excretion  of  Urine        .... 

.     581 

D.  Excretion  by  the  Skin           .... 

.     582 

Sweat  Glands         ..... 

.     583 

Sebaceous  Glands  ..... 

.     585 

SECTION    VIIL 

The  Regulation  of  Growth  and  Function. 

I.  Heredity 
II.  The  Nervous  System 
III.  Chemical  Regulation  :  The  Endoci 
Developed — 

(a)  From  the  Nervous  System 

1.  Chromaffin  Tissue 

2.  Hypophysis     . 
{h)  From  the  Buccal  Cavity— 

3.  Thyreoid 

4.  Pituitary 
(c)  From  Intestinal  Mucosa — 

5.  Pancreas 

6.  Mucosa  of  Small  Inte; 


.     586 

.     587 

rinetes   . 

■     .     588 

— 

.     590 

.     594 

.     595 

.     599 

.     601 

stine 

.     601 

CONTENTS 


{(l)  From  the  Branchial  Arches — 

7.  Thymus           .             .             .             .  .602 

8.  Parathyreoids .  ....     603 
(e)  From  the  Mesothelium  of  the  Genital  Ridge — 

9.  Gonads             .             .             .             .  .605 
10.  Inter-renals     .             .             .             .  .609 

The  Interaction  of  the  Endocriuetes           .             .  .     611 

The  Mode  of  Action  of  the  Internal  Secretions      .  .611 

Protective  Modifications  of  Metabolism — Immunity              .  .     612 

PART   IV. 


THE   ANIMAL   AS   PART   OF   THE   SPECIES. 

A.  Reproduction. 

I.  Determination  of  Sex 

II.  The  Gonads 

III.  The  Secondary  Sexual  Organs 

IV.  The  (Estrous  Cycle 
V.  Impregnation 

B.  Development. 
I.  Early  Stages  of  Development 

II.  Attachment  of  Embryo  to  Mother 

III.  The  Nourishment  of  the  Foetus 

IV.  Growth  of  the  Fcetus 
V.  Metabolism  in  Pregnancy 

VI.  The  Young  Animal  at  Birth 
VII.  Fojtal  Circulation 
VIII.  Gestation  and  Delivery    . 
IX.  Lactation  . 

Appendices 

Index  .... 


617 
618 
621 
622 
623 


624 
626 
628 
630 
631 
631 
631 
633 
634 

641 
655 


PART   I. 

SECTION    I. 

The  Growth  of  Physiology. 

Physiology  formerly  embraced  the  study  of  all  nature 
((pua-ig),  but  it  is  now  restricted  to  the  study  of  life  and 
the  activities  of  living  things.  It  is  really  an  older  science 
than  anatomy,  for  even  before  any  idea  of  pulling  to  pieces, 
or  dissecting  plants  and  animals  had  suggested  itself  to  our 
forefathers,  speculations  in  regard  to  the  causes  and  nature 
of  the  various  vital  phenomena  were  indulged  in,  some  of 
which  foreshadowed  in  a  truly  wonderful  way  the  scientific 
discoveries  of  to-day.  Thus,  about  500  b. c. ,  Empedocles  not 
only  formulated  a  doctrine  of  evolution,  but  indicated  that 
the  struggle  for  existence  played  a  part  in  the  process. 
About  a  century  later  Hippocrates  insisted  on  the  importance 
in  the  treatment  of  disease  of  studying  the  normal  action  of 
the  body,  and  recognised  the  importance  of  the  vis  medicatrix 
natiircB.  His  followers  adopted  the  idea  of  a  pneiwia,  a 
subtile  agent  attracted  to  the  lungs  and  distributed  to  the 
body  as  the  basis  of  all  vital  phenomena.  The  physiology 
of  to-day  is  the  offspring  of  such  speculations. 

Organ  and  Function. — The  first  great  and  true  advance 
was  through  anatomy.  Galen,  about  200  a.d.,  dissected  and 
made  observations  on  the  physiology  of  animals.  He  described 
various  organs  and  endeavoured,  more  or  less  successfully, 
to  ascertain  their  mode  of  action  by  experiments  on  living 
animals.      He  may  well  be  called  the  father  of  physiology. 

The  Dark  Ages  fell  upon  Europe,  and  little  further 
advance  was  made  till,  in  the  sixteenth  century,  the  group 
of  Italian  anatomists  showed  how  the  body  is  built  up  of 
1  1 


2  VETERINARY   PHYSIOLOGY 

definite  organs  which  they  described  in  detail,  and  thus 
prepared  the  way  for  the  work  of  Harvey,  which  led  to  such 
important  discoveries,  and  which  established  the  relationship 
of  function  to  organ. 

The  connection  between  organ  and  function  having  been 
demonstrated,  the  question  of  why  these  various  functions  are 
connected  with  the  respective  organs — why  the  liver  should 
secrete  bile  and  the  biceps  muscle  contract,  next  forced  itself 
upon  the  attention. 

Tissues  and  Function. — Again  anatomy  paved  the  way 
to  the  explanation.  The  dissecting  knife  and  the  early  and 
defective  microscope  showed  that  the  organs  are  composed 
of  certain  definite  structures  or  tissues,  differing  widely  from 
one  another  in  their  physical  characters  and  appearance,  and, 
as  physiologists  soon  showed,  in  their  functions.  By  the 
end  of  the  seventeenth  century  Leuwenhoek  and  Malpighi 
had  so  advanced  the  knowledge  of  the  tissues  that  Haller,  in 
the  middle  of  the  eighteenth  century,  was  able  to  indicate 
that  the  function  of  an  organ  is  really  the  function  of  the 
tissue  of  which  it  is  composed. 

Early  in  the  nineteenth  century  Johannes  Miiller,  taking 
a  comprehensive  survey  of  a  great  mass  of  observations  which 
had  accumulated,  and  adding  to  them  the  results  of  his  own 
investigations,  created  the  modern  science  of  physiology. 

Cells  and  Function. — ^Physiologists  and  anatomists  alike 
devoted  their  energies  to  the  study  of  these  various  tissues, 
and,  as  the  structure  of  the  microscope  improved,  greater  and 
greater  advances  were  made  in  their  analysis,  till  at  length 
Schwann  was  enabled  to  make  his  world-famous  generalisa- 
tion, that  all  the  tissues  are  composed  of  certain  similar 
elements  more  or  less  modified,  which  he  termed  cells,  and 
it  became  manifest  that  the  functions  of  the  dif event  tissues 
are  de/pendent  on  the  activities  of  their  cells. 

The  original  conception  of  the  cell  was  very  different 
from  that  which  we  at  present  hold.  By  early  observers  it 
was  described  as  composed  of  a  central  body  or  nucleus, 
surrounded  by  a  granular  cell  substance  with,  outside  all,  a 
cell  membrane.  As  observations  in  the  structure  of  the  cell 
were  extended,  it  soon  became  obvious  that  the  cell  membrane 


THE   GROWTH   OF   PHYSIOLOGY  3 

was  not  an  essential  part ;  and  later,  the  discovery  of  cells 
without  any  distinct  nucleus  rendered  it  clear  that  the 
essential  part  is  the  cell  substance.  This  substance  von 
Mohl  named  protojjlasm,  by  which  name  it  has  been  since 
generally  known. 

Protoplasm  and  Function. — So  far,  physiology  had  followed 
in  the  tracks  of  anatomy,  but  now  another  science  became 
her  guide.  Chemistry,  which  during  the  early  part  of  the 
nineteenth  century  advanced  Avith  enormous  strides,  and 
which  threw  such  important  light  upon  the  nature  of  organic 
substances,  lent  her  aid  to  physiology  ;  and,  morphologists 
having  shown  that  the  vital  unit  is  essentially  a  mass 
of  protoplasm,  the  science  of  life  has  become  the  science  of  the 
chemistry  of  irrotoplasm. 

The  prosecution  of  physiology  on  these  lines  has  changed 
the  whole  face  of  the  science.  Physiology  is  no  longer  the 
follower  of  anatomy.  It  has  become  its  leader,  and  at  the 
present  time,  as  we  shall  afterwards  see,  not  only  the  various 
activities,  but  also  the  various  structural  differences  of  the 
different  tissues,  are  to  be  explained  in  terms  of  variations 
in  the  chemical  changes  in  protoplasm. 

Already  these  chemical  studies  have  shown  that  proto- 
plasm is  not  a  single  substance,  but  a  mixture  of  many 
substances  in  a  constant  state  of  flux  and  change,  and  that 
its  condition  is  largely  determined  by  the  physical  relations 
of  the  substances  in  the  mixture. 

Within  recent  years  the  application  of  molecular  physics 
to  physiology  has  greatly  advanced  the  knowledge  of  many 
of  the  obscure  characters  of  living  matter. 

In  the  study  of  physiology  the  order  of  its  development 
must  be  reversed,  and  from  the  study  of  protoplasm  the 
advance  must  be  made  along  the  following  lines  : — 

1.  Protoplasm — the   physical  basis  of  life  ;    its  activities 

and  nature. 

2.  Cells. — The   manner   in  which   protoplasm   forms  the 

vital  units  of  the  body. 

3.  Tissues. — The  manner  in  which   these  are  formed  by 


4  VETERINARY   PHYSIOLOGY 

cells.      Their  structure,  physical  and  chemical  pro- 
perties, and  vital  manifestations. 
a.  The    Vegetative    Tissues,   supporting,     binding 
together,  protecting  and  nourishing  the  body. 

h.  The  Master  Tissues — nerve  and  muscle — through 
which  the  external  world  acts  upon  the 
body,  and  the  body  reacts  upon  the  surround- 
ings. 

4.  Nutrition  of  Tissues. 

a.  The  manner  in  which  substances  necessary  for 
the  tissues  are  supplied — 
Food. 
Digestion. 
h.  The  manner  in  which  the  nourishing  fluids  are 
brought  into  relationship  with  tissues — 
Circulation, 
c.   The  fluids  bathing  the  tissues — 

Blood  and  Lymph. 
cl.  The   supply  of   oxygen   and   the   elimination  of 
carbon  dioxide. 
Respiration. 
e.  The  manner  in  which  the  waste  products  of  tissues 
are     eliminated — ^Excretion,    Hepatic,    Renal, 
Pulmonary,  Cutaneous. 

5.  Reproduction  and  Development. 

Students  who  have  not  the  knowledge  of  Chemistry  and 
Physics  necessary  for  the  Study  of  Physiolog-y  are  referred 
to  the  Appendices. 


PART   II. 

SECTION   I. 
Protoplasm. 

I.  Nature  of  Protoplasmic  Activity. 

The  first  step  in  the  study  of  physiology  must  be  to 
acquire  as  clear  and  definite  a  conception  as  possible  of  the 
nature  of  protoplasmic  activity  in  its  most  simple  and 
uncomplicated  form,  for  in  this  way  an  idea  of  the  essential 
and  non-essential  characteristics  of  life  may  best  be  gamed. 

The  common  yeast  (Saccharomyces  Cerevisios)  aftbrds 
such  a  simple  form  of  living  matter. 

This  plant  consists  of  very  minute  oval  or  spherical  bodies 
frequently  connected  to  form  chains,  each  composed  of  a 
harder  outer  covering  or  capsule,  and  of  a  softer  inner 
substance  which  has  all  the  characters  of  protoplasm. 

1.  Manifestations  of  Life. — Its  vital  manifestations  may 
be  studied  by  placing  a  few  torula?  in  a  solution,  containing 
glucose,  CgHi^Og,  some  nitrogen- containing  substances  such 
as  urea,  CONoH^,  or  ammonium  nitrate,  NH^NOg,  with 
traces  of  disodium  phosphate,  Na2HP04,  and  of  potassium 
sulphate,  K2SO4  (Practical  Physiology). 

If  the  vessel  be  kept  all  night  in  a  warm  place,  the 
clear  solution  will  in  the  morning  be  seen  to  be  turbid,  and 
probably  covered  with  froth.  An  examination  of  a  drop  of 
the  fluid  shows  that  the  turbidity  is  due  to  the  presence  of 
myriads  of  torulse.  In  a  few  hours  the  few  torulse  placed  in 
the  fluid  have  increased  many  hundredfold.  The  whole 
mass  of  yeast  has  grown  in  amount  by  the  growth  and 
multiplication  of  the  individual  units. 


6  VETERINARY    PHYSIOLOGY 

This  power  of  growth  and  reproduction  under  suitable 
conditions  is  an  essential  characteristic  of  living  matter. 

2.  Conditions  necessary  for  Manifestations  of  Life. — 
(a)  If  the  yeast  be  mixed  with  the  solid  constituents  of 
the  solution  in  a  dry  state,  no  growth  or  reproduction 
occurs.      Water  is  essential. 

(b)  If  the  yeast,  mixed  with  the  solution,  be  kept  at  the 
freezing  point  no  growth  takes  place,  but  this  proceeds 
actively  at  about  36°  C.  A  certain  range  of  temperature  is 
necessary  for  the  vitality  of  protoplasm. 

In  the  absence  of  these  conditions,  protoplasm  is  only 
potentially  alive,  and  in  this  state  it  may  remain  for  long 
periods  without  undergoing  any  change,  as  in  the  seeds  of 
plants  and  in  dried  bacteria. 

3.  Essentials  for  Growth. — In  order  that  the  growth  of 
the  yeast  may  take  place,  there  must  be  : — 

{a)  A  Supply  of  Material  from  which  it  can  be 
formed.  The  chemical  elements  in  protoplasm  are  carbon, 
hydrogen,  oxygen,  nitrogen,  sulphur,  and  phosphorus.  These 
elements  are  contained  in  the  ingredients  of  the  solution 
used.  If  yeast  be  sown  in  distilled  water,  even  if  it  be  kept 
at  a  temperature  of  36°  C,  it  does  not  grow. 

(6)  A  Supply  of  Energy  to  bring  about  the  construc- 
tion. The  source  of  the  energy  is  indicated  by  an  examina- 
tion of  the  fluid  in  which  the  yeast  has  grown.  The  sugar, 
CgHioC^e'  ^^^  decreased  in  amount,  being  converted  into 
alcohol,  CgHgO,  and  carbon  dioxide,  CO2 — 
C6Hi206=2COo  +  2C2H60. 
It  is  this  oxidation  of  the  carbon  to  carbon  dioxide  which 
yields  the  energy,  just  as  the  combustion  of  the  coals  in  the 
furnace  yields  the  energy  for  an  engine.  But  in  this  case 
the  Oo  comes  from  the  CgH^.jOe'  ^'^^  "^^^  from  outside,  and 
the  energy  evolved  in  the  reaction  is  only  about  a  tenth  of 
that  liberated  by  the  complete  oxidation  to  CO,  and  HjO 
which  occurs  in  the  animal  body.  The  formation  of  alcohol 
from  sugar  does  not  lead  to  the  liberation  of  energy.  So 
far  as  this  is  concerned,  the  alcohol  may  be  considered  as  a 
by-product. 


PROTOPLASM 


The  behaviour  of  the  yeast  plant  shows  that  such 
protoplasm,  when  placed  in  suitable  conditions,  has  the 
power  of  breaking  down  certain  coviplex  substances,  and  of 
utilising  the  energy  so  liberated  for  building  itself  up,  and 
thus  increasing  and  growing.  It  is  this  power  of  using  this 
energy  for  growth  which  has  enabled  living  matter  to  exist 
and  to  extend  over  the  earth. 

4.  Liberation  of  Energy  by  Protoplasm. — How  is  this 
oxidation  and  liberation  of  energy  effected  ? 

The  answer  to  this  question  has  been  given  by  the 
demonstration  by  Biichner  that  the  expressed  juice  of  the 
yeast    torulce   acts   on    the   sugar   in   the  same  way   as  the 


1 

~~ — --^^ 

•••■ 

.•••'' 

\ 

\ 

'■•A 

-r 

,■■■'-'" 

•■.\ 

Fig.  1. — To  show  the  relationship  of  the  rate  of  enzyme  action  to  tempera- 
ture. The  vertical  lines  represent  different  temperatures.  The 
dotted  line  represents  the  rate  of  enzyme  action  as  modified  by 
temperature.  The  continuous  line  shows  the  destruction  of  the 
enzyme  as  the  temperature  rises.  The  dash  line  shows  the  actual 
rate  of  enzyme  action  as  modified  by  these  two  factors. 

living  yeast.  The  yeast  therefore  manufactures  something 
which  splits  the  sugar.  This  something  belongs  to  the 
group  of  Enzymes  or  Zymins  which  play  so  important  a 
part  in  physiology  generally. 

(1)  Conditions  of  Enzyme  Action. — For  the  manifestation 
of  their  activity  these  enzymes  require  the  presence  of 
water  and  a  suitable  temperature — in  the  case  of  the  yeast 
enzyme  about  36°  C.  is  the  best.  At  lower  temperatures 
the  reaction  becomes  slower  and  is  finally  stopped,  and  at 
a  higher  temperature  it  is  delayed  and  finally  arrested  by 
the  destruction  of  the  enzyme  (fig.  1). 


8  VETERINARY   PHYSIOLOGY 

(2)  Nature  of  Enzyme  Action. — (i.)  Enzymes  act  by 
hastening  reactions  which  go  on  slowly  without  their 
presence,  but  they  do  not  themselves  take  any  direct  part 
in  the  reaction  ;  they  modify  its  rate  but  not  its  extent. 
Hence,  a  very  small  quantity  may  bring  about  an  extensive 
change  in  the  substance  acted  upon,  (ii.)  The  reaction 
does  not  pass  beyond  the  point  of  equilibrium.  Thus,  when 
the  enzyme  maltase  acts  upon  malt  sugar,  it  converts  it 
only  in  part  into  dextrose,  (iii.)  With  certain  enzymes  at 
least,  the  action  may  actually  be  reversed ;  the  enzyme, 
which  splits  esters  into  their  component  acid  and  alcohol, 
may  cause  a  linking  of  those  components  to  form  esters. 
(iv.)  The  rapidity  of  the  reaction  is  retarded  as  it  proceeds 
sometimes  by  the  accumulation  of  H  ions,  sometimes  by  the 
accumulation  of  the  products  of  the  change.  When  these 
are  removed  the  action  may  again  be  accelerated. 

The  general  action  of  enzymes  is  catalytic.  It  may  be  com- 
pared to  the  action  of  an  acid  in  the  inversion  of  cane  sugar — 

Cx2H220n  +  H,0  =  2(CeH,A)- 

Here  an  acid  merely  hastens  a  reaction  which  would  go  on 
slowly  in  the  presence  of  water  alone. 

(3)  Mode  of  Action  of  Enzymes.  —  (a)  The  precise  way  in 
which  such  catalytic  actions  are  brought  about  is  still  not 
quite  clear,  but  there  is  evidence  that  the  catalyser  acts  as 
a  middleman  between  the  reacting  substances — in  the  case 
of  HoO.,  taking  up  the  0  and  then  giving  it  off  and  in  the  case 
of  the  decomposition  of  cane  sugar  taking  up  H^O  and 
handing  it  on.  In  the  same  way  in  the  oxidation  of 
glucose  which  occurs  when  it  is  boiled  with  an  alkali,  a 
metallic  oxide,  such  as  cuprous  oxide,  may  take  oxygen 
from  the  air,  becoming  cupric  oxide,  and  then  hand  the 
oxygen  on  to  the  glucose,  thus  making  the  oxidation  more 
rapid. 

(b)  While  such  catalysers  as  the  inorganic  acids  act  upon 
many  different  substances,  the  enzymes  have  generally  a 
specific  action  upon  one  substance  alone,  the  substrate 
of  the  enzyme.  It  is  as  if  each  enzyme  fitted  one  special 
substrate  as  a  key  fits  one  special  lock.     They  are  generally 


PROTOPLASM  9 

designated  by  attaching  the  suffix  ase  to  the  name  of  their 
substrate,  thus  malta.se  is  the  enzyme  that  acts  upon  malt  sugar. 

Our  knowledge  of  the  chemistry  of  enzymes  is  not 
complete,  because  when  attempts  are  made  to  isolate  them 
it  is  frequently  found  difficult  to  separate  them  completely 
from  their  substrate.  Maltase  has  been  shown  not  to  be  of 
the  nature  of  a  protein. 

(4)  Essential  Nature  of  Enzymes. — They  are  colloids  (see 
p.  12),  and  their  colloidal  character  helps  to  explain  their 
activity. 

The  energy  liberated  by  the  enzyme  of  yeast  is  used  by 
the  protoplasm  for  growth. 

5.  Metabolism  of  Protoplasm While  ordinary  proto- 
plasm gets  its  energy  by  breaking  down  complex  molecules 
and  liberating  their  stored  energy,  green  plants,  by  the  action 
of  their  chlorophyll,  are  able  to  store  the  energy  of  the  sun's 
rays  by  building  up  complex  molecules  such  as  sugar  and 
starch.  It  is  through  these  green  plants  that  the  energy  of 
the  sun  is  made  available  for  all  living  things  upon  the 
earth. 

Protoplasm  is  not  only  growing,  it  is  also  constantly 
breaking  down,  and,  if  yeast  Ido  kept  at  a  suitable  tempera- 
ture in  water  without  any  supply  of  material  for  construction, 
it  gives  off  carbon  dioxide  and  decreases  in  bulk  on  account 
of  these  disintegrative  changes.  These  are  as  essential  a 
part  of  its  life  as  the  building-up  changes,  and  it  is  only 
when  they  are  in  progress  that  the  latter  are  possible. 

Protoplasm  {living  matter)  is  living  only  in  virtue  of  its 
constant  chemical  changes  {metabolism),  and  these  changes 
are  on  the  one  hand  destructive  {katabolic),  on  the  other 
constructive  {anabolic). 

Living  matter  thus  differs  from  dead  matter  in  this 
respect,  that,  side  by  side  with  destructive  changes,  con- 
structive changes  are  always  going  on,  luhereby  its  amount 
is  maintained  or  increased,  so  that  it  has  been  able  to 
spread  all  over  the  surface  of  the  earth. 

Hence  our  conception  of  living  matter  is  not  of  a  definite 
chemical  substance,  but   of  a  set   of  substances    constantly 


10  VETERINARY  PHYSIOLOGY 

undergoing  internal  changes.  It  might  be  compared  to  a 
whirlpool  constantly  dragging  things  into  its  vortex,  and 
constantly  throwing  them  out  more  or  less  changed,  but 
itself  continuing  apparently  unchanged  throughout.  Hoppe- 
Seyler  expresses  this  by  saying  :  "  The  life  of  all  organisms 
depends  upon,  or,  one  can  almost  say,  is  identical  with,  a 
chain  of  chemical  changes."  Foster  puts  the  same  idea  in 
more  fanciful  language  :  "  We  may  speak  of  protoplasm  as  a 
complex  substance,  but  we  must  strive  to  realise  that  what 
we  mean  by  that  is  a  complex  whirl,  an  intricate  dance,  of 
which  what  we  call  chemical  composition,  histological 
structure,    and    gross    configuration    are,    so    to   speak,    the 


Death, — While  the  continuance  of  these  chemical  changes 
in  protoplasm  is  life,  their  stoppage  is  death.  For  the  con- 
tinuance of  life  the  building-up  changes  must  be  in  excess 
of,  or  equal  to,  the  breaking-down — the  evolution  of  energy 
must  be  sufficient  for  growth  or  maintenance.  It  is  only 
the  surplus  over  this  which  is  available  for  external  work. 
In  the  young  the  surplus  energy  is  largely  used  for  growth 
and  develojDment ;  in  adult  life  for  work.  When  failure  in 
the  sup2)ly  or  in  the  utilisation  of  the  energy-yielding 
material  occurs,  the  protoplasm  dwindles  and  disintegrates. 
Death  is  sudden  when  the  chemical  changes  are  abruptly 
stopped,  slow  when  the  anabolic  changes  are  interfered  with. 

The  series  of  changes  which  occur  between  the  infliction 
of  an  incurable  injury  and  complete  disintegration  of  the 
protoplasm  constitute  the  processes  of  Necrobiosis,  and  their 
study  is  of  importance  in  pathology. 

Stimuli. — The  rate  of  the  chemical  processes  in  protoplasm 
may  be  quickened  or  slowed  by  changes  in  the  surroundings, 
and  such  changes  are  called  stimuli.  If  the  stimulus  increases 
the  rate  of  change,  it  is  said  to  excite ;  if  it  diminishes  the 
rate  of  change,  it  is  said  to  depress.  Thus  the  activity  of 
the  changes  in  yeast  may  be  accelerated  by  a  slight  increase 
of  the  temperature  of  the  surrounding  medium,  or  it  may  be 
depressed  by  the  addition  of  such  a  substance  as  chloroform. 


PROTOPLASM  11 

II.  Structure  of  Protoplasm. 

Protoplasm  occurs  as  a  semifluid  transparent  viscous 
material,  usually  in  small  individual  particles — Cells — more 
or  less  associated.  It  may,  however,  occur  as  larger  con- 
fluent masses — Plasmodia. 

Protoplasm  in  its  simplest  state  may  be  regarded  as  a 
fluid,  since  fine  particles  in  it  are  seen  to  move  freely  in 
Brownian  movement  (p.  13  (3)),  and  if  it  contains  drops  of 
water  they  assume  a  spherical  form ;  certain  plasmodial 
masses  of  protoplasm  among  the  myxomycetes  in  which 
granules  exist  may  creep  through  cotton  wool  and  emerge 
without  their  granules,  having  actually  filtered  them  off. 


i^)      CJ^^<v^^     'rvS-,-vv<:i         ('') 


(c) 


Fig.  2.— (a)  Foam  structure  of  a  mixture  of  Olive  Oil  and  Cane  Sugar; 
(6)  Reticulated  structure  of  Protoplasm  ;  (c)  Reticulated  structure 
of   Protoplasm    after  fixation   in    the   cell   of   an    earth-worm   (after 

Bt'TSCHLl). 

But  while  protoplasm  may  thus  be  looked  upon  as 
sssentially  a  fluid,  a  reticulated  appearance  can  frequently  be 
made  out  even  in  the  living  condition  (fig.  2),  and  from  this 
it  has  been  concluded,  chiefly  as  the  result  of  Butschli's 
investigations,  that  there  is  a  somewhat  more  solid  part 
arranged  like  the  films  of  a  mass  of  soap-bubbles,  with  a 
more  fluid  interstitial  part,  a  sort  of  foam  structure  which 
might  be  compared  to  an  emulsion  of  oil  in  a  colloidal  gum 
solution.  A  certain  amount  of  organisation  is  thus  present 
in  most  protoplasm,  and  in  certain  cells  this  organisation 
becomes  very  marked.  This  conception  of  the  structure  of 
protoplasm    leads    us   to  regard   each   vesicle  as    a   minute 


12  VETERINARY   PHYSIOLOGY 

independent  chemical  laboratory,   separated  from  others  by 
more  or  less  permeable  colloidal  walls. 

There  is  good  evidence  that  when  protoplasm  is  coagu- 
lated, a  marked  netted  appearance  may  be  observed  which 
may  vary  greatly  in  character  according  to  the  fixing  reagent 
by  which  the  coagulation  has  been  produced,  and  which  is 
purely  an  artifact. 


III.  Chemical  and  Physical  Conditions. 

Protoplasm  is  not  a  substance,  but  a  mixture  of  various 
substances,  a  heterogeneous  complex,  in  a  constant  state  of 
flux  and  change. 

A.  WATER. 

Water,  holding  or  held  in  a  Colloidal  Complex  with  certain 
crystalloids,  constitutes  something  over  75  per  cent.,  and  acts 
as  the  solvent  or  suspender  of  the  other  materials. 

B.  COLLOIDAL  COMPLEX. 

The  series  of  substances  which  constitute  the  mass  of  all 
protoplasm  exist  in  a  colloidal  state. 

Colloids. 

(1)  Colloids  were  long  ago  distinguished  by  Graham 
from  crystalloids  by  the  fact  that  they  do  not  dialyse 
through  an  animal  membrane  when  mixed  with  water, 
and  he  concluded  that  this  is  due  to  the  large  size  of 
the  molecule  which  constitutes  such  colloids.  More  recent 
investigations  have  shown  that  this  explanation  is  insufficient, 
since  substances  of  small  molecular  weight  may  at  one  time 
exist  in  a  crystalloid  state,  and  at  another  in  a  colloid 
condition,  e.g.,  silicic  acid. 

The  essential  character  of  the  colloidal  state  consists  in 
the  existence  of  matter  in  two  conditions,  "  phases,"  often  so 
finely  subdivided  as  to  render  the  detection  of  the  con- 
dition very  difficult.  The  natures  of  the  external  or 
continuous   phase    and  of   the  internal   or   dispersed  phase 


PROTOPLASM 


13 


suspended  in  the  other  may  vary  greatly.      Bayli 
following  table  to  illustrate  this  : — 


the 


Internal  or 

External  or 

Example. 

Dispersed  Phase. 

Continuous  Phase. 

1.  Gas 

Liquid 

Foam 

2.  Liquid 

Gas 

Fog 

3.  Liquid 

Another  immis- 
cible Liquid 

Emulsion  or  Emulsoid  ;  mists 

4.  Liquid 

Solid 

Jellv,  as  Gelatine  in  some  forms 

5.  Solid 

Gas 

Tobacco  Smoke 

6.  Solid 

Liquid 

Ordinary  Colloid  Solution  or  Suspen- 

soid 
Ruby  Glass 

7.  Solid 

Another  Solid 

Protoplasm  may  be  regarded  as  an  emulsoid. 

The  emulsoids  are  characterised  by  showing  considerable 
viscosity. 

Colloids  have  sometimes  been  divided  into  sols  when  they 
are  fluid,  and  gels  Avhen  they  are  more  solid,  and  there  is 
some  evidence  that  in  the  latter  condition  the  dispersed 
phase  is  the  more  fluid.  (2)  It  is  now  known  that  if  a  beam 
of  light  is  allowed  to  pass  through  a  colloid  it  is  rendered 
visible — the  Faraday-Tyndall  phenomenon — whereas  when 
it  passes  through  crystalloids  in  water  it  is  not  seen.  (3)  If 
a  suspensoid  be  placed  under  a  microscope,  •  and  a  ray  of 
light  be  passed  into  it  along  the  plane  of  the  stage,  it  may- 
be seen  to  be  full  of  shining,  dancing  particles.  These  last 
two  tests  indicate  the  two  phases  in  the  colloid. 

This  fine  subdivision  of  the  two  phases  introduces  an 
element  in  the  behaviour  of  a  colloid  which  is  not  present  in 
the  case  of  a  solution,  namely,  the  presentation  of  an 
enormous  extent  of  surface  between  the  two  phases.  It  has 
been  calculated  that  a  sphere  of  gold  with  a  radius  of  1 
millimetre,  if  subdivided  into  a  gold  sol  or  suspensoid,  would 
present  a  surface  of  no  less  than  100  square  metres.  This 
allows  of  extensive  interactions  between  the  phases,  and 
these  interactions  will  depend  upon  chemical  changes  in 
each.      Thus  great  activity  of  change  is  rendered  possible. 

The  phenomena  of  surface  tension    between    the   phases 


14  VETERINARY   PHYSIOLOGY 

must  manifest  themselves  on  account  of  the  large  surfaces 
exposed.  Surface  tension  is,  of  course,  most  easily  studied, 
at  the  surface  bounding  a  fluid  and  air.  At  such  a 
surface  a  skin  of  increased  tension  exists.  Its  condition  is 
modified  by  many  factors,  among  others  by  the  solutes  in  the 
fluid.  Inorganic  salts  generally  increase,  while  organic  salts 
decrease  it.  The  solutes  which  lower  surface  tension  tend 
to  be  more  concentrated  at  the  surface,  those  raising  it  tend 
to  be  less  concentrated. 

The  amount  of  concentration  at  such  surfaces  is  often 
greater  than  can  be  accounted  for  by  the  condition  of  the 
fluid.  A  process  which  has  been  called  adsorption  occurs, 
one  substance  tending  to  deposit  in  large  amounts  upon  the 
surface  of  another.  This  seems  to  depend  mainly  upon  the 
electric  charge  of  the  two  substances,  those  of  opposite 
charges  tending  to  cling  together.  This  may  occur  between 
substances  in  solution  and  colloids,  or  between  different 
colloid  particles  in  a  compound  sol,  and  may  lead  to  an 
increase  in  the  size  of  the  particles  and  to  precipitation.  It 
may  also  explain  the  membrane-like  covering  of  many  cells. 

It  is  impossible  to  analyse  such  an  ever-changing  substance 
as  protoplasm,  and,  although  what  is  left  when  these  chemical 
changes  are  stopped  can  be  examined,  such  analyses  give 
little  insight  into  the  essential  nature  of  the  living  matter. 

The  chief  constituents  found  after  the  death  of  protoplasm 
are  the  proteins,  along  with  small  amounts  of  fatty  substances, 
carbohydrates,  and  crystalloids. 

Substances  entering  into  the  Colloidal  Complex. 
1.  Proteins. 

1.  Physical  Characters.— The  Native  Proteins — those  which 
may  be  separated  from  the  residue  of  living  matter — have  a 
white,  yellow,  or  brownish  colour  when  dried.  In  structure 
they  are  usually  amorphous,  but  many  have  been  prepared 
in  a  crystalline  condition,  and  it  is  probable  that  all  may  take 
a  crystalline  form.  The  crystals  vary  in  shape,  being  usually 
small  and  needle-like,  but  sometimes  forming  larger  rhombic 


( 


PROTOPLASM  15 

plates.  In  protoplasm  proteins  form  part  of  the  emulsoid 
complex.  But  some  purified  proteins  form  colloidal  suspensions 
— hydrosols — in  water,  some  require  the  presence  of  neutral 
inorganic  salts,  others  of  an  acid  or  alkali,  while  some  are 
completely  insoluble  and  uususpendable  without  a  change  in 
their  constitution.  The  animal  proteins  are  insoluble  in 
alcohol ;  all  proteins  are  insoluble  in  ether. 

All  proteins  rotate  the  plane  of  polarised  light  to  the  left 
on  account  of  the  action  of  the  amino  acids  of  which  they 
are  composed  (p.  16), 

2.  Chemistry.— (1)  Percentage  Composition. — Proteins 
contain  the  following  chemical  elements  : — Carbon,  hydrogen, 
oxygen,  nitrogen,  and  sulphur,  in  about  the  following 
percentage  amounts  : — 

c.  H.  N.  s.  o. 

52  7  16  1  24 

It  is  important  to  remember  the  amounts  of  nitrogen  and 
carbon,  since  proteins  are  the  sole  source  of  the  former 
element  in  the  food  and  an  important  source  of  the  latter. 

(2)  Size  of  Molecules.— -As  regards  the  number  of 
atoms  of  these  elements  which  go  to  form  a  single  molecule, 
information  is  afforded  by  the  percentage  of  sulphur  in  the 
molecule,  and  by  the  number  of  atoms  of  various  metals 
which  combine  with  a  molecule  of  the  protein.  The  follow- 
ing probable  formula  for  the  molecule  of  the  chief  protein  of 
the  white  of  egg  is  given  simply  to  shoAV  how  complex  these 
substances  are  : — C204H322N52O66S0. 

(o)  Constitution. — The  constitution  of  the  protein 
molecule  has  been  investigated  (A)  by  studying  the  products 
of  the  decomposition  of  the  molecule  by  various  agents,  and 
(B)  by  attempting  to  build  up  the  molecule  by  the  synthesis 
of  the  products  of  disintegration. 

(A)  The  native  proteins  have  very  large  molecules,  and 
they  tend  to  break  down  into  simpler  proteins.  This 
decomposition  is  accompanied  by  hydration,  and  it  is 
accelerated  by  the  action  of  acids,  and  by  a  group  of 
enzymes  which  may  be  named  proteolytic  enzymes. 

Under  the  action  of  alkalies  and  under  the  influence  of 


16 


VETERINARY  PHYSIOLOGY 


certain  bacteria  other  lines  of  breaking  down  may  occur,  but 
these  have  thrown  less  important  light  upon  the  constitution 
of  the  proteins. 

As  the  molecules  become  smaller  the  colloidal  characters 
decrease,  dittusion  through  animal  membranes  becomes  more 
marked,  and  the  tendency  to  precipitate  is  lessened.  This 
latter  property  has  been  used  to  classify  the  proteins  into 
three  main  groups — 

Precipitated  by — 
1.  Native  Proteins     boiling  and  by  saturation  with  (NH4)2S04. 
not   by  boiling   but   by   saturation    with 

(NH,),SO,. 
not   by  boilina;,  nor   by   saturation    with 


3.  Peptones  . 


Products  of  Disintegration. — Under  the  influence  of  a 
prolonged  action  of  acids  and  of  certain  enzymes  such  as 
the  erepsin  of  the  intestine  (p.  828)  the  splitting  of  the 
molecule  is  carried  further,  till  finally  the  greater  part  of 
the  nitrogen  of  the  protein  comes  to  be  distributed  in  bodies 
of  three  different  classes — 


1.  Mon-amino  acids  . 

2.  Di-amino  acids 

3.  Amides  and  ammonia  compounds 


about  70  per  cent. 
»     20       „ 
„     10       „ 


1.  Mon-amino  Acids  are  thus  the  bodies  from  which  the 
proteins  are  chiefly  formed,  and  the  character  of  the  proteins 
of  different  animals  and  of  different  tissues  depends  largely 
upon  the  mon-amino  acids  which  predominate. 

The  different  proportions  of  these  which  occur  in 
different  proteins  is  indicated  by  the  following  table  : — 


Glycin. 

Alanin. 

Leucin, 
&c. 

Arginin. 

Trypto- 
phan. 

Tyrosin. 

Serum  Albumen 

0-0 

2"7 

20 

? 

+ 

2-1 

Excelsin  (from  Brazil 

Nut)      . 

0-6 

2-3 

8-7 

16-0 

+ 

31 

Gelatin     . 

16-5 

0-8 

2-1 

76 

0-0 

0-0 

Salmin  (from   Sper- 

matozoa of  Salmon) 

00 

0-0 

-t- 

87-4 

? 

0-0 

PROTOPLASM  17 

In  most,  iso-amino-caproic  acid,  or  leucin,  is  the  most 
abundant,  and  in  disintegrative  diseases  of  the  liver,  such 
as  acute  yellow  atrophy,  it  may  appear  in  the  urine, 
separating  out  as  oily-looking  spherules. 

Some  of  the  mon-amino  acids  split  off  from  the  protein 
molecule  attached  to  the  benzene  ring,  or  to  the  benzene  ring 
linked  to  a  pyrrol  ring.  In  Tryptophan,  amino-propionic 
acid  (Alanin)  is  linked  to  such  a  complex — 

H  NH„  0 

I    Til 

C-C— C— 0— H 

I      I 
H    H 
NH 

In  tyrosin,  alanin  is  linked  to  an  hydroxy  benzene  ring. 

H  NH  0 

/— \       I       I       II 
0H<  \_C— C— C— OH 

\_/      I      I 
H    H 

Like  leucin,  it  ma}^  appear  in  the  urine  in  disintegrative 
diseases  of  the  liver.  It  takes  the  form  of  rosettes  ol 
acicular  crystals. 

2.  Di-amino  Acids, — In  most  proteins,  the  di-amino  acids 
are  less  abundant  than  the  mon-amino.  But,  in  a  simple 
form  of  protein,  which  occurs  linked  with  nucleic  acid  in 
the  heads  of  spermatozoa,  called  by  Kossel  protamine,  these 
di-amino  acids  constitute  about  80  per  cent,  of  the  molecule. 
They  are — 

Lysin — di-amino-caproic  acid. 

Arginin — amino-valerianic  acid  linked  to  guanidin 
(p.  209). 

Histidin — amino-propionic  acid  linked  to  the  iminazole 
ring. 

Of  these  three  arginin  is  the  most  abundant. 

3.  Amides  are  always  present  in  small  amount,  linked 
together  as  in  biuret  H.N— CO— NH— CO— NH,,  and  it  is 


18 


VETERINARY   PHYSIOLOGY 


these  which  give  the  biuret  test — the  pink  or  violet  when 
sodium  hydrate  is  added  to  the  protein  with  which  a  trace  of 
cupric  sulphate  has  been  mixed. 

The  different  stages  of  the  disintegration  of  the  native 
protein  molecule  may  be  arranged  as  follows,  according  to 
their  reactions  to  heat,  ammonium  sulphate,  and 
alcohol :  ^ — 


Native  Proteins. 

Proteoses. 

Peptones. 

Polypeptides. 

Dipeptides. 
Final  Products. 


-o 

c3    >^. 

'^ 

.  rrt 

13 

a-^ 

III 

ll 

^^  '^ 

.r"^ 

Sriffi 

S^ 

£.-a 

(^ 

^ 

rt 

o 

>^ 

CJ 

1 

*j 

._ 

rt 

rt 

^ 

_a. 

r  ^ 

o 

2 

1 

PL, 

Sh 

o 

o 

o 

o 

"c 

^ 

^2; 

'^ 

o 


For  a  "  Classification  of  Proteins,"  see  Appendix.  The 
tests  for  the  Proteins  must  he  learned  practically. 

B.  Synthesis  of  Proteins. — Emil  Fischer  and  his  co-workers 
have  succeeded  in  building  up  from  the  amino  acids  a  series 
of  bodies  containing  several  of  the  amino  acid  molecules, 
linked    to    one    another    in    series,    the    hydroxyl,    OH,    of 

1  For  a  short  account  of  the  chemistry  of  these  products  of  disintegra- 
tion, see  Appendix. 


PROTOPLASM  19 

one  being  linked  on  to  the  amidogen  of  the  other  with  the 
giving  off  of  HjO,  thus  : — 


:^>- 


H     0 

1  OH  H  j 

H    H 

u 

1 

H 

t      t 

1       1 
N— C- 

1 
H 

0 
-C— OH 


Amino-acetic  acid.         Amino-acetic  acid. 

Glyciu.  Glycin. 

Glycyl-glycin. 

This  he  calls  glycyl-glycin, — the  amino  acids  which  have 
lost   the  OH  of  the  acid  being  designated  by  the  terminal 

yi-  _^ 

Such  compounds  he  calls  peptides,  characterising  them, 
according  to  the  number  of  molecules  linked,  as  di-,  tri-, 
tetra-,  and  poly-peptides. 

Some  of  the  higher  of  these  give  the  biuret  test  for 
proteins  from  the  presence  of  the  linked  CO.NHo  group  ;  and 
if  an  acid  with  the  benzene  ring  is  in  the  chain,  they  also 
give  the  xantho-proteic  test. 

He  has  also  succeeded  in  building  the  pyrrol  derivative, 
pyrrolidine-carboxylic-acid,  or  prolin,  into  polypeptides. 

2.  Fats  and  Lipoids. 

In  addition  to  ordinary  fats  (see  p.  41),  protoplasm  also 
contains  a  group  of  substances  which  are,  like  the  fats, 
soluble  in  alcohol  and  ether,  and  which  have  been  called 
Lipoids.  These  are  generally  most  abundant  in  the  outer 
layers  of  protoplasmic  units,  where  they  help  to  form  a 
covering  membrane. 

One  of  the  most  important  is  Cholesterol,  This  is  a  mono- 
hydric  unsaturated  alcohol,  which,  when  dissolved  in  hot 
alcohol,  tends  to  crystallise  out  on  cooling  in  flat  square 
plates,  generally  with  a  notch  out  of  a  corner. 

Some  of  the  lipoids  contain  phosphorus,  and  have  been 
grouped  as  Phosphatides.  The  most  important  of  these  is 
Lecithin. 

This  is  a  fat  in  which  one  of  the  acid  radicles  is  replaced 


20  VETEEINARY   PHYSIOLOGY 

by    phosphoric     acid     linked     to     cholin  —  hydroxyethyl- 
trimethyl-ammonium-hydroxide. 


C  Fatty  acid. 
<  Fattv  aci( 


Lecithin.  Glycerol  <  Fatty  acid. 

(  Phosphoric  acid. 

I 

Cholin. 


Cholin. 


H     H 

OH 

1        1 
HO— C— C- 

1    /cu. 

1       1 

\CH3 

H     H 

ydroxyethyl 

trimethyl-ammonium 

hydroxide. 

Cholin  has  some  action  upon  visceral  muscle,  and  some 
of  the  symptoms  occurring  in  degenerative  changes  of  the 
nervous  system  may  be  due  to  its  presence.  It  is  closely 
allied  to  muscarin,  a  very  powerful  jjoison  of  vegetable  origin. 

In  protoplasm,  the  lipoids  are  intimately  associated 
with  the  proteins,  and  form  part  of  the  colloidal  complex. 
By  their  tendency  to  adsorb  to  the  surface,  and  thus  to  form 
membranes,  they  help  to  differentiate  masses  of  protoplasm 
from  their  surroundings  and  to  present  a  more  or  less  per- 
meable membrane,  through  which  exchanges  between  the 
living  matter  and  its  surroundings  go  on. 

3.  Carbohydrates. 

Closely  connected  with  the  proteins  and  fats,  and  some- 
times actually  built  into  the  molecule  of  the  former,  are 
small  quantities  of  carbohydrates,  bodies  belonging  to  the 
class  of  starches  and  sugars  (p.  285). 

C.   CRYSTALLOIDS. 

These  may  either  exist  in  true  solution  or  be  combined 
with  the  proteins  and  lipoids  in  the  colloidal  state.      Those 


PROTOPLASM  21 

in  solution  may  not  be  ionised,  e.g.  glucose,  or  they  may  be 
ionised  into  an-ions  and  cat-ions.  Among  the  more 
important  of  tlie  cat-ions  are  potassium,  calcium,  and 
sodium.  The  presence  of  such  crystalloids  free  in  true 
solution  chiefly  determines  the  osmotic  pressure  of  the  mass 
of  protoplasm,  and  hence  this  may  vary  from  time  to  time 
according  to  whether  these  substances  are  united  to  the 
proteins  or  are  free. 

The  osmotic  pressure  in  the  protoplasm  of  a  cell  may  be 
ascertained  by  subjecting  it  to  fluids  of  different  osmotic 
equivalents  and  determining  whether  it  swells  by  the  passage 
of  fluid  inwards  or  shrinks  by  the  passage  of  water  outwards, 
thus  ascertaining  the  molecular  concentration  of  the 
surrounding  fluid  and  so  of  the  cell  itself.  This  has  been 
called  the  method  of  Plasmolysis.  The  red  cells  of  the 
blood  have  a  very  definite  osmotic  pressure,  and  when 
subjected  to  a  fluid  of  lower  osmotic  pressure,  they  swell, 
while  in  a  fluid  of  higher  osmotic  pressure,  they  shrink. 
This  is  called  the  method  of  Hcemolysis. 


IV,  Protoplasmic  Activity. 

This  complex  of  substances  called  protoplasm  is,  during 
life,  in  a  constant  state  of  active  chemical  change.  All 
its  conditions  make  for  great  instability  :  its  colloidal 
nature,  its  demarcation  from  its  surroundings  as  the  result 
of  surface  tension  with  adsorption,  its  frequent  division  into 
innumerable  vesicles  separated  from  one  another  by  very 
unsubstantial  and  temporary  septa  ever  changing  as  the  result 
of  internal  chemical  changes,  all  of  these  combine  to  produce 
a  very  labile  condition.  Thus  oxidation  may  be  going  on  in 
one  part  of  the  mass,  drawing  oxygen  from  another,  and  thus 
leading  to  a  simultaneous  process  of  reduction.  Such  a 
mechanism  is  pregnant  with  possibilities  as  a  transformer 
of  energy  and  as  a  producer  of  movement. 


22  VETERINARY   PHYSIOLOGY 

That  such  movements  really  can  be  produced,  even  in 
dead  matter,  by  the  combination  of  changes  brought  about 
by  osmosis  and  by  alteration  in  surface  tension,  may  be 
demonstrated  by  the  behaviour  of  mixtures  of  such 
substances  as  camphor  and  water. 


SECTION     11. 

The  Cell. 

Protoplasm  occurs  in  the  animal  body  as  small  separate 
masses  of  Cells.  These  vary  considerably  in  size,  but  in  the 
higher  animals,  on  an  average,  they  are  from  7  to  20  micro- 
millimetres^  in  diameter.  The  advantage  of  this  subdivision 
is  obvious.  It  allows  nutrient  matter  to  reach  every  particle 
of  the  protoplasm.  In  all  higher  animals  each  Cell  has  a 
perfectly  definite  structure.  It  consists  of  a  mass  of  proto- 
plasm, in  which  is  situated  a  more  or  less  defined  body,  the 
nucleus. 

A.  Cell  Protoplasm. 

1.  Structure. — The  general  characters  of  protoplasm  have 
been  already  described  (p.  11).  In  some  cells  condensation 
at  the  surface  is  marked,  and  a  so-called  membrane 
surrounds  them. 

At  some  point  in  the  protoplasm  of  many  cells,  one  or 
two  small  spherical  bodies,  the  centrosomes  (fig.  3),  are  found, 
from  which  rays  pass  out  in  ditferent  directions.  For  the 
detection  of  these  bodies  special  methods  of  staining  and  the 
use  of  very  high  magnifying  powers  are  required.  They  will 
be  again  considered  when  dealing  with  the  reproduction  of 
cells. 

The  cell  protoplasm  frequently  contains  granules,  either 
formed  in  the  protoplasm,  or  consisting  of  material  ingested 
by  the  cells. 

In  the  protoplasm,  vacuoles  are  sometimes  found,  and 
from  a  study  of  these  vacuoles  in  protozoa,  it  appears  that 

1  The  micro-millimetre  is  the  j  oW*-^'  ^^  ^  millimetre. 

23 


24 


VETERINARY   PHYSIOLOGY 


they  are  often  formed  round  material  which  has  been  taken 
into  the  protoplasm,  and  that  they  are  filled  with  a  fluid 
which  can  digest  the  nutritious  part  of  the  ingested  particles. 
In  some  cells,  vacuoles  may  appear  in  the  process  of 
disintegration. 

2.  Activities. — (a).  Many  cells  have  the  power  of  ingest- 
ino"     foreign     material.      This    phagocytic     action     plays    an 


Fia.  3.— Diagram  of  a  Cell  to  show  structure  of  Protoplasm  and  Nucleus.  In 
the  protoplasm — ^,  attraction  sphere  enclosing  two  centrosomes  ;  V, 
vacuole  ;  C,  included  granules  ;  B,  plastids,  present  in  some  cells.  In 
the  nucleus — B,  nucleolus  ;  F,  chromatin  network  ;  G,  linin  network  ; 
H,  karyosome  or  nodal  swelling.      (Wilson.) 


enormously    important    part    both   in   physiological    and    in 
pathological  processes  in  the  body. 

(b)  In  certain  cells,  protoplasm  undergoes  changes  in  shape 
(aTtioeboid  movement).  This  may  be  studied  in  the  white 
cells  in  the  blood  of  the  frog  or  newt.  Processes  (pseudo- 
podia)  are  pushed  out,  and  these  are  again  withdrawn,  or 
the  whole  cell  may  gradually  follow  the  process,  and  thus 
change  its  position. 


THE   CELL  25 

In  some  unicellular  organisms  movements  take  place  along 
some  definite  line,  and  fibrils  are  found  arranged  more  or  less 
parallel  to  the  line  of  movement.  Such  contractile  processes, 
from  their  resemblance  to  muscles,  have  been  termed  myoids. 
In  other  protozoa  the  pseudopodia  manifest  a  to-and-fro 
rhythmic  waving  movement,  which  may  cause  the  cell  to  be 
moved  along,  or  may  cause  the  adjacent  fluid  to  move  over 
the  cell.  Such  mobile  processes,  when  permanent,  have  been 
called  cilia.  These  movements  are  the  result  of  chemical 
changes  in  the  protoplasm,  by  which  alterations  in  the 
osmotic  pressure  and  changes  in  the  surface  tension  of  the 
various  parts  are  produced. 

The  movements  are  modified  by  the  various  Stimuli 
which  alter  the  activity  of  the  chemical  changes  (p.  10). 
The  stimulation  may  be  (a)  General. —  Cooling  diminishes 
and  finally  stops  them.  Gentle  heat  increases  them,  but 
when  a  certain  temperature  is  reached  they  are  stopped. 
Drying  and  various  drugs,  such  as  chloroform,  also  arrest 
the  movements. 

(6)  Unilateral. —  Changes  in  the  surroundings  may  cause 
either  contraction  or  expansion,  may  repel  or  attract.  When 
an  attracting  or  a  repelling  influence,  a  positive  or  a  negative 
stimulus,  acts  at  one  side  of  the  cell — unilateral  stimulation — 
it  may  lead  to  movement  of  the  cell  away  from  it  or  towards 
it.  If  the  action  is  towards  the  stimulus,  it  is  said  to  be 
'positive  ;  if  away  from  it,  negative. 

Chemiotaxis  is  the  attraction  or  repulsion  produced  by 
one-sided  application  of  chemical  stimuli.  This  is  well  seen 
in  the  plasmodial  masses  of  cethalium  septicum,  which  grows 
on  tan.  Oxygen  and  water  both  attract  it,  exercising  a 
positive  chemiotaxis.  It  is  also  seen  in  the  streaming  of 
the  white  cells  of  the  blood  to  disintegrating  tissues,  or  to 
various  micro-organisms  introduced  into  the  tissues  which 
have  to  be  destroyed  to  prevent  their  poisoning  the  organism, 
and  in  the  attraction  exercised  by  the  ovum  upon  the  male 
element  in  reprodiiction. 

Barotaxis  is  the  effect  of  unilateral  pressure  or  mechanical 
stimulation.  Many  protozoa  appear  quite  unable  to  leave 
the  sohd  substance — e.g.  the  microscope  slide — with  which 


26  VETERINARY   PHYSIOLOGY 

they  are  in  contact,  the  unilateral  pressure  seeming  to  cause 
a  positive  attraction  in  that  direction.  It  is  well  seen  in 
climbing  plants. 

Phototaxis. — Light,  which  plays  so  important  a  part  in 
directing  the  movements  of  the  higher  plants,  also  acts  posi- 
tively or  negatively  on  many  unicellular  organisms.  Thus, 
the  swarm  spores  of  certain  sdgss  are  positively  attracted 
by  moderate  illumination,  streaming  to  the  source  of 
light,  while  they  are  negatively  stimulated  by  strong 
light,  and  stream  away  from  it.  Light  also  plays  an 
important  part  in  directing  the  movements  of  certain 
bacteria. 

Thermotaxis. — The  unilateral  influence  of  temperature  is 
well  seen  in  the  plasmodium  of  cethalium  septicum  which 
streams  from  cold  water  towards  water  at  a  temperature  of 
about  30°  C. 

Galvanotaxis. — A  current  of  electricity  has  a  marked  effect 
in  directing  the  movements  of  many  cells.  Certain  infusoria, 
when  brought  between  the  poles  of  a  galvanic  battery,  stream 
towards  the  negative  pole,  while  other  organisms  move  to  the 
positive. 


The  etfects  of  this  unilateral  stimulation  are  of  great 
importance  in  physiology  and  pathology,  since  they  explain 
the  streaming  of  leucocytes  to  the  intestine  during  digestion 
and  to  parts  of  the  body  infected  by  micro-organisms  and 
other  poisons.  They  also  explain  the  apparently  volitional 
acts  of  unicellular  organisms.  Many  of  these  organisms 
appear  definitely  to  select  certain  foods,  but  in  reality  tliey 
are  simply  impelled  towards  them  by  this  unilateral 
stimulation. 


B.   Nucleus. 

(1)  Structure. — The  nucleus,  seen  with  a  moderate  magni- 
fying power,  appears  in  most  cells  as  a  well-defined  circular 
or  oval  body  situated  towards  the  centre  of  the  cell  (figs.  2  (c) 


THE   CELL  27 

and  3).  Sometimes  it  is  obscured  by  the  surrounding  proto- 
plasm. It  has  a  granular  appearance,  and  usually  one  or 
more  clear  refractile  bodies  —  the  nucleoli  —  are  seen 
within  it.  It  stains  deeply  with  many  reagents  of  a 
basic  reaction,  such  as  bjematoxylin,  carmine,  methylene 
blue,  etc. 

In  some  cells  the  nucleus  is  irregular  in  shape,  and  in 
some  it  is  broken  up  into  a  number  of  pieces,  giving  the  cell 
a  multi-nucleated  character. 

It  is  usually  composed  of  (a)  fibres  arranged  in  a  compli- 
cated network  (fig.  3).  These  fibres  appear  to  be  of  two  kinds: 
(1)  those  forming  a  fine  network — the  linin  network  (G)  ; 
and  (2)  those  forming  generally  a  coarser  network,  the  fibres 
of  which  have  a  special  affinity  for  basic  stains — the  chromatin 
network  (F). 

The  chromatin  fibres  vary  in  their  arrangement  in  dif- 
ferent cells.  Usuall}'  they  form  a  network,  but  occasionall}'- 
they  are  disposed  as  a  continuous  skein.  In  nuclei  with  the 
former  arrangement  of  fibres,  swellings  may  be  observed 
where  the  fibres  unite  with  one  another — the  nodal  swellings, 
or  karyosomes,  distinct  from  the  nucleolus.  The  resting 
nucleus  appears  to  be  surrounded  by  a  distinct  nuclear 
membrane,  which  is  either  a  basket-like  interlacement  of 
the  fibres  at  the  periphery,  or  a  true  membrane  produced 
by  adsorption. 

Between  the  fibres  is  (/>)  a  more  fluid  material  which  may 
be  called  the  nuclear  plasma  or  haryoplasm.  Digestion  in 
the  stomach  removes  the  nuclear  plasma,  but  leaves  the 
network  unacted  upon. 

(2)  Chemistry. — The  nuclear  network  is  composed  of 
Nucleins.  These  are  combinations  of  protamines  (p.  17) 
with  nucleic  acid. 

Protamines  constitute   about   one-third    and  nucleic  acid 

about  two- thirds  of  the  nucleins.      The  chief  di-amino  acid 

in  them  is  Arginin,  which  constitutes  nearly  90  per 
cent. 

The  nucleic  acid  may  be  broken  down  into — 

1.  Purins,  such  as  guanin  and  adenin  (see  Appendix). 


28  VETERINARY   PHYSIOLOGY 

2.  Pyrimidin  Bases,  such  as  thymin,  which  contain  the 

asymmetric  ring 

— N— C- 

I      I 
=  C    C— 

I    II 

— N— 0— 

3.  Hexoses  (CeHiA)'  (P-  285). 

4.  Metaphosphoric  Acid  (HPO3). 

(3)  Functions. — The  part  taken  by  the  nucleus  in  the 
general  life  of  the  cell  is  not  fully  understood. 

1st.  It  exercises  an  influence  on  the  nutritive  processes, 
since  it  has  been  observed  in  certain  of  the  large  cells  in 
lower  organisms  that  a  piece  of  the  protoplasm  detached 
from  the  nucleus  ceases  to  grow,  and,  after  a  time,  dies. 
In  certain  cells,  e.g.  cells  of  the  nervous  S3^stem,  it  has  been 
found  to  shrink  and  to  become  displaced  from  its  central 
position  as  a  result  of  continued  activity.  Important  inter- 
changes of  material  go  on  between  the  nucleus  and  the 
protoplasm. 

2nd.  It  is  the  great  reproductive  organ  of  the  cell  play- 
ing an  important  part  in  transmitting  inherited  characters 
(p.  617). 

C.    Reproduction  of  Cells. 

Cells  do  not  go  on  growing  indefinitely.  When  they 
reach  a  certain  size  they  generally  either  divide  to  form 
two  new  cells,  or  die  and  undergo  degenerative  changes. 
The  reason  of  this  is  possibly  to  be  found  in  the  well-known 
physical  fact,  that,  as  a  sphere  increases  in  size,  the  mass 
increases  more  rapidly  than  the  superficies.  Hence,  as  a 
cell  becomes  larger  and  larger,  the  surface  for  nourishment 
becomes  smaller  and  smaller  in  relation  to  the  mass  of 
material  to  be  nourished.  Probably  the  altered  metabolism 
so  produced  sets  up  the  changes  which  lead  to  the  division 
of  the  cell.  These  changes  have  now  been  very  carefully 
studied  in  a  large  number  of  cells,  and  it  has  been  shown 
that  the  nucleus  generally  takes  a  most  important  part  in 
division. 


THE  CELL 


29 


A.  Mitotic  Division. — In  a  cell  about  to  divide,  the  first 
change  is  a  general  enlargement  of  the  nucleus.  At  the 
same  time  the  centrosome  becomes  double,  and  the  two 
portions  travel  from  one  another,  but  remain  united  by 
delicate  lines  to  form  a  spindle-shaped  structure  (fig.  4  (1)). 
The  spindle  passes  into  the  centre  of  the  nucleus,  and  seems 
to  direct  the  changes  in  the  reticulum.  The  nuclear  mem- 
brane  disappears,   and   the   nucleus   is   thus  not  so   sharplj' 


(3) 

Fig.  4.  — Nucleus  in  Mitosis.     (1)  Convoluted  stage  ;  (2)  Monaster 
stage  ;  (3)  Dyaster  stage  ;  (4)  Complete  division. 

marked  off  from  the  cell  protoplasm.  The  nucleoli  and 
nodal  points  also  disappear,  and  with  them  all  the  finer 
fibrils  of  the  network,  leaving  only  the  stouter  fibres,  whicli 
are  now  arranged  either  in  a  skein,  or  as  loops  with  their 
closed  extremity  to  one  pole  of  the  nucleus  and  their  open 
extremity  to  the  other.  The  nucleus  no  longer  seems  to 
contain  a  network,  but  appears  to  be  filled  with  a  convoluted 
mass  of  coarse  fibres,  and  hence  this  stage  of  nuclear  division 
is  called  the  convoluted  stage. 

The  spindle  continues  to  grow  until  it  occupies  the  whole 


30  VETERINARY   PHYSIOLOGY 

length  of  the  nucleus.  The  two  centrosomes  are  now  very 
distinct,  and  from  them  a  series  of  radiating  lines  extends  out 
into  the  protoplasm  of  the  cell. 

The  nuclear  loops  of  fibres  break  up  into  short,  thick 
pieces  ;  and  these  become  arranged  around  the  equator  of 
the  spindle  in  a  radiating  manner,  so  that  when  the  nucleus 
is  viewed  from  one  end  it  has  the  appearance  of  a  rosette  or 
a  conventional  star.  This  stage  of  the  process  is  hence  often 
called  the  single  star  or  monaster  stage  (fig.  4  (2)). 

Each  loop  now  splits  longitudinally  into  two,  the  divisions 
lying  side  by  side  (tig.  4  (2)). 

The  next  change  consists  in  the  separation  ironi  one 
another  of  the  two  halves  of  the  split  loops — one  half  of 
each  passing  up  towards  the  one  polar  body,  the  other  half 
passing  towards  the  other.  It  is  the  looped  parts  which  first 
separate  and  which  lead  the  way — the  open  ends  of  the  loops 
remaining  in  contact  for  a  longer  period,  but,  finally,  also 
separating.  In  this  way,  around  each  polar  body,  a  series  of 
looped  fibres  gets  arranged  in  a  radiating  manner,  so  that  the 
nucleus  now  contains  two  rosettes  or  stars,  and  this  stage  of 
division  is  hence  called  the  dy aster  stage  (fig.  4  (3)). 

The  single  nucleus  is  now  practically  double.  Gradually 
in  each  half  finer  fibres  develop  and  produce  the  reticular 
appearance.  Nuclear  nodes,  nucleoh,  and  the  nuclear  mem- 
brane appear,  and  thus  two  resting  nuclei  are  formed  from 
a  single  nucleus.  Between  these  two  nuclei  a  delicate  line 
appears,  dividing  the  cell  in  two,  and  the  division  is  com- 
pleted (fig.  4  (4)). 

The  network  of  the  nucleus  of  actively  dividing  cells  is 
rich  in  nucleic  acid,  but  in  cells  which  have  ceased  to  divide, 
in  which  the  nucleus  has  ceased  to  exercise  its  great  repro- 
ductive function,  the  amount  of  nucleic  acid  diminishes,  and 
may  be  actually  less  than  the  amount  in  the  cell  protoplasm. 

B.  Amitotic  Division. — In  some  cells  the  nucleus  does  not 
appear  to  take  an  active  part,  the  cell  dividing  without  the 
characteristic  changes  above  discussed. 


SECTION     III. 

The  Tissues. 

All  the  tissues  of  the  body  are  formed  from  a  single  cell — 
the  ovum. 

In  unicellular  organisms  the  functions  of  nutrition  and 
of  reproduction  are  performed  by  the  one  cell.  In  the 
metazoa  there  is  a  differentiation  into  gametic  or  reproductive 
cells,  and  somatic  or  body  cells  which  form  the  various 
tissues  of  the  individual.  The  latter  are  primarily  developed  to 
nourish  and  protect  the  gametic  cells  which  are  potentially 
eternal,  going  on  from  generation  to  generation,  while  the 
somatic  cells  perish  with  the  death  of  the  individual. 

The  mammalian  ovum  is  holoblasdc  and  undergoes  com- 
plete division.  The  cells  get  arranged  in  three  layers,  the 
epiblast,  mesoblast,  and  hypoblast,  and  from  these  the 
tissues  are  developed. 

The  structure  of  the  tissues  nuist  be  studied  practically 
in  the  class  of  Histology.  Here  all  that  will  be  given  is  a 
brief  summary  of  their  development,  and  of  their  structural 
and  chemical  characters. 


(A)  THE    VEGETATIVE    TISSUES. 

The  Vegetative  Tissues  are  those  which  support,  bind 
together,  protect,  and  nourish  the  body.  They  may  be 
divided  into  the  Epithelial  Tissues,  formed  from  the  epiblast 
or  hypoblast,  and  consisting  of  cells  placed  upon  surfaces, 
and  the  Connective  Tissues  developed  from  the  mesoblast, 
and  consisting  chiefly  of  formed-material  between  cells. 


VETERINARY   PHYSIOLOGY 


I.    EPITHELIUM. 

1.  Squamous  Epithelium— 

(a)  Simple  Squamous  Epithelium. — This  is  seen  lining  the 
air  vesicles  of  tlie  lungs.  It  consists  of  a  single  layer  of  flat, 
scale-like  cells,  each  with  a  central  nucleus.  The  outlines  of 
these  cells  may  be  made  visible  by  staining  with  nitrate 
of  silver,  which  blackens  the  cement  substance  between  the 
cells. 

(h)  Stratified  Squamous  Epithelium  (tig.  5). — The  skin  and 
the  lining  membrane  of  the  mouth  and  gullet  are  covered  by 
several  layers  of  cells.  The  deeper  cells  divide,  and,  as  the 
young  ones  get  pushed  upwards  towards  the  surface  and  away 
from  the  nourishing  fluids  of  the  body,  their  nutrition  is 
modified,  and  the  protoplasm  undergoes  a  change  into 
keratin,  a  substance  belonging  to  the  group  of  sclero-proteins 
(Appendix).  It  is  a  hard,  horny 
material.  It  composes  the  nails 
and  hair,  and  the  horns  and 
hoofs  of  certain  animals.  It 
first  makes  its  appearance  as  a 
number  of  little  masses  or  gran- 
ules in  the  cells,  and  these  run 
together  to  fill  the  cells  which 
„      _     c-      -^  ■,  c,  ..  ■     become  flattened  out  into  thin 

J^iG.  0.  —  btratmed  Squamous  Epi- 

thelium  from  the  cornea.  SCaiCS. 

It  forms  an  admirable  pro- 
tective covering  to  the  body,  not  only  on  account  of  its  hard- 
ness and  toughness,  but  because  poisons  cannot  readily  pass 
through  it,  and  also  because  it  is  not  easily  acted  on  by 
chemicals.  It  is  characterised  by  the  large  proportion  of 
loosely  combined  sulpluir  which  it  contains.  Hence,  lead 
solutions  colour  keratin  black  by  forming  the  black  sulphide 
of  lead,  and  are  much  used  as  hair  dyes  (see  Chemical 
Physiology). 

The     sulphur     is     largely     in     the     form     of    cystin — 


EPITHELIUM 


33 


two     thio  -  amino  -  propionic     acid     molecules     linked     to- 
gether. 

H   NH., 

I       I 
H— C— C— CO.OH 

I       I 
S    H 

I 

S    H 

I       I 
H—C—C— CO.OH 

I      I 
H   NH, 

Tyrosin  (p,  17)  is  also  relatively  abundant. 
(c)  Transitional  Epithelium. — A  slightly  modified  stratified 
squamous  epithelium  lines  the  urinary  passages.      It  is  char- 


FiG.  6 


-(«)  Columnar  Epithelium  from  the  small  intestine  ;  (b)  Cili 
Epithelium  from  the  trachea. 


acterised  by  the  more  columnar  or  pear-like  shape  of  the  cells 
of  the  deeper  layers,  and  by  the  cells  being  very  elastic,  so  that 
they  may  be  stretched  or  compressed  according  to  the  state 
of  the  viscus  they  line, 

2.  Columnar  Epithelium  (fig.  6,  a). — The  cells  lining 
the  stomach  and  intestine  in  the  embryo  elongate  at  right 
angles  to  their  plane  of  attachment,  and  become  columnar 
in  shape.  The  free  border  of  the  cells  has  an  appearance 
like  a  hem,  due  to  a  series  of  short  rods  placed  side  by  side. 
The  great  function  of  this  form  of  epithelium  is  to  absorb 
the  digested  matter  from  the  intestine,  and  to  pass  it 
on  to  the  blood. 
3 


34 


VETERINARY   PHYSIOLOGY 


Among  these  occur  some  larger,  somewhat  pear-shaped, 
cells,  attached  by  their  small  extremity.  Their  protoplasm 
is  collected  at  their  point  of  attachment,  while  the  body  of  the 
cell  is  filled  with  mucin,  a  clear,  transparent  material  These 
chalice  cells  producing  mucin  lead  to  the  study  of  the  next 
type  of  epithelium. 

3.  Secreting  Epithelium.  —  This  type  of  epithelium, 
which  has  as  its  function  the  production  of  some  material 
which  is  to  be  excreted  from  the  cell,  is  generally  arranged 
as  the  lining  of  depressions  or  pits — the  glands. 

The  simplest  form  of  gland  is  the  shnyle  tubular — a  test- 
tube-like  depression,  lined  by  secreting  cells.  Instead  of  being 
simple,  the  tube  may  be  branched,  when  the  gland  is  described 


Fig.  "i.- — A  Zymin-secreting  Gland,  to  show  an  acinus  lined  by  secreting 
cells  containing  zymogen  granules,  and  the  duct. 

as  racemose.  In  many  glands  the  secreting  epithelium  is 
confined  to  the  deeper  part  of  the  tube,  acinus  (fig.  7),  while 
the  more  superficial  part  is  lined  by  cells  which  do  not 
secrete,  forming  the  duct. 

In  many  situations  several  simple  glands  are  grouped 
together,  their  ducts  opening  into  one  common  duct,  and  a 
compound  gland  results. 

Secreting  epithelium  varies  according  to  the  material  it 
produces, 

(A)  Mucin-secreting  Epithelium. — Many  glands  have  for 
their  function  the  production  of  mucus,  a  slimy  substance  of 
use  in  lubricating  the  mouth,  stomach,  intestine,  etc.      The 


EPITHELIUM  35 

acini  containing  such  cells  are  usually  large.  The  cells 
themselves  are  large,  and  are  placed  on  a  dehcate  basement 
membrane,  a  condensation  of  the  subjacent  tibrous  tissue, 
which  bounds  the  acinus.  The  nuclei  are  situated  near  to 
the  attached  margin  of  the  cells.  These  are  somewhat 
irregular  in  shape,  and  are  packed  close  together. 

Their  appearance  varies  according  to  whether  the  gland 
has  been  at  rest  or  has  been  actively  secreting. 

Resting  State.— In  the  former  case,  in  the  fresh  condition, 
the  cells  are  large,  and  pressed  closely  together.  Their 
protoplasm  is  filled  with  large  shining  granules.  After  treat- 
ment with  reagents,  each  cell  becomes  distended  with  clear, 
transparent  mucin  formed  by  the  swelling  and  coalescence  of 
the  granules,  and  the  cells  tend  to  burst,  so  that  the  lumen 
of  the  gland  becomes  filled  with  the  glairy  mass. 

After  Activity. — After  the  gland  has  been  actively 
secreting,  the  cells  are  smaller  and  the  granules  are  much 
less  numerous,  being  chiefly  situated  at  the  free  extremity  of 
the  cell,  and  leaving  the  nucleus  much  more  apparent. 

This  form  of  epithelium,  during  the  resting  condition  of 
the  gland,  takes  up  nourishing  matter  and  forms  this  mucin- 
yielding  substance.  During  the  active  state  of  the  gland 
the  mucin-yielder  is  changed  to  mucin,  and  is  extruded  from 
the  cells  into  the  lumen  of  the  gland. 

Mucin  is  a  substance  which  occurs  in  many  tissues. 
When  precipitated  and  freed  from  water  it  is  white  and 
amorphous.  On  the  addition  of  water  it  swells  up  and  forms 
a  glairy  mass.  In  the  presence  of  alkalies  it  forms  a 
more  or  less  viscous  emulsoid,  and  from  this  it  is  pre- 
cipitated by  acetic  acid.  It  is  a  conjugated  protein 
(Appendix) — a  protein  linked  to  glucosamiiie — a  glucose 
molecule  in  which  one  of  the  hydroxyls  is  replaced  by 
amidogen — NH., — 


/3 


It     is     therefore     called     a     glyco-protein.      When    boiled 


36  VETERINARY   PHYSIOLOGY 

with     an     acid     it    yields     a    sugar    (see    Chemical    Physi- 
ology). 

(B)  Zymin-secreting  Epithelium. — Another  kind  of  secret- 
ing epithelium  forms  the  various  juices  which  act  upon  the 
food  to  digest  it.  These  juices  owe  their  activity  to  the 
presence  of  enzymes  or  zymins  (p.  7). 

A  zymin-forming  gland,  after  a  prolonged  period  of  rest, 
shows  cells  closely  packed  together,  so  that  it  is  difficult  to 
make  out  their  borders.  The  protoplasm  is  loaded  with 
granules  which  are  much  smaller  than  those  seen  in  the 
mucin- forming  cells,  and  which  do  not  swell  up  in  the  same 
way,  under  the  action  of  reagents.  The  nucleus  is  often 
obscured  by  the  presence  of  these  granules. 

When  the  gland  has  been  actively  secreting,  the  granules 
become  fewer  in  number,  and  are  confined  to  the  free 
extremity  of  the  cell  ;  they  are  obviously  passing  out.  The 
cell  becomes  smaller,  and  its  outline  is  more  distinct  and  the 
nucleus  more  apparent. 

The  granules  which  fill  the  cells  are  not  comj^osed  of  the 
active  enzyme.  If  extracts  of  the  living  cells  be  made,  they 
are  inert,  and  it  is  only  after  the  granules  have  left  the  cell, 
or  are  in  the  process  of  leaving,  that  they  become  activated. 
Hence,  the  granules  are  said  to  be  composed  of  zymin-form- 
iug  substance  or  zymogen. 

The  series  of  changes  are  parallel  to  those  described  in 
the  mucin-forming  cells.  During  the  so-called  resting  state 
of  the  gland,  the  cells  are  building  up  zymogen.  When 
the  gland  is  active,  the  cells  throw  off  the  material 
they  have  accumulated,  and  it  undergoes  a  change  to 
zymin. 

(C)  Excreting  Epithelium  does  not  manufacture  materials 
of  use  in  the  animal  economy,  but  passes  substances  out  of 
the  body.  Such  epithelium  is  seen  in  the  kidneys  and 
sweat  glands,  and  probably  in  the  liver.  The  cells  are 
composed  of  a  granular  protoplasm,  in  which  the  presence 
of  the  material  to  be  excreted  either  in  its  fully  elaborated 
condition,  or  in  process  of  preparation,  may  frequently  be 
demonstrated — e.g.  iron-containing  particles.  These  cells 
do    not  merely  take  up  material  from  the  blood   and   pass 


CONNECTIVE  TISSUES  37 

it  out,  but  tbey  may  profoundly  alter  it  before  getting 
rid  of  it. 

4.  Ciliated  Epithelium  (fig.  (5  (b),  p.  83). — The  cells  are 
usually  more  or  less  columnar,  and  the  free  border  is  pro- 
vided with  a  series  of  hair-like  processes,  the  cilia,  which 
vary  in  size  in  different  situations. 

In  the  living  state  the  cilia  are  in  constant  rhythmic 
motion,  each  cilium  being  suddenly  whipped  or  bent  down 
in  one  direction,  and  then  again  assuming  the  erect  posi- 
tion. 

All  the  cilia  on  a  surface  work  harmoniously  in  the  same 
direction,  and  the  movement  passes  from  the  cilia  of  one  cell 
to  those  of  the  next  in  regular  order,  beginning  at  one  end 
of  the  surface  and  passing  to  the  other. 

As  a  result  of  this  constant  harmonious  rhythmic  move- 
ment, any  matter  lying  upon  the  surface  is  steadily  whipped 
along  it  ;  and,  since  the  cilia  usually  work  from  the  inner 
parts  of  the  body  to  the  outside,  this  matter  is  finally 
expelled  from  the  body.  They  line  the  respiratory  passages, 
and  their  movement  plays  an  important  part  in  getting  rid 
of  dust  which  has  been  inhaled. 

The  movements  of  the  cilia  are  dependent  on  the  changes 
in  the  protoplasm,  and  everything  which  influences  the  rate 
of  chemical  change  modifies  the  rate  of  ciliary  movement, 
which  may  thus  be  taken  as  an  index  of  the  protoplasmic 
activity. 


II.   CONNECTIVE    TISSUES. 

These  are  the  binding  and  supporting  tissues  of  the  body 
— fibrous  tissue,  cartilage,  and  bone.  They  are  formed 
from  the  mesoblast  of  the  embryo,  and  most  of  them  contain 
blood-vessels. 

1.  Mucoid  Tissue. — The  cells  of  the  mesoblast  of  the 
embryo,  which  at  first  lie  in  close  apposition  with  one 
another,  become  separated,  remaining  attached  by  elongated 
processes.      Between  the  cells  a  clear,  transparent  substance 


38 


VETERINAEY   PHYSIOLOGY 


makes  its  appearance,  forming  a  soft,  jelly-like  tissue.  It 
contains  an  abundance  of  mucin  (p.  35).  This  tissue  is 
widely  distributed  in  the  embryo  as  a  precursor  of  the  con- 
nective tissues,  and  after  birth  it  is  still  to  be  seen  in  the 


X.-,i 


/ 


Fig.   8. — Mucoid  Tissue  from  an  embiyo  rabbit. 

pulp  of  a  developing-  tooth  and   in  the  vitreous  humour  of 
the  eye  (fig.  8). 

2.   Fibrous  Tissue. — As  development  advances,  the  cells  of 
mucoid  tissue  elongate  and   become  spindle-shaped,  and  are 


Fig.   9. — Fibroblasts  from  young  fibrous  tissue. 

continued  at  their  ends  into  fibres  (fig.    9).      These  cells  are 
often  called  fibroblasts. 

The  connective  tissues  are  thus  clearly  distinguished  from 
the   epithelia    by  having  the  formed  material    betiueen   and 


CONNECTIVE   TISSUES  39 

not  in    the    cells.      They    are    composed    of    the    following 
parts  : — 

I.  Formed  material. 
(a)  Fibres. 
(6)  Matrix. 
II,   Spaces  (Connective  Tissue  Spaces). 
III.   Cells. 

I.  FoR.MED  Material. — (a)  Fibres  (fig.  10). — 1st.  Non- 
elastic  (White  Fibres).  These  are  delicate,  transparent  fibrils 
arranged  in  bundles.  They  do  not  branch,  and  they  have  a 
mucin-like  matrix  between  them.  They  are  composed  of  a 
non-elastic  substance,  collagen.  This  is  a  sclero-protein 
(Appendix),  and  it  gives  the  biuret  reaction  but  not  the  tests 


Fig.    10. — Bundles  of  White  Fibres,  with  Fibroblasts  (a)  and  Elastic  Fibres 
anastomosing  with  one  another  (//). 

for  the  proteins  depending  on  the  presence  of  the  benzene 
nucleus.  It  contains  neither  tryptophan  nor  tyrosin  (p.  17), 
but  it  is  rich  in  amino-acetic  acid — glycin.  It  is  insoluble 
in  cold  water,  but  swells  up  and  becomes  transparent  in 
acetic  acid.  It  has  a  great  aflinity  for  carmine,  and  stains  a 
pink  colour  with  it.  When  boiled,  it  takes  up  water  to  form 
a  hydrate,  gelatin,  a  substance  soluble  in  hot  water,  and 
forming  a  jelly  on  cooling  (see  Chemical  Physiology). 

2nd.  Elastic  Fibres.  These  are  highly  refractile  elastic 
fibres,  which  branch  and  anastomose  with  one  another. 
They  are  composed  of  Elastin,  a  sclero-protein  which  is  very 
poor  in  tyrosin,  and  hence  gives  the  xantho-proteic  test  very 
faintly.  It  is  insoluble  in  both  cold  and  hot  water  and  is 
not  acted  on  by  acetic  acid.  It  stains  j'ellow  withjpicric 
acid  and  it  has  no  affinity  for  carmine. 


40  VETERINARY   PHYSIOLOGY 

(6)  Matrix. — This  is  composed  of  the  miicus-Uke  material 
which  is  so  abundant  in  the  fcetal  mucoid  tissue. 

According  to  the  arrangement  of  the  fibres,  and  to  the 
preponderance  of  one  or  other  variety,  various  types  of  fibrous 
tissue  are  produced. 

When  a  padding  is  required,  as  under  the  skin  and  under 
mucous  membranes,  the  fibres  are  arranged  in  a  loose  felt- 
work  to  constitute  areolar  tissue. 

In  fascia,  in  tendon  sheaths,  and  in  fiat  tendons,  the 
fibres  are  closely  packed  together  to  form  more  or  less 
definite  layers.  In  tendons  and  ligaments  the  fibres  run 
parallel  and  close  together.  In  the  tendons  of  muscles,  where 
elasticity  is  not  required,  the  fibres  are  of  the  white  or  non- 
elastic  variety.  In  ligaments,  where  elasticity  is  desirable, 
the  elastic  fibres  preponderate. 

II.  The  SPACES  of  fibrous  tissue  vary  with  the  arrange- 
ment of  the  fibres.  In  the  loose  areolar  tissue  under  the 
skin  they  are  very  large  and  irregular,  in  fascia  they  are 
flattened,  while  in  tendon,  where  the  fibres  are  in  parallel 
bundles,  they  are  long  channels. 

III.  The  CELLS  of  fibrous  tissue  (Fibroblasts)  vary  greatly 
in  shape.  In  the  young  tissue  they  are  elongated  spindles, 
from  the  ends  of  which  the  fibres  extend.  In  some  of  the 
loose  fibrous  tissues  they  retain  this  shape,  but  in  the 
denser  tissues  they  get  squeezed  upon,  and  are  apt  to  be 
flattened  and  to  develop  processes  thrust  out  into  the 
spaces. 

In  certain  situations,  peculiar  modifications  of  fibroblasts 
occur. 

(A)  Endothelium. — When  these  cells  line  the  larger 
connective  tissue  spaces  they  become  flattened,  and  form  a 
covering  resembling  simple  squamous  epithelium.  Such  a 
layer  lines  all  the  serous  cavities  of  the  body,  and  the 
lymphatics,  blood-vessels,  and  heart,  all  of  which  are 
primarily  large  connective  tissue  spaces.  To  demonstrate  the 
outlines  of  these  cells  it  is  necessary  to  stain  with  nitrate  of 
silver,    which     has    a    special     affinity    for    the     interstitial 


CONNECTIVE   TISSUES 


41 


substance,  and  which  thus  forms  a  series  of  black  lines 
between  the  cells. 

(B)  Fat  Cells. —  In  the  areolar  tissue  of  many  parts  of  the 
body,  fat  makes  its  appearance  in  the  cells  round  the  smaller 
blood-vessels,  and  when  these  cells  occur  in  masses  Adipose 
Tissue  is  produced. 

Little  droplets  of  oil  first  appear,  and  these  become  larger, 
run  together,  and  finally  form  a  large  single  globule,  distend- 
ing the  cell,  and  pushing  to  the  sides  the  protoplasm  and 
nucleus  to  form  a  sort  of  capsule  (fig.  11). 


Fig.    11.  — Fat  Cells  stained  with  osmic  acid,  and  lying  alongside 
a  small  blood-vessel. 


If  the  animal  be  starved,  the  fat  gradually  disappears  out 
of  the  cell,  and  in  its  place  is  left  a  clear  albuminous  fluid 
which  also  disappears,  and  the  cell  resumes  its  former 
shape. 

Fats. — The  ordinary  fats  are  esters  of  the  triatomic 
alcohol,  glycerol  (see  Appendix) — 


OH 

C3H5-;  OH 

(oh 


i 


formed  by  the  replacement  of  the  hydrogen  of  the  hydroxyls 
by  the  radicles  of  the  fatty  acids. 


42  VETERINARY   PHYSIOLOGY 

The  most  abundant  fatty  acids  of  the  body  are  : — 

(Palmitic  Acid,  CisHgiCOOH  ; 
batm-ated— ^g^^^^.^  Acid,  C17H35COOH  ; 

Unsaturated — Oleic  Acid,  C17H33COOH  ; 

The  unsaturated  acids  are  more  readily  oxidised  than  the 
saturated.  They  tend  to  break  at  the  double  link  ia  the 
chain  thus — 

H    H      H    H 

I       I     1     I       I 
— C— C=C— C— 


H  I  H 

The  three  fats  are — 

Palmitin,  C,}I,(O.C,Jl,,CO),  =  Q^HgA' 
Stearin,  C3H,(O.C,,H3,CO)3  =  C,,;H,,oOe, 
Olein,  C3H,(O.C\,H33CO)3  =  C,,H,o A- 

It  will  be  observed  that  the  molecules  of  these  fats  are 
very  rich  in  carbon  and  hydrogen,  and  very  poor  in 
oxygen,  containing  only  about  12  per  cent.,  i.e.  they  contain 
a  large  amount  of  material  capable  of  being  oxidised,  and 
thus  capable  of  affording  energy  in  the  process  of  combustion. 

The  fats  resemble  one  another  in  being  insoluble  in 
water,  but  soluble  in  ether  and  in  hot  alcohol.  As  the 
alcohol  cools,  they  separate  out  as  crystals.  They  differ 
from  one  another  in  their  melting  point,  palmitin  melting  at 
the  highest  and  olein  at  the  lowest  temperature.  Fat  Avhich 
is  rich  in  palmitin  and  stearin,  as  ox  fat,  is  thus  hard  and 
solid  at  the  ordinary  temperature  of  the  air,  while  fat  rich  in 
olein,  as  dog  fat,  is  semi-fluid  at  the  same  temperature. 
Olein  acts  as  a  solvent  for  the  fats  of  a  higher  melting 
point.      (For  tests,  see  Chemical  Physiology.) 

The  functions  of  adipose  tissue  are  twofold  ;- — 

1st.  Mechanical. — The  mass  of  adipose  tissue  under  the 
skin  is  of  importance  in  protecting  the  deeper  structures 
from  injury.  It  is  a  cushion  on  which  external  violence 
expends    itself      Further,    this    layer    of    subcutaneous    fat 


CONNECTIVE  TISSUES  43 

prevents   the  loss  of  heat  from   the  body,  being,  in   fact,  an 
extra  garment. 

2nd.  Chemical. — Fat,  on  account  of  its  great  quantity  of 
unoxidised  carbon  and  hydrogen,  is  the  great  storehouse  of 
energy  in  the  body. 

(C)  Pigment  Cells. — In  various  parts  of  the  eye  the 
connective  tissue  and  other  cells  contain  a  black  pigment — 
Melanin.  The  precise  mode  of  origin  of  this  pigment  is  not 
known.  It  contains  carbon,  hydrogen,  nitrogen,  oxygen, 
and  it  may  also  contain  iron.  Melanin  is  closely  related  to 
a  series  of  dark  pigments  which  are  produced  by  heating 
protein  with  mineral  acids — the  melanoidins — and  like  them, 
when  heated  with  potash,  it  yields  indol  and  skatol  (see  p. 
330;.  It  is  therefore  probably  connected  with  tyrosin 
or  Avith  tryptophan  (see  p.  17).  It  has  nothing  to  do 
with  the  blood-pigments.  Melanin-like  i3igments  are  widely 
distributed  in  nature,  occurring  not  only  in  the  connective 
tissue  pigment-cells  of  animals,  but  also  in  epithelial  cells  of 
the  epidermis,  hair,  and  eye,  and  in  the  tissues  of  some 
plants.  Its  function  in  the  eye  is  to  prevent  the  passage  of 
light  through  the  tissues  in  which  it  is  contained. 

The  fibrous-tissue  cells  containing  the  pigment  are 
branched,  and  in  many  cases  they  possess  the  power  of  move- 
ment. This  is  specially  well  seen  in  such  cells  in  the  skin 
of  the  frog,  where  contraction  and  expansion  may  be  studied 
under  the  microscope.  By  these  movements  the  skin  is 
made  lighter  or  darker  in  colour.  The  movements  of  these 
cells  are  under  the  control  of  the  central  nervous  system. 

3.  Cartilage. — AVhile  fibrous  tissue  is  the  great  binding 
medium  of  the  body,  support  is  afforded  in  foetal  life  and  in 
certain  situations  in  adult  life  by  cartilage. 

Where  cartilage  is  to  be  formed,  the  embryonic  cells  become 
more  or  less  oval,  and  secrete  around  them  a  clear  pellucid 
capsule.  This  may  become  hard,  and  persist  through  life,  as 
in  the  so-called  iiavenchymatous  cartilage  of  the  mouse's  ear. 

(1)  Hyaline  Cartilage. — Development,  liowever,  usually 
goes  further,  and  before  the  capsule  has  hardened,  the 
cartilage    cells    again    divide,   and   each    half    forms   a  new 


44  VETERINARY   PHYSIOLOGY 

capsule  which  expands  the  original  capsule  of  the  mother 
cell,  and  thus  increases  the  amount  of  the  formed  material. 
This  formed  material  has  a  homogeneous,  translucent 
appearance,  and  a  tough  and  elastic  consistence,  and  cuts 
like  cheese  with  the  knife  (fig.  12). 

The  formed  material  of  cartilage,  chondro-onucoid,  is  not 
a  special  substance,  but  a  mixture  of  chondroitin-sulphuric 
acid  with  collagen  in  combination  with  proteins.  Chondroitin, 
when  decomposed,  yields  glucosamine,  a  sugar-like  substance 
containing  nitrogen  (p.  35)  ;  glycuronic  acid,  a  sugar  with 
the  terminal  carbon  oxidised  to  the  carboxyl  state  ;  and 
acetic  acid  probably  derived  from  the  amino-acetic  acid 
of  collagen. 

Cartilage  is  surrounded  by  a  tibrous  membrane,  the  peri- 
chondrium, and  frequently  no  hard-and-fast  line  of  demarca- 
tion can  be  made  out  between  them.  The  fibrous  tissue 
gradually  becomes  less  fibrillated,  the  cells  become  less 
elongated  and  more  oval  and  the  interfibrillar  substance 
increases  in  amount  and  becomes  of  the  same  refractive 
^.  index  as  the  fibres.      During 

old     age,    a     fibrillation      of 
=^  •  ""  the       homogeneous  -  looking 

cartilage    is    made    manifest, 

^  ^_.  especially  in  costal  cartilage, 

'^^^^=^'"'  --'''.  l^y    the    deposition    of    lime 

;''■?'  salts  in  the  matrix,  between 

the  fibres.      It  was  long  ago 

shown  that  in  inflammation  of 

cartilage       this       fibrillation 

,,;■.  •  appears;     and    by    digesting 

,/^\  it  i'^  baryta  water,  a  similar 

structure     may    be     brought 

Fig.    12.-Hyaluie  CartUage  covered      ^^^^_         rpj^^    ^j^^^    connection 
by  perichondnun). 

of      cartilage     with      fibrous 
tissue  is  thus  clearly  demonstrated. 

Such  homogeneous  hyaline  cartilage  precedes  most  of  the 
bones  in  the  embryo,  and  covers  the  ends  of  the  long  bones 
in  the  adult  (articular  cartilage),  forms  the  framework  of 
the  larynx  and  trachea  and  constitutes  the  costal  cartilages. 


9 


CONNECTIVE   TISSUES 


45 


(2)  Elastic  Fibro-Cartilage. — In  certain  situations — e.g.  in 
the  external  ear — a  specially  elastic  form  of  cartilage  is 
developed,  elastic  fibres  appearing  in  the  cartilaginous 
matrix,  and  forming  a  network  through  it. 

(o)  White  Fibro-Cartilage. — In  other  situations — e.g.  the 
intervertebral  discs — a  combination  of  the  binding  action  of 
fibrous  tissue  with  the  padding  action  of  cartilage  is  required  ; 
and  here  strands  of  white 
fibrous  tissue  with  little 
islands  of  hyaline  cartil- 
age are  found.  It  is  also 
found  where  white  fibrous 
tissue,  e.g.  tendon,  is  in- 
serted into  hyaline  cartil- 
age, and  it  is  reall}-  a 
mixture  of  two  tissues — 
wdiite  fibrous  tissue  and 
cartilag-e. 


4.  Bone. — The  great 
supporting  tissue  of  the 
adult  is  Bone. 

(1)  Development  and 
Structure.  —  Bone  is 
formed  by  a  deposition  of 
lime  salts  in  layers  or 
lameUcc  of  white  fibrous 
tissue.  But  while  some 
bones,  as  those  of  the 
cranial    vault,     face,     and 


Fig.  13. — Intra-membranous  Bone  De- 
velopment in  the  lower  jaw  of  a  foetal 
cat.  Above,  the  process  of  ossification 
is  seen  shooting  out  along  the  fibres, 
and  on  the  lower  surface  the  process 
of  absorption  is  going  on.  Two  osteo- 
clasts— large  multi-nucleated  cells — 
are  shown  to  the  left. 


clavicle,  are  produced  entirely  in  fibrous  tissue,  others  are 
preformed  in  cartilage,  which  acts  as  a  scaffolding  upon 
which  the  formation  of  bone  goes  on. 

A.  Intra-membranous  Bone  Development. — This  may  be 
studied  in  any  of  the  bones  of  the  cranial  vault  where 
cartilage  is  absent  (fig.  13). 

At  the  centre  of  ossification,  the  matrix  between  the 
fibres  becomes  impregnated  with  lime  salts,  chiefly  the  phos- 
phate  and  carbonate.      How  this   deposition   takes    place  is 


46  VETERINARY   PHYSIOLOGY 

not  known,  and  how  far  it  is  dependent  on  the  action  of 
cells  has  not  been  clearly  determined  ;  but  in  front  of  the 
process,  as  it  shoots  outwards  from  the  centre  in  all  direc- 
tions, accumulations  of  cells  are  to  be  seen,  and  these  cells 
have  been  called  osteoblasts.  The  cells  get  enclosed  in 
definite  spaces,  lacunce,  and  become  bone  cells.  Narrow 
branching  channels  of  communication  are  left  between  these 
lacunee,  the  canaliculi. 

The  fully  formed  adult  bone,  however,  is  not  a  solid 
block,  but  is  composed  of  a  compact  tissue  outside,  and  of  a 
spongy  bony  tissue,  cancellous  tissue,  inside.  This  cancellous 
tissue  is  formed  as  a  secondary  process.  Into  the  block  of 
calcareous  matter,  formed  as  above  described,  processes  of 
the  surrounding  fibrous  tissue  burrow,  carrying  in  blood- 
vessels, lymphatics,  and  numerous  cells  (fig.  13,  lower  surface). 
This  burrowing  process  seems  to  be  carried  on  by  the  cells, 
which  eat  up  the  bony  matter  formed.  In  doing  this  they 
frequently  change  their  appearance,  becoming  large  and 
multi-nucleated  (osteoclasts).  Thus  the  centre  of  the  bone  is 
eaten  out  into  a  series  of  channels,  in  which  the  marrow  of 
the  bone  is  lodged,  and  between  which  narrow  bridges  of 
bone  remain. 

It  is  by  the  extension  of  the  calcifying  process  outwards, 
and  the  burrowing  out  of  the  central  part  of  the  bone,  that 
the  dense  diploe  and  spong}^  cancellous  tissue  are  produced. 

B.  Intra-cartilaginous  Bone  Development. —  In  the  bones 
preformed  in  cartilage,  the  process  is  somewhat  more  complex. 
But  all  the  bone  is  developed  in  connection  with  fibrous 
tissue,  and  the  cartilage  merely  plays  the  part  of  a  scatfblding 
and  is  all  removed. 

Where  the  adult  bone  is  to  be  produced,  a  minute  model 
is  formed  in  hyaline  cartilage  in  the  embryo,  and  this  is 
surrounded  by  a  fibrous  covering,  the  'pevicltondriihtn.  In 
the  deepest  layers  of  this  perichondrium  the  process  of  calcifi- 
cation takes  place  as  described  above,  and  spreads  outwards, 
thus  encasing  the  cartilage  in  an  ever-thickening  layer  of 
bone  (fig.  14,  a). 

At  the  same  time,  in  the  centre  of  the  cartilage,  at  what  is 
called   the   centre  of  ossification,  the   cells   begin  to  divide 


CONNECTIVE   TISSUES  47 

actively,  and,  instead  of  forming  new  cartilage,  eat  away  the 
material,  and  thus  open  out  spaces  (fig.  14,  6).  Into  these 
spaces  processes  of  the  perichondrium  bore  their  way,  carry- 
ing with  them  blood-vessels,  and  thus  rendering  the  cartilage 
vascular.  The  vascularisation  of  the  centre  of  the  cartilao-e 
having  been  effected,  the  process  of  absorption  extends 
towards  the  two  ends  of  the  shaft  of  cartilage,  which  con- 
tinues to  elongate.  The  cartilage  cells  divide  and  again 
divide,  and,  by  absorbing  the  material  between   them,  form 


'**"-»  -s*. 


^^ 


..^' 


Fig.  14. — Intra-cartilaginous  Bone  Development.  A  phalanx  of  a  fcetal 
finger  showing  the  formation  of  periosteal  bone  round  the  shaft  (a) ; 
the  opening  up  of  the  cartilage  at  the  centre  of  ossification  and  the 
vascularisation  of  the  cartilage  by  the  invasion  of  fibrous  tissue  {b) ; 
and  the  calcification  of  the  cartilage  round  the  spaces  (c). 

long  irregular  canals  running  in  the  long  axis  of  the  bone, 
with  trabeculse  of  cartilage  between  them.  Into  these 
canals  the  processes  of  the  periosteum  extend,  and  fill  them 
with  its  fibrous  tissue.  A  deposition  of  lime  salts  takes 
place  upon  the  trabeculse,  enclosing  cells  of  the  invading 
fibrous  tissue,  and  thus  forming  a  crust  of  bone,  while  the 
cartilage  also  becomes  calcified.  If  this  calcification  of  the 
cartilage  and  deposition  of  bone  were  to  go  on  unchecked, 
the  block  of  cartilage  would  soon  be  converted  to  a  sohd 
mass  of  calcified  tissue.  But  this  does  not  occur.  For,  as 
rapidly  as  the  trabeculis  become  calcified,  they  are  absorbed. 


48 


VETERINARY  PHYSIOLOGY 


while  the  active  changes  extend  further  and  further  from  the 
centre  to  the  ends  of  the  shaft.  The  centre,  which  was  once 
formed  by  the  embryonic  cartilage,  is  thus  changed  to  a 
space  filled  by  fibrous  tissue  which  afterwards  becomes  the 
bone  marrow. 

The  process  of  absorption  does  not  stop  at  the  original 
block  of  cartilage ;  but  after  all  of  this  has  been  absorbed, 
the  bone  formed  outside  the  cartilage  in  the  fibrous  tissue  is 
attacked    by  burrowing    processes   from    inside   and  outside, 


Fig.  15.— Cross  section  through  part  of  the  shaft  of  an  adult  long  bone  to 
show  the  arrangement  in  lanielte  distributed  as  Haversian  (1),  inter- 
stitial (2),  periphei-al  (3),  and  medullary  (4). 


which  hollow  out  long  channels  running  in  the  long  axis  of 
the  bone.  These  are  the  Haversian  spaces  (fig.  15). 
Round  the  inside  of  each,  calcification  occurs,  spreading 
inwards  in  layers,  and  enclosing  connective  tissue  cells,  until 
at  length  only  a  small  canal  is  left,  an  Haversian  canal, 
containing  some  fibrous  tissue,  blood-vessels,  lymphatics,  and 
nerves,  with  layer  upon  layer  of  bone  concentrically  arranged 
around  it.  This  constitutes  an  Haversian  system.  In  this 
way  the  characteristic  appearance  of  the  shaft  of  a  long  bone 
is  produced,  with  layers  of  calcified  fibrous  tissue,  the  bone 


CONNECTIVE  TISSUES  49 

lamelke,  arranged  as  Haversian,  interstitial,  peripheral,  and 
medullary  lamellae  (fig.  15). 

One  important  function  performed  by  the  cartilage  is  in 
bringing  about  the  increase  in  length  of  the  bones.  As 
growth  progresses,  the  cartilage  grows  in  length,  and  the 
formation  of  bone  outside  the  cartilage  spreads  to  each  end, 
and  thus  the  shaft  of  the  bone  is  formed.  But,  in  addition 
to  the  centre  of  ossification  in  the  shaft — the  dicqjhysis,  one 
or  more  similar  centres  of  ossification  form  at  each  end 
of  the  bone.  These  are  the  epiphyses.  Between  these  and 
the  diaphysis  a  zone  of  actively  growing  cartilage  exists  until 
adult  life,  when  the  bones  stop  lengthening.  In  this  zone, 
the  cells  arrange  themselves  in  vertical  rows,  divide  at  right 
angles  to  the  long  axis  of  the  bone  and  form  cartilage. 
This  cartilage,  as  it  is  formed,  is  attacked  by  the  bone-form- 
ing changes  at  the  diaphysis  and  epiphyses.  But  the 
amount  of  new  cartilage  formed  is  at  first  proportionate  to 
this,  and  thus  a  zone  of  growing  cartilage  continues  to  exist 
until  early  adult  life,  when  epiphyses  and  diaphysis  join  and 
growth  in  length  is  stopped.  The  rate  and  extent  of  this 
growth  of  the  cartilage  determines  the  length  of  the  limbs. 
It  is  influenced  by  many  factors,  such  as  the  general  nutrition 
of  the  animal,  and  also  by  the  influence  of  the  internal 
secretions  of  various  structures,  such  as  the  thyreoid  and 
pituitary  body  (see  pp.  595,  599). 

(2)  Chemistry. — The  composition  of  adult  bone  is 
roughly  as  follows  : — 

Water,  10  per  cent. 
Solids,  90  per  cent. 

Organic,  35  per  cent. — chiefly  collagen. 
Inorganic,  65  per  cent. 
Calcium  phosphate,  51. 
carbonate,  11. 
fluoride,  0-2. 
Magnesium  phosphate,  1. 
Sodium  salts,  1. 

The  most  important  points  are  the  small  amount  of  water, 
4 


50  VETERINARY  PHYSIOLOGY 

the  large  amount  of  inorganic  matter,  chiefly  calcium  phosphate, 
and  the  nature  of  the  organic  matter — collagen. 

(3)  Metabolism. — Although  bone  consists  so  largely  of 
inorganic  matter  it  is  permeated  by  blood-vessels  and  living 
cells,  and  it  undergoes  metabolic  changes  not  only  during 
development  but  in  the  full  grown  animal.  In  fasting  and 
in  the  conditions  of  acidosis  (p.  481)  the  lime  salts  may 
be  removed  from  the  bone  to  the  blood  and  excreted.  In 
rickets  the  metabolism  of  growing  bone  is  modified,  the  lime 
salts  not  being  properly  laid  down  or  being  too  rapidly 
removed  ;  possibly  both  these  changes  go  on  together.  The 
bones  are  thus  softened  and  undergo  deformity.  In  osteo- 
malacia there  is  a  softening  of  the  bones  due  to  the 
removal  of  lime  salts. 


(5)  THE    MASTER    TISSUES —NERVE    AND 
MUSCLE. 


By"  means  of  the  epithelial  and  connective  tissues  the 
body  is  protected,  supported  and  nourished.  It  performs 
purely  vegetative  functions,  but  it  is  not  brought  into  active 
relationship  with  its  environments.  By  the  development  of 
Nerve  and  Muscle  the  surroundings  are  able  to  act  upon  the 
body,  and  the  body  can  react  upon  its  surroundings. 

These  tissues  may  therefore  be  called  the  Master   Tissues, 
and  it    is    as   their  servants  that  all   the 
other  tissues  of  the  soma  functionate. 

Upon  them  the  very  existence  of  an 
animal  depends.  It  lives  in  a  world  of 
constant  change.  The  surrounding  con- 
ditions are  not  always  compatible  with  life  ; 
the  temperature  may  be  too  high  or  too 
low,  or  it  may  find  itself  plunged  in  a 
medium  in  which  it  cannot  breathe  and  it 
must  escape  ;  food  may  be  wanting,  and  it 
has  to  be  obtained.  A  thousand  changfinw 
conditions  have  to  be  daily,  hourly,  almost 
momentarily  met  by  adjustments  of  the 
body,  and  to  make  these  appropriate  the 
various  ditferent  kinds  of  change  must 
each  produce  ditferent  effects. 

Some  means  by  which  they  can  do  so 
is  the  first  essential  for  the  continuance  of 
the  animals'  existence.  Not  only  must  each  produce  its  special 
effect,  but  means  must  exist  by  which  each  different  effect 
or  combination  of  effects  may  produce  an  appropriate 
reaction. 

In  unicellular  organisms  changes  in  the  surroundings  act 

51 


Fig.  16.  —  Poterio- 
dendron  in  its  cap- 
sule, to  illustrate 
the  first  stage  in 
the  evolution  of 
a  neuro-niuscular 
system. 


52  VETERINARY   PHYSIOLOGY 

directly  on  the  cell  protoplasm,  e.g.  an  amoeba,  when  touched, 
draws  itself  together.  But,  even  in  these  simplest  organisms, 
certain  kinds  of  external  conditions  will  produce  one  kind 
of  change,  while  others  will  produce  a  different  one,  as  has 
been  shown  in  considering  unilateral  stimulation  (p.  25). 
Even  among  unicellular  organisms— e.(/.  among  the 
infusoria — animals  are  found  in  which  the  cell  is  differ- 
entiated into  a  receiving  and  a  reacting  part.  Poteriodendron, 
a  little  infusorian  sitting  in  a  cup-like  frame,  consists  of  a 
long  process  or  cilium  extending  up  from  a  cell,  while  a 
contractile  myoid  attaches  the  cell  to  the  floor  of  the  cap. 
When  the  cilium  is  touched  the  myoid  contracts,  and 
draws  the  creature  into  the  protection  of  its  covering 
(fig.  16). 

In  multicellular  organisms  the  result  is  secured  by  the 
development  at  the  surface  of  the  body  of  special  structures 
or  Receptors,  each  kind  of  which  is  stimulated  or  made  to 
undergo  a  change,  more  especially  by  some  one  kind  of 
external  change. 

Thus  one  kind  is  specially  stimulated  by  the  contact  of 
gross  matter,  another  by  the  addition  of  heat,  another  by  its 
withdrawal ;  another  kind  is  specially  acted  on  by  the 
vibration  of  air  called  by  physicists  "  sound  waves,"  another 
by  the  ethereal  "  waves  of  light "  ;  yet  another  by  the 
presence  of  substances  suspended  in  air  or  in  solution  in 
water. 

But  before  the  reaction  of  the  body  upon  its  surroundings 
can  be  appropriate,  the  effects  of  these  various  changes  must 
be  brought  together  and  harmonised  and  integrated,  and 
this  is  secured  by  the  existence  of  strands  of  living  matter, 
nerves,  which  pass  from  each  receptor  and  lead  to  a  common 
or  to  several  common  receiving  arrangements  or  stations  in 
the  central  nervous  system. 

The  combined  effect  of  all  the  different  stimuli  from 
without  call  into  play  a  series  of  the  other  stations  in  the 
central  nervous  system  which  set  in  action  the  great  re- 
acting or  effector  arrangements,  the  Muscles. 

It  is  impossible  to  study  the  mode  of  action  of  the 
receptors  without  also  considering  the  mode  of  action  of  the 


NERVE  53 

transmitting  nerves  and   of   the    receiving   stations   in    the 
central  nervous  system. 

This  involves  the  study  of  the  motor  reactions  which 
may  follow  any  given  set  of  stimuli. 

Before  proceeding  to  study  the  physiology  of  the  complex 
set  of  receptors,  nerves,  central  nervous  system,  and  effectors, 
it  will  be  well  first  to  consider — 

1.  How  the  nervous  system  is  developed. 

2.  The  physiology  of  single  nerve  units. 

I.  Development  of  the  Nervous  System. 

To  form  the  nervous  mechanism,  a  part  of  the  epithelial 
covering  of  the  embryo  sinks  inwards  as  a  canal,  and  the 
cells  of  this  form  functional  connections  with  the  surface 
on  the  one  hand,  and  with  the  reacting  structures  on  the 
other. 

At  first  the  cells  composing  this  tube  are  undifferenti- 
ated and  alike  {neurohlasts),  but  later  some  of  them  throw  out 
processes  (a)  towards  the  surface,  and  others  (6)  towards  the 
reacting  structures.  These  cells  with  their  outgrowths  form 
the  units  of  which  the  nervous  system  is  built  up — the 
Neurons.  They  are  separate  from  one  another,  but  are 
associated  by  the  close  propinquity  of  their  branching  twig- 
like processes  or  dendrites,  such  an  association  being  known 
as  a  synapse  (fig.  17). 


Fig.  17. — To  show  a  recdring  (c)  and  a  reacting  Neuron  (a),  each  with  den- 
drites at  its  extremities,  and  their  connection  to  one  another  through 
a  Synapsis  {h). 

1.  Neurons  to  and  from  the  Body  Wall- — The  nerve  fibres 
are  formed  as  outgrowths  from  the  nerve  cells.  This  has 
been  demonstrated  by  the  histological  investigations  of 
Ramon-y-Cajal,  who    used    a    method    of  impregnating   the 


54  VETERINARY   PHYSIOLOGY 

fibres  with  silver,  and  still  more  strikingly  by  the  experi- 
mental investigations  of  Ross  Harrison  upon  the  tadpole. 
The  nerves  which  come  from  the  central  nervous  system 
each  consist  of  two  roots,  one  coming  from  the  back  of  the 
neural  canal,  the  other  from  the  antero-lateral  aspect. 
Harrison  was  able  to  remove  the  cells  from  the  back  of  the 
neural  canal,  and  he  then  found  that  the  posterior  roots  of 
the  nerves  did  not  grow,  but  that  the  anterior  roots  did 
grow.  He  also  removed  the  cells  from  the  front  part  of  the 
neural  canal,  leaving  those  at  the  back,  and  he  then  found 
that  processes  grew  out  in  the  position  of  the  posterior  roots 
only.  He  also  found  that  from  these  posterior  cells  were 
derived  certain  covering  cells  which  afterwards  formed  the 


«^— ^ f-<  4-^. <  4-< 


6 

Fig.  18. — To  show  the  formation  of  a  Visceral  Nerve,  a,  the  neurons 
which  have  travelled  out  to  form  the  terminal  ganglion;  b,  an  emi- 
grated neuron  which  has  come  to  rest  in  a  sympathetic  ganglion  :  c,  a 
neuron  which  has  remained  in  the  spinal  cord  and  sent  out  a  process  so 
that  it  may  act  upon  the  emigrated  neurons. 

sheaths  of  the  nerve  fibres.  Lastly,  he  excised  part  of  the 
neural  canal  before  the  cells  had  differentiated,  and  kept  it 
alive  for  five  weeks  in  lymph,  and  he  was  able  to  observe 
the  outgrowth  of  the  processes  which  afterwards  form  the 
axons  of  the  nerve  fibres.  The  evidence  thus  seems  to  be 
conclusive,  that  these  somatic  nerve  fibres  are  essentially 
outgrowths  from  nerve  cells. 

2.  Neurons  to  the  Viscera. — While  the  nerves  to  and  from 
the  body  ivall  are  formed  as  described  above,  those  passing 
to  the  viscera  are  developed  by  a  migration  outwards  of 
neuroblasts.  Some  of  these  come  to  rest  in  the  sympathetic 
ganglia  in  front  of  the  spinal  column,  others  travel  on  and 
come  to  rest  in  more  remote  ganglia,  while  a  large  number 
pass  right  out  into  the  tissues,  there  to  throw  out  processes 


NERVE 


55 


and  to  form  a  network  which  may  be  called  a  terminal 
2)lexus.  These  are  kept  in  connection  with  the  central 
nervous  system  by  the  outgrowths  of  processes  from  cells 
which  remain  in  the  spinal  cord  (fig.  18).  In  certain 
positions,  e.g.  in  the  wall  of  the  gut,  these  terminal  plexuses 
control  the  activity  of  the  structure  in  which  they  are 
placed  (see  fig.  48,  p.  109). 

It  is  as  if    the    central    nervous   svstem    had    devolved 


Fig.    19.— (a)  A  Nerve  Cell  with  Nissl's  Granules  ;  (b)  a  similar  cell 
showing  changes  on  section  of  its  axon. 

the    power    of    local    government     upon     these     emigrated 


II.  Structure. 

1.  Cells. — The  shape  and  characters  of  the  cells,  and 
their  position  upon  the  processes  vary  greatly,  but  the}^  have 
all  the  following  features  in  common  : — They  are  nucleated 
protoplasts,  the  protoplasm  of  which,  after  fixing  and 
staining,  shows  a  well-marked  network,  in  the  meshes  of 
which  a  material  which  stains  deeply  with  basic  stains,  and 
which  seems  to  be  used  up  during  the  activity  of  the 
neuron,  may  accumulate  in  granules.  The  granules  formed 
of  this  material  are  generally  known  as  Nissl's  granules 
(fig.  19). 

Mott  has  failed  to  find  such  a  structure  in  living  nerve 
cells,  and  by  the  use  of  the  ultra-microscope  has  observed 
particles    moving    freely     in    the    colloidal    iluid     contents, 


56  VETERINARY   PHYSIOLOGY 

which    they   could    not    have    done    had    there    been    such 
structures. 

These  cells  give  off  at  least  one  process,  which  continues 
for  some  distance,  as  the  axon.  Frequently  other  processes 
are  given  off,  which  form  a  branching  system  of  dendrites. 
The  axons  end  in  dendrites,  so  that  all  the  processes  are 
essentially  the  same.  These  processes  appear  to  be  fibrillated, 
and  in  fixed  specimens  the  tibrillse  may  be  traced  through 
the  protoplasm  of  the  cells,  but  this  appearance  is  probably 
an  artefact  due  to  the  action  of  the  fixing  agents  employed. 
In  many  cases  the  dendrites  show  little  buds  or  gemmules 
upon  their  course,  and,  according  to  some  observers,  it  is 
through  these  that  one  neuron  is  brought  into  definite 
relationship  at  one  time  with  one  set  of  neurons,  and  at 
another  with  other  adjacent  neurons.  There  is  also  some 
evidence  that  the  dendrites  as  a  whole  may  expand  and 
contract,  and  thus  become  connected  with  those  of  adjacent 
neurons. 

2.  Axon. — The  axon  process,  as  it  passes  away  from  the 
cell,  becomes  a  Nerve  Fibre,  and  acquires  one  or  two  cover- 
ings. 

(1)  A  thin  transparent  membrane,  the  primitive  sheath  or 
neurolemma,  is  present  in  all  peripheral  nerves.  Between  it 
and  the  axis  cylinder  there  are  a  number  of  nuclei 
surrounded  by  a  small  quantity  of  protoplasm,  the  nerve 
corpuscles.  Fibres  with  this  sheath  alone  have  a  grey  colour, 
and  they  may  be  called  grey  or  non-medullated  jihres. 
They  are  abundant  in  the  visceral  nerves.  This  sheath 
is  absent  from  the  nerve  fibres  of  the  central  nervous 
system. 

(2)  A  thick  white  sheath — the  medullary  sheath  or  white 
sheath  of  Schwann — which  gives  the  white  colour  to  most 
of  the  nerves  of  the  body  appears  somewhat  late  in  the 
development  of  many  nerve  fibres.  It  lies  between  the 
primitive  sheath  with  its  nerve  corpuscles  and  the  axon.  It 
is  not  continuous,  but  is  interrupted  at  regular  intervals  by 
constrictions  of  the  neurolemma  at  the  nodes  of  Ranvier 
(fig.  20).      It  is  composed  of  a  sponge-work  or  felt-work  of 


NERVE 


57 


a  horn-like  substance — neuro-keratin — the  meshes  of  which 
are  filled  with  a  peculiar  fatty  material. 

The  nerve  fibres  run  together  in  bundles  to  constitute 
the  nerves  of  the  body,  and  each  bundle  is  surrounded  by 
a  dense  fibrous  sheath,  the  'perineurium.  When  a  bundle 
divides,  each  branch  has  a  sheath  of  perineurium,  and  in 
many  nerves  this  sheath  is  continued,  as  the  sheath  of  Henle, 
on  to  the  single  fibres  which  ultimately  branch  off"  from 
the  nerve. 

Not  only  do  nerves  branch  and  anastomose  in  the  great 
nerve  plexuses,  but  in  the  nerves  themselves  a  similar  plexus- 
like rearrangement  of  the  bundles  and  fibres  takes  place. 


Fig.   20. — Pieces  of  two  white  Nerve  Fibres. 

Each  nerve  fibre  ends  in  a  series  of  dendritic  expansions, 
which  vary  greatly  in  character  according  to  the  structures 
to  which  they  pass. 


III.  Chemistry  of  Nerve. 

The  chemistry  of  neuron  cells  and  their  processes  has  been 
deduced  from  a  study  of  the  chemistry  of  the  grey  matter  of 
the  brain  where  they  preponderate,  while  the  chemistry  of  the 
white  fibres  is  indicated  by  the  analysis  of  the  white  sub- 
stance of  the  brain,  which  consists  chiefly  of  medullated  fibres. 

The  grey  matter  contains  over  80  per  cent,  of  water. 
The  solids  consist  of  rather  less  than  1 0  per  cent,  of  proteins. 
Two  globulins,  one  coagulating  at  a  low  and  the  other  at  a 
higher  temperature,  and  a  nucleo -protein  have  been  isolated. 
Lecithin  and  cholesterol  each  constitute  about  3  per  cent. 

The  white  matter  contains  only  about  70  per.  cent  of 
water.  The  proteins,  similar  to  those  in  the  grey  matter, 
constitute  between  7   and   8   per  cent.      Lecithin   occurs   in 


58  VETERINARY   PHYSIOLOGY 

about  the  same  amount  as  in  the  grey  matter,  but 
cholesterol  and  lipoids,  other  than  lecithin,  constitute  more 
than  15  or  16  per  cent. 

From  the  fatty  material  of  the  white  sheaths  various 
mixtures  of  lipoid  substances  have  been  isolated.  These 
have  been  named  Cerebrosides  and  have  been  classified  into — 

1.  Galactosides  yielding  a  sugar — galactose,  nitrogen,  but 
no  phosphorus. 

2.  Phosphatides,  of  which  lecithin  is  the  most  important 
(p.  20). 

3.  Cholesterol,  a  monohydric  alcohol,  belonging  to  the 
group  of  terpenes. 

IV.  Physiology  of  Neurons. 

The  neurons  form  a  most  intricate  labyrinth  throughout 
all  parts  of  the  body,  and  more  especially  throughout  the 
central  nervous  system.  Each  is  brought  into  relationship 
with  many  others  by  its  dendritic  terminations,  and  there  is 
a  continued  interaction  between  them,  the  activity  of  any 
one  influencing  the  activity  of  many  others.  In  this  way 
the  constant  activity  of  the  nervous  system,  which  goes  on 
from  birth  to  death,  during  consciousness  and  in  the  absence 
of  consciousness,  is  kept  up. 

It  is  unnecessary  and  gratuitous  to  invoke  the  conception 
of  automatic  action  on  the  part  of  any  portion  of  the  nervous 
system.  Throughout  life  these  neurons  are  constantly  being 
acted  upon  from  without ;  and  activity,  once  started  by  any 
stimulus,  sets  up  a  stream  of  action  which  may  be 
coexistent  with  life. 

A.  SINGLE  NEURONS, 
OR  Neurons  lying  side  by  side  in  Nerves. 
It  has  been  shown  that  the  great  purpose  of  neurons  is  to 
enable  external  changes  to  produce  appropriate  reaction 
(p.  52).  The  changes  set  up  in  the  receptors  at  the  surface 
must  be  conducted  to  the  stations  in  the  central  nervous 
system,  and  again  conducted  out  to  the  muscles.  Conduction 
is  thus  the  great  property  of  nerve. 


NERVE  59 

1.  Manifestations  of  the  Activity  of  Neurons. 

When  conducting,  nerve,  like  a  telegraph  wire,  manifests 
no  visible  change.  Its  activity  is  shown  chiefly  by  changes 
in  the  structures  to  which  it  passes,  but  also  by  certain 
electrical  disturbances  in  the  conducting  part. 

(1)  Action  on  other  Structures. — The  activity  of  the  out- 
going neurons — neurons  conducting  impulses  from  the  central 
nervous  system  to  muscles,  glands,  etc. — is  manifested  by 
changes  in  the  muscles  or  other  structures  to  which  they 
go  :  while  the  activity  of  ingoing  neurons  is  made  evident  (a)  by 
their  action  on  outgoing  neurons   to  muscles,   etc.   (see  fig. 


A  B 

Fig.  21. — To  show  the  union  of  the  vagus  A  to  the  anterior  end  of  the  sym- 
pathetic B  in  the  neck.  The  part  of  the  central  nervous  s3-steni  which 
normally' acted  upon  the  abdominal  viscera  becomes  trained  to  act  upon 
the  structures  in  the  face  and  head. 

17,  p.  5  3),  and  (6)  sometimes  by  modifications  in  the  state 
of  consciousness  which  may  be  of  the  nature  of  a  simple 
brief  sensation,  or,  by  the  implication  of  a  number  of  other 
neurons,  may  develop  into  a  series  of  changes  accompanied 
by  a  corresponding  series  of  sensations. 

Very  interesting  results  follow  from  this  fact,  that  the 
activity  of  neurons  is  made  raanifest  by  changes  in  the 
structure  to  which  they  pass.  Langley  has  demonstrated 
that,  if  the  vagus,  which  conducts  downwards  to  the 
abdominal  viscera,  be  cut,  and  the  cervical  sympathetic, 
which  conducts  upwards  to  the  head,  be  also  cut,  and  the 
central  end  of  the  vagus  united  to  the  peripheral  end  of  tlie 
sympathetic  (fig.  21),  fibres  grow  outwai'ds  from  the   vagus 


60        VETERINARY  PHYSIOLOGY 

into  the  sympathetic,  and  when  the  vagus  is  stimulated,  the 
resuks  which  naturally  follow  stimulation  of  the  sympathetic 
occur. 

Kennedy  has  shown  that,  if  the  nerves  to  the  flexors  and 
the  nerves  to  the  extensors  of  a  dog's  forelimb  be  cut,  and 
the  central  end  of  the  former  united  to  the  peripheral  end  of 
the  latter,  and  vice  versa,  the  normal  co-ordinate  movements 
of  the  limb  are  restored,  and  that,  if  that  part  of  the  brain 
which  naturally  causes  extension  be  stimulated,  flexion 
occurs.  He  has  applied  the  information  thus  gained  to  the 
treatment  of  abnormal  conditions  in  the  human  subject.  In 
a  woman  who  suffered  from  spasmodic  action  of  the  muscles 
of  the  face  supplied  by  the  seventh  cranial  nerve,  he  divided 
this  nerve  and  connected  its  peripheral  end  with  the  central 
end  of  the  spinal  accessory  and  thus  secured  a  complete 
recovery.  Such  observations  are  of  great  interest,  since  they 
indicate  that  the  activity  of  the  cells  and  synapses  from 
which  the  fibres  come  may  undergo  profound  alteration  in 
function,  that  they  may  in  fact  be  trained  or  educated  to 
take  on  activities  not  natural  to  them. 

(2)  Electrical  Changes. — The  part  of  the  neuron  in  action 
is  electro-positive  to  the  rest  of  the  neuron.  This  simply 
means  that  the  parts  in  action  become  to  the  rest  of 
the  neuron  what  the  zinc  plate  (the  electro-positive 
element)  in  a  zinc  and  copper  galvanic  cell  is  to  the  copper. 
The  flow  of  current  set  up  along  the  wire  connecting  the 
elements  of  the  cell  is  made  manifest  by  the  deflection  of  a 
galvanometer  needle.  In  the  same  way  the  zinc-like 
(electro-positive)  action  of  the  acting  part  of  the  neuron  is 
made  manifest  if  wires  are  led  off  from  the  nerve  round 
a  galvanometer  (fig.  22). 

In  order  that  neurons  may  produce  their  effect  they  must 
be  capable  of  Excitation  and  Conduction,  and  these  must 
now  be  studied. 

2.   Excitation  of  Neurons. 

Neurons,  like  all  other  protoplasm,  react  to  changes  in 
external  conditions  ;  they  are  capable  of  stimulation. 


NERVE 


61 


A.  A  neuron  is  usually  stimulated  from  one  or  other  of 
its  terminal  dendritic  endings,  either  by  changes  set  up  in 
the  tissues  round  these,  or  by  changes  in  other  neurons. 
Thus  (fig.  17)  one  set  of  neurons  may  be  thrown  into  action 
by  changes  in  the  tissue  at  their  extremity,  and  a  second  set 
may  be  stimulated  by  the  activity  of  the  first. 

B.  Neurons  may  also  be  stimulated  at  any  part  of  their 


course,  as 


may  be  demonstrated  by  pinching  the  ulnar 


behind  the  internal  condyle  of  the  humerus,  when  a  sensation 
localised  on  the  ulnar  side  of  the  hand  is  experienced,  and 
by  the  contraction  produced  in  the  gastrocnemius  muscle  of 


Fig.  22. — A,  To  show  a  galvanic  cell  with  a  zinc  and  copper  element  and  the 
flow  of  the  electric  current  passing  round  a  galvanometer.  B,  A  nerve 
in  which  the  dark  part  is  in  action  and  is  acting  to  the  rest  of  the 
nerve  as  the  zinc  elements  in  the  cell. 


the  frog  when  the  sciatic  nerve  is 
{Practical  Physiology). 


stimulated  in  its  middle 


Means   of  Stimulation. 

knj sudden  change  tends  to  excite  to  activity,  whether  it  be 
mechanical,  as  in  pinching  a  nerve,  or  a  change  in  the 
temjjerature,  or  in  the  electric  conditions,  or  in  the  chemiccd 
surroundings  of  the  neurons  ;  agents  which  withdraw  water, 
like  glycerine,  stimulating  strongly. 

The  electrical  method  of  stimulating  nerve  is  constantly 
used  in  medicine,  and  it  must  be  studied  carefully.      It  is  a 


62  VETERINARY   PHYSIOLOGY 

matter  of  no  importance  how  the  electricity  is  procured,  but 
most  usually  it  is  obtained  either — 

Isf.  Directly  from  a  galvanic  battery,  accumulator,  or 
electric  main  ;  or 

2nd.    From  an  induction  coil. 

list.    Galvanic  Stimulation. 

A.  Exposed  Nerve. 

The  sciatic  nerve  of  the  frog  passing  to  the  gastrocnemius 
muscle  may  be  placed  upon  the  wires  from  a  galvanic 
battery,  and  the  contraction  of  the  muscle  may  be  taken 
as  the  index  of  the  stimulation  of  the  nerve. 

It  will  be  found  that — (a)  On  making  the  current, 
and  upon  breaking  the  current,  a  contraction  results. 
While  the  current  is  flowing  through  the  nerve,  the 
muscle  iisiially  remains  at  rest  ;  but  if  the  current  is 
suddenly  increased  in  strength,  or  suddenly  diminished 
in  strength,  the  muscle  at  once  contracts.  With  strong 
currents,  a  sustained  contraction — galvanotonus — may  persist 
while  the  current  flows  {Practical  Physiology). 

It  is  the  siicldenness  in  the  variation  of  the  strength  of 
the  current,  rather  than  its  absolute  strength,  which  is  the 
factor  in  stimulating,  as  may  be  shown  by  inserting  some 
form  of  rheonome  into  the  circuit  by  which  the  current  may 
be  either  slowly  or  rapidly  varied.  This  fact  is  of  great 
importance  in  applying  galvanic  currents  in  the  treatment 
of  various  diseases  in  the  human  subject.  Great  care  is 
necessary  to  increase  sloivly  and  to  decrease  slowly  the 
strength  of  the  current,  or  painful  stimulation  may  be  pro- 
duced (Practical  Physiology). 

(b)  The  stimulus  on  making  is  stronger  than  that  on 
breaking,  so  that,  if  a  current  be  made  weaker  and  weaker, 
breaking  ceases  to  cause  a  contraction,  while  making  still 
produces  it  (Practiced  Physiology). 

(c)  The  two  poles  do  not  produce  the  same  effect.  The 
negative  pole  or  cathode  stimulates  on  making  ;  while  the 
positive    pole  or  anode  stimulates  at  breaking.      This  may 


NERVE  63 

be  stated — the  nerve  is  always  stimulated  at  the  ])oint 
tvhere  the  current  leaves  it.  On  making  this  is  at  tlie 
cathode ;  on  breaking  at  the  anode  (fig.  23)  {Practical 
Physiology). 

These  results  may  be  summarised  as  follows  : — 

1.  Stimulation  on  closing  (making);  stimulation  on  open- 
ing (breaking). 

2.  Closing  stimulation  stronger  than  opening  stimulation. 

3.  Stimulation  at  cathode  on  closing,  at  anode  on 
opening. 

Or,  taking  the  contraction  of  the  muscle  as  the  sign  of 
stimulation,  and  representing  it  by  C,  the  law  of  galvanic 
stimulation  may  be  expressed  thus  : — 

1.  C.C  •  CO 

2.  CO  CO 

3.  CCC     CAO 

Explanation  of  Elec- 
tric Stimulation.  —  A 
study  of  the  influence 
of  the  current  while  it 
is  flowing  throws  im- 
portant light  on  this 
point.  This  condition  Yig.  23.— To  show  stimulation  at  the 
of    the    tissue    is    known  cathode  on  closing  and  at  the  anode 

-,,      .       .  .-rt  on  opening,  at  the  point  where  the 

as    EleCtrotonUS    {Prac-  current  lef^es  the  nerve. 

tical  Physiology). 

While  the  current  simply  flows  through  a  nerve  no 
stimulation  is  produced,  but  the  excitability  is  profoundly 
modified. 

Round  the  cathode  the  nerve  becomes  more  easily  stimu- 
lated, while  round  the  anode  or  positive  pole  it  becomes  less 
easily  stimulated.  This  may  be  expressed  by  saying  that 
the  part  of  the  nerve  under  the  influence  of  the  cathode  is 
in  a  state  of  catelectrotonus,  of  increased  excitability,  while 
the  part  of  the  nerve  under  the  influence  of  the  anode  is  in 
a  state  of  anelectrotonus,  of  decreased  excitability  or  of  more 
stable  equilibrium.  This  is  easily  demonstrated  by  passing 
a  galvanic  current  along  a  nerve  going  to  a  muscle,  and 


64  VETERINARY   PHYSIOLOGY 

stimulating  first  in  the  region  of  the  cathode  and  then  in  the 
region  of  the  anode.  A  much  stronger  stimulus  will  be 
found  necessary  to  cause  contraction  of  the  muscle  at  the 
second  point,  in  the  region  of  the  anode. 

It  is  the  sudden  production  of  increased  excitability  at 
the  cathode  on  closing  which  causes  an  explosion,  a  stimu- 
lation. The  sudden  removal  of  the  increased  stabiUty  round 
the  anode  when  the  current  is  broken  is  sufficient  to  cause 
an  explosion  if  the  current  is  strong  enough. 

The  study  of  electrotonus  thus  explains  (1)  why  any 
sudden  change  in  the  flow  of  electricity  through  a  muscle 
stimulates  it  ;  (2)  why  the  stimulation  starts  from 
the  cathode  on  closing  and  from  the  anode  on  opening ; 
and  (3)  why  the  closing  contraction  is  stronger  than  the 
opening. 

This  law  of  Polar  Excitation,  Avhile  it  applies  to   normal 


+ 


Fig.  24. — To  show  the  passage  of  a  galvanic  current  along  a  nerve  so  that 
distinct  polar  effects  are  produced. 

muscle  and  nerve,  does  not  apply  to  all  protoplasm.  Thus, 
amoiba  shows  contraction  at  the  anode  and  expansion  at  the 
cathode  when  a  galvanic  current  is  passed  through  it. 

B.   Nerve  under  the  Skin. 

In  practice  the  galvanic  current  may  be  used  to 
stimulate  nerves  and  muscles  in  situ  under  the  skin.  To 
use  the  current  for  this  purpose  an  electrode  is  placed  over 
the  nerve  or  muscle  to  be  investigated  and  the  other  over 
some  indifferent  part  of  the  body.  The  very  considerable 
electrical  resistance  of  the  skin  has  to  be  overcome  by  using 
rather  large  electrodes  usually  covered  with  chamois  leather 
well  soaked  in  saturated  salt  solution. 

Applied  in   this   way,   the  current  passes   from   pole  to 


NERVE 


65 


pole,  not  along  the  nerve  or  muscle  (fig.  24),  but  more  or 
less  across  it  (fig.  25). 

The  nerve,  if  under  the  cathode,  is  thus  under  the 
influence  of  the  cathode  on  the  side  near  the  pole,  and 
under  the  influence  of  the  anode  on  the  side  away  from  the 
pole,  and,  vice  versa,  under  the  anode  (fig.  25). 


-•^r-**-^"^ 
i^-^ 


,!*■•*- ti  t  ^^ 


B 


S7^ 


yViiVN^j. 


^ 


Fig.  25. — Electrical  Stimulation  of  human  muscle  or  nerve  to  show  the 
passage  of  the  current  across  the  structure,  and  the  consequent 
combination  of  effects  under  each  pole. 


Hence  there  will  be  a  stimulation  both  at  making 
and  at  breaking  (p.  63)  under  both  cathode  and 
anode. 

The  cathodal  closing  contraction  is  the  stronger  because 
of  its  dependence  upon  the  more  effective  pole,  the  cathode, 
as  it  is  a  closing  contraction,  and  also  because  the  excitation 
begins  at  A,  which  is  near  the  stimulating  pole.  The 
cathodal  opening  contraction  has  the  less  effective  pole — the 
anode  as  its  origin,  since  it  is  an  opening  contraction,  and 
the  excitation  is  at  the  less  effective  position  B,  which  is 
separated  from  the  stimulating  pole — the  anode  by  a 
5 


m 


VETERINARY   PHYSIOLOGY 


greater  distance  than  is  A  from  the  cathode.  Similarly,  the 
anodal  closing  contraction  has  the  better  pole,  the  cathode  as 
the  stimalating  pole,  but  in  the  worst  position  D,  far 
from  the  cathode.  The  anodal  opening  contraction  has 
the  less  effective  pole — the  anode  as  the  origin  of  excita- 
tion, but  in  the  better  position  G,  close  to  the  stimulating 
pole. 


Position. 


Strength  of  Current 
necessary  to  stimulate. 


-c.c. 


Cathode,  Closing  A    \ 
Anode,  Closing  D       J 
Anode,  Opening  C     t  .  p, 
Cathode,  Opening  B  j^-^- 


Better 
Better 
Worse 
Worse 


Better 
Worse 
Better 
Worse 


Weakest 
Medium 
Medium 
Strongest 


The  strength  of  the  current  required  to  stimulate  is 
measured  in  milliamperes.  The  effective  strength  varies 
o-reatly,  even  in  normal  individuals  of  the  same 
species. 

Changes  in  Disease  and  Injury. — In  tetany  in  children, 
and  after  the  removal  of  the  parathyreoids  in  man  and 
animals  (p.  603),  a  condition  of  increased  excitability  of 
the  nerves  to  mechanical  and  galvanic  stimulation  occurs, 
and  this  is  used  in  the  diagnosis  of  these  conditions  when 
other  symptoms  are  latent. 

When  the  nerve  to  a  muscle  is  cut  it  rapidly  loses  its  power 
of  responding  to  electrical  stimulation  (p.  76),  but  the  muscle 
continues  to  respond,  and  its  response,  although  at  first  it 
may  be  decreased,  is  afterwards  increased  and  becomes 
peculiarly  slow.  Some  neurologists  have  maintained  that 
there  is  a  qualitative  change  in  the  response,  that  the 
response  to  anodal  closing — A.C.  becomes  greater  than  that 
to  cathodal  closing — C.C.  With  fine  electrodes  applied  to  ex- 
posed muscle  this  does  not  occur.  It  is  probably  due  to  the 
fact  that  in  normal  muscle  it  is  the  nerve  endings  which  are 
stimulated,  but  that  in  degeneration  the  response  depends 
upon  the  number  of  muscular  fibres  stimulated  rather  than 
upon   the  pole  applied.      Hence  in  anodal  closing  the  worst 


NERVE 


67 


position,  far  from  the  pole,  acts  upon  more  fibres  than  the 
cathode  acts  upon  in  closing. 


Fig.  26. — To  illustrate  the  reason  for  the  increase  in  the  anodal  closing 
stimulation  in  a  muscle  after  degeneration  of  the  nerve  and  nerve 
endings. 


2.  Faradic  Stimulation. 
When  nerve  is  stimulated  by  induced  or  faradic  electricity 


Electric 
Current. 


Make. 


Break. 


Make.       Break. 


Contraction 
of  Muscle. 


Cathode.  Anode. 

Galvanic. 


Cathode.     Anode. 
Induced. 


Fig,  27. — To  show  separation  of  make  and  break  stimuli  and  of  anodal  and 
cathodal  effects  when  a  galvanic  current  is  used,  and  their  combina- 
tion when  the  induction  coil  is  used  (faradic). 

(fig.  28),  with  each  make  and  break,  or  with  each  sudden 
alteration  in  the  strength  of  the  primary  circuit,  there  is  a 
sudden  appearance  and  equally  sudden  disappearance  of  a 
flow  of  electricity  in  the  secondary  coil.  If,  therefore,  wires 
from  the  secondary  coil  are  led  ofif  to  a  nerve,  each  change 
in  the  primary  circuit  causes  the  sudden  and  practically 
simultaneous    appearance  and    disappearance  of  an  electric 


68 


VETERINARY  PHYSIOLOGY 


current  in  the  nerve,  and  this,  of  course,  causes  a  contraction. 
But  here  the  effects  of  closing  and  opening  the  current  are 
practicall}^  fused,  and  hence  the  influence  of  the  anode  and 
cathode,  and  of  closing  and  opening,  need  not  be  considered 
(fig.  27)   {Practical  Physiology). 

It  must,  of  course,  be  remembered  that  in  an  induction 
coil  the  opening  of  the  primary  circuit  produces  a  more  power- 
ful current  in  the  secondary  coil  than  the  closure  of  the 
primary  circuit,  and  therefore  a  more 
powerful  stimulation  of  the  nerve  (fig. 
28). 

Relationship  of  the  Excita- 
tion to  the  Strength  of  the 
Stimulus. — A  nerve  is  made  up  of  a 
series  of  axons  placed  side  by  side. 
The  ditlerence  in  the  effect  of  a  weak 
and  of  a  strong  stimulus  as  indicated 
by  the  contraction  of  the  muscle 
supplied  may  be  due  either  to  a 
graded  effect  of  the  stimulus  on  every 
fibre  or  to  the  number  of  fibres 
stimulated  by  the  different  strengths 
of  stimulus.  The  cutaneus  dorsi 
nerve  of  the  frog  is  composed  of  only 
ten  fibres,  and,  as  the  strength  of 
stimulus  is  steadily  increased,  the 
resulting  contractions  increase  in  ten 
stages.  The  conclusion  is  that  the 
result  depends  upon  the  number  of 
fibres  stimulated,  and  that,  when  a 
stimulus  excites  a  fibre,  it  does  so  to  call  forth  its  full 
action — the  stimulation  of  each  axon  is  either  all  or  nothing. 
Variations  in  Excitability.  —  The  influence  of  the 
galvanic  current  upon  the  excitability  of  nerve  has  been 
already  considered  (Electrotonus,  p.  63). 

Many  other  factors  modify  its  excitability.  It  may  be 
increased  by  a  slight  cooling,  but  it  is  decreased  at  lower 
temperatures.  It  is  increased  by  warming  up  to  a  certam 
point.      Drying  at  first  increases  excitability,  then  abolishes 


Fig.  28.— Course  of  Elec- 
tric Current  in  primary 
circuit  (lower  line),  and  in 
secondary  circuit  (upper 
line)  of  an  induction  coil. 
Observe  that  in  the 
secondary  the  make  (up- 
stroke) and  break  (down- 
stroke)  are  combined,  and 
that  a  stronger  current 
is  developed  in  the 
secondary  circuit  upon 
breaking  than  upon  mak- 
ing the  primary  circuit. 


NERVE 


69 


it.  It  is  influenced  by  many  chemical  substances,  some  of 
which  increase  its  excitability  in  small  doses,  and  diminish 
it  in  larger  doses  ;  some  again  even  in  the  smallest  dose 
depress  its  activity,  e.g.  potassium  salts,  and  such  drugs  as 
chloroform  and  ether. 

Continued  activity  has  no  effect  on  the  excitability  of  axons, 
and  the  phenomena  of  fatigue  are  not  manifested  in  them. 

This  may  be  proved  by  taking  two  nerve-muscle  prepar- 
ations, A  and  B,  and  stimulating  both  nerves  repeatedly  with 


Stimulating  Current. 


\    t  f  Blocking  Current. 


A  B 

Fig.  29. — Experiment  to  show  that  a  nerve  cannot  be  fatigued.  Two  muscle 
nerve  preparations,  A  and  B,  are  stimulated  by  the  faradic  current. 
B  is  blocked  by  a  galvanic  current  or  by  cooling,  till  the  muscle  of  A 
no  longer  contracts.     The  block  is  then  removed,  and  B  contracts. 

the  same  electric  current,  but  preventing  the  stimulus  from 
reaching  one  of  the  muscles,  B,  by  blocking  its  passage  by 
passing  a  galvanic  current  through  the  nerve  (see  p.  71)  or 
by  applying  ice  to  it  (fig.  29). 

If,  after  muscle  A  no  longer  contracts,  the  block 
is  removed,  muscle  B  will  contract,  showing  that  the  nerve 
is  not  fatigued. 

3.  Conduction  in  Neurons. 

When  a  neuron  is  stimulated  at  any  point,  some  time 
elapses  before  the  result  of  the  stimulation  is  made  manifest, 


70 


VETERINARY  PHYSIOLOGY 


and  the  further  the  point  stimulated  is  from  the  structure 
acted  upon,  the  longer  is  this  latent  period.  This  indicates 
that  the  change  does  not  develop  simultaneously  throughout 
the  neuron,  but,  starting  from  one  point,  is  conducted  along  it. 
(a)  The  rate  of  conduction  may  be  determined — 
1st  By  stimulating  a  nerve  going  to  a  muscle  at  two 
points  at  a  known  distance  from  one  another,  and  measuring 
the  difference  of  time  which  elapses  between  the  contraction 


Fig.  30. — M,  Muscle  attached  to  crank  lever  marking  on  revolving  drum.  The 
secondary  circuit  of  an  induction  coil  is  connected  with  a  commutator, 
with  the  crossed  wires  removed  so  that  the  current  may  be  sent  either 
through  the  wires  going  to  the  nerve  at  A  far  from  the  muscle,  or  at  B, 
a  point  at  a  measured  distance  nearer  the  muscle.  On  the  drum,  A 
represents  the  onset  of  contraction  on  stimulating  at  A,  and  B  the  onset 
on  stimulating  at  B.  To  secure  stimulation  in  each  case  with  the  drum 
in  the  same  position,  the  make  and  break  of  the  primary  circuit  is 
caused  by  the  point  of  K  touching  and  quitting  the  point  P. 

resulting  from  stimulation  at  each  (fig.  30)  (Practical 
Physiology). 

2ncl.  By  taking  advantage  of  the  fact  that  the  conducting 
part  of  a  neuron  is  electro-positive,  i.e.  like  the  zinc  element  in 
a  galvanic  cell,  to  the  rest  (p.  60),  and  by  finding  how  long 
after  stimulation  at  one  point  this  electric  change  reaches 
another  point  at  a  measured  distance  from  it  (fig.  8 1 ). 

The  rate  of  conduction  varies  considerably;  everything 
stimulating  protoplasmic  activity  accelerating,  and  everything 


NERVE 


71 


depressing  protoplasmic  activity  diminishing  it.  Under 
normal  conditions  in  the  fresh  nerve  of  the  frog,  the  velocit}' 
is  about  33  metres  per  second.  In  man  it  is  about  100  to 
150  metres  per  second,  and  in  the  octopus  only  about  2 
metres. 

(h)  Factors  modifying  Conduction. — Conduction  is  modified 
by  temperature.  Cooling  a  nerve  lowers  its  power  of  con- 
duction ;  gently  heating  it  increases  it.  Various  drugs 
which  diminish  protoplasmic  activity — e.g.  chloroform, 
ether,  carbon  dioxide,  etc. — diminish  conduction.  But 
while  the  excitability  of  the  part  of  the  nerve  under  the 


^-^<M^<vI] 


Fig.  31. — N,  a  piece  of  a  Nerve  connected  by  non-polarisable  electrodes  to  the 
galvanometer,  G.  By  an  induction  coil  it  may  be  stimulated  at  A. 
When  the  nerve  impulse  reaches  a,  a  deflection  of  the  galvanometer 
needle  takes  place. 

influence  of  these  agents  undergoes  a  steady  decrement,  i.e. 
the  contractions  of  the  muscles  supplied  steadily  decrease, 
the  conduction  of  the  impulse  is  either  completely  blocked, 
or,  if  it  is  transmitted,  it  regains  its  full  strength  in  the  part 
of  the  nerve  peripheral  to  the  block.  Very  important 
conclusions  as  to  the  nature  of  the  nerve  impulse  have  been 
based  upon  this  fact.  The  electric  current,  too,  acts  differently 
on  conduction  and  on  excitability  (p.  63).  While  a  weak 
current  has  little  or  no  effect,  a  strong  current  markedly 
decreases  conductivity  round  the  positive  pole,  and  to  a  less 
extent  decreases  it  at  the  negative  pole,  so  that  the  general 
effect  of  a  strong  current  is  to  decrease  conductivity  and  to 
block  the  passage  of  impulses  (fig.  32). 


72  VETERINARY   PHYSIOLOGY 

From  this  influence  of  the  electric  current  upon  (a)  excit- 
ability and  (6)  conductivity  certain  differences  are  to  be 
observed  in  the  effects  of  stimulating  an  exposed  nerve  with 
currents  of  different  strengths  and  of  different  direction — 
downwards,  to,  the  muscle  or  upwards,  from,  the  muscle. 
These  have  been  formulated  as  Pfliiger's  Law.  But 
since  they  have  no  bearing  upon  the  stimulation  of 
unexposed  nerves  in  animals  they  need  not  be  considered 
here. 

(c)  When  a  neuron  is  stimulated  in  the  middle,  the  change 
travels  in  both  directions,  although  its  result  is  only  made 
manifest  by  the  action  of  the  structure  at  the  end  on  which 
it  normally  acts.       (i.)  This    two-way   conduction    may    be 


Fig.  32. — To    show  the  effect  of    a  galvanic  current    upon  the    excitability 
(continuous  line)  and  upon  the  conductivity  (broken  line)  of  a  nerve. 

demonstrated  by  using  the  electric  changes  as  an  index  of 
nerve  action  connecting  a  galvanometer  with  the  nerve  on 
each  side  of  the  point  stimulated  instead  of  with  only  one 
side,  as  in  fig.  31.  (ii.)  It  is  also  demonstrated  by  the 
experiment  of  "paradoxical  contraction,"  in  which  the 
electric  variation,  set  up  by  stimulating  the  branch  of  the 
sciatic  nerve  of  the  frog  going  to  the  muscles  of  the  thigh, 
stimulates  the  nerve  fibres  to  the  gastrocnemius  lying  along- 
side of  it,  and  causes  that  muscle  to  contract.  That  this 
does  not  occur  when  impulses  from  the  central  nervous 
system  pass  along  the  nerve  is  because  the  strength  of  these 
impulses,  i.e.  the  number  of  fibres  implicated,  as  indicated 
by  the  electrical  change,  is  very  much  weaker  than  that 
of  impulses  caused  by  direct  stimulation  {Practical 
Physiology). 


NERVE  73 

4.  Classification  of  Neurons  by  the  Direction  of  Conduction. 

Since  a  nerve  is  normally  stimulated  from  either  the 
one  or  the  other  end,  and  hence  conducts  in  one  direction, 
and  since  the  passage  of  impulses  along  it  is  made  manifest  by 
changes  in  the  structure  to  which  it  goes  (p.  59),  it  is  possible 
to  classify  nerve  fibres  according  to  whether  they  conduct  to  or 
from  the  central  nervous  system, and  according  to  the  structure 
upon  which  they  act. 

To  find  out  the  direction  of  conduction  and  the  special 
mode  of  action  of  any  nerve,  after  its  anatomical  distribution 
has  been  determined,  two  methods  of  investigation  may  be 
employed — 

1st.  The  nerve  may  be  cut,  and  the  results  of  section 
studied. 

2nd.  The  nerve  may  be  stimulated,  and  the  result  of 
stimulation  noted. 

Usually  these  methods  are  used  in  conjunction ;  the 
nerve  is  cut,  and  when  the  changes  thus  produced  have  been 
noted,  first  the  upper  or  central  end  and  then  the  lower  or 
peripheral  end  of  the  cut  nerve  are  stimulated. 

It  is,  of  course,  only  if  a  nerve  is  constantly  transmitting 
impulses  that  section  reveals  any  change.  If  the  nerve  is 
not  constantly  in  action,  stimulation  alone  will  demonstrate 
its  functions. 

A.  Outgoing  or  Efferent  Nerves. — Section  of  certain  nerves 
produces  a  change  of  action  in  muscles,  glands,  etc.,  or,  if 
the  nerve  is  not  constantly  acting,  stimulation  of  the  peri- 
pheral end  of  the  cut  nerve  causes  some  change  in  the 
activity  of  these  structures.  Stimulation  of  the  central  end 
of  such  nerves  produces  no  effect.  These  nerves  there- 
fore conduct  impulses  outward  from  the  central  nervous 
system. 

The  nerves  going  to  the  skeletal  muscles  may  cause  either 
an  increased  activity  or  a  decreased  activity  according  to  the 
way  in  which  they  are  called  into  action,  so  that  at  one  time 
the  nerve  may  be  an  excitor  of  the  muscle,  at  another  time 
an  inhibitor. 

The  nerves  going  to  the  visceral  muscles,  on  the  other 


74  VETERINARY   PHYSIOLOGY 

hand,  are  either  augmentor  or  inhibitory,  but  they  do  not  act 
at  one  time  in  the  one  way,  at  another  in  the  other. 

B.  Ingoing  or  Afferent  Nerves. — Section  of  another  set  of 
nerves  may  produce  loss  of  sensation  in  some  part  of  the 
body.  When  the  peripheral  end  of  the  cut  nerve  is  stimu- 
lated no  result  is  obtained.  When  the  central  end  is 
stimulated,  sensations,  with  or  without  some  kind  of  action  may 
result.  Such  nerves  obviously  conduct  inwards  to  the  central 
nervous  system.  Those  which,  when  stimulated,  give  rise  to 
sensations  may  be  called  sensory ;  those  which  give  rise  to 
some  action  are  called  excito- reflex,  because  the  action  which 
results  is  produced  by  reflex  action  (p.  82).  But  these  two 
are  not  distinct  from  one  another,  and  a  nerve  which  at  one 
time,  when  stimulated,  causes  a  sensation,  may  at  another 
time  cause  a  reflex  action  without  sensation.  The  branch  of 
the  fifth  cranial  nerve  which  passes  to  the  conjunctiva  of  the 
eye  is  an  example  of  such  a  nerve.  When  the  conjunctiva 
is  touched — i.e.  when  this  nerve  is  stimulated — the 
orbicularis  palpebrarum  is  brought  into  action  through  the 
seventh  cranial  nerve,  and  the  eye  is  closed.  The  conjunctival 
branch  of  the  fifth  cranial  nerve  is  thus  an  excito-motor 
nerve. 

When  the  terminations  of  the  lingual  branch  of  the  fifth 
nerve  in  the  tongue  are  stimulated,  the  result  is  a  free  flow 
of  saliva,  through  the  action  of  the  secretory  fibres  of  the 
seventh  nerve  and  of  the  glosso-pharyngeal.  The  lingual 
nerve  is  thus  excito -secretory. 

Stimulation  of  the  nerves  from  any  part — e.g.  by  a 
mustard  blister — causes  relaxation  of  the  blood-vessels  of  the 
part,  and  such  afferent  nerves  may  be  called  excito- vaso- 
dilator. 

C.  Many  nerves  of  the  body  contain  both  afferent  and 
efferent  nerve  fibres,  and  are  called  mixed  nerves. 


5.   The  Nature  of  the  Nerve  Impulse. 

The  impulse  which  passes  along  a  nerve  is  due  to 
changes  in  the  axis  cylinder,  since  this,  without  its  sheaths, 
can  conduct.      Further,  it  is  dependent  on   the  vitality  of 


NERVE  75 

the  nerve.  Death  of  the  nerve,  as  when  it  is  heated  to  47° 
C,  at  once  stops  the  transmission  of  an  impulse. 

We  may  at  once  dismiss  the  idea  that  the  impulse 
is  due  to  a  mere  flow  of  electricity.  Electricity  travels 
along  a  nerve  at  about  300  million  metres  per  second,  a 
velocity  much  higher  than  that  of  the  nerve  impulse. 

Two  possibilities  remain.  The  impulse  may  be  of  the 
nature  of  the  molecular  vibration,  such  as  occurs  in  the 
stethoscope  which  conducts  sound  vibration,  or  it  may 
consist  of  a  series  of  chemical  changes,  such  as  cause  the 
activity  of  protoplasm  generally. 

In  considering  this  matter  it  must  be  remembered  that 
the  amount  of  energy  evolved  in  a  nerve  impulse  need  not 
be  great.  All  it  has  to  do  is  to  start  the  activity  of  the 
part  to  which  it  goes.  Hence,  if  chemical  changes  are  the 
basis  of  the  impulse,  these  may  be  extremely  small  in 
amount  and  difficult  to  detect,  while  at  the  same  time 
recovery  may  be  extremely  rapid. 

As  a  matter  of  fact,  the  evidence  of  chemical  changes  in 
nerve  fibres  is  slight.  No  change  in  reaction,  no  heat  pro- 
duction, and  no  phenomena  of  fatigue  can  be  demonstrated. 
But  two  bits  of  evidence  point  to  the  existence  of  chemical 
changes.  Oxygen  is  required  ;  and  the  fact  that,  if  an  impulse 
succeeds  in  passing  through  a  piece  of  nerve  in  which  con- 
duction is  decreased  by  cold  or  by  drugs,  it  regains  its 
original  strength  must  mean  that  the  impulse  is  due 
to  a  chemical  change  similar  to  that  which  occurs  as  a 
spark  passes  along  a  trail  of  gunpowder.  The  spark  may  be 
delayed  and  reduced  to  a  minimum  in  a  damp  part  of  the 
trail,  but,  if  it  passes  this,  it  regains  its  original  strength 
when  a  dry  part  is  reached. 


NERVE    CELLS. 

1.  Automatic  Action? — It  has  sometimes  been  supposed 
that  nerve  cells  originate  nerve  impulses,  that  they  have  an 
automatic  action.  The  nerve  cells  which  send  fibres  to 
muscles  may  be  isolated  from  the  influence  of  incoming 
neurons  by  cutting  the  posterior  roots  of  the   spinal   nerves 


76  VETERINARY   PHYSIOLOGY 

(p.  107),  and  when  this  is  done  the  muscles  supplied  remain 
soft,  flaccid,  and  motionless.  Under  normal  conditions  no 
impulses  pass  from  the  cells. 

2.  Conduction? — That  the  cells  do  not  play  an  essential 
part  in  conduction  has  been  most  clearly  demonstrated  by 
experiments  upon  the  common  crab,  where  it  is  possible  to 
remove  the  nerve  cell  from  the  fibre  with  which  it  is 
connected  without  injuring  the  fibre.  For  some  time  after 
this  is  done  the  tibre  continues  to  conduct. 

3.  Nutrition  of  the  Neuron. — The  function  of  the  cell 
is  to  preside  over  the  nutrition  of  the  neuron.  It  appears 
to  have  the  power  of  accumulating  a  reserve  of  material 
which,  when  the  cells  are  fixed,  appears  as  Nissl's  granules, 
for  it  has  been  found  that  after  continued  action  these 
granules  diminish  in  amount.  The  nucleus,  too,  gives  off 
material  for  the  nourishment  of  the  neuron,  and  in  conditions 
of  excessive  activity  it  has  been  found  shrunken  and  dis- 
torted. 

If  any  part  of  the  neuron  is  cut  off  from  its  connection 
with  the  cell,  it  dies  and  degenerates,  and  later  the  cell  may 
also  undersro  changes. 

T  1 

(a)  Degeneration  and  Regeneration  of  Nerves. — in  the  cat, 
excitabihty  disappears  after  three  days  (p.  66),  and  the  white 
sheath  shows  degenerative  changes  in  eight  days.  The  fatty 
matter  runs  into  globules  and  stains  black  with  osmic  acid, even 
after  treatment  with  chrome  salts  (Marchi's  method).  This 
seems  to  be  due  to  the  fact  that  osmic  acid  acts  upon  the 
unsaturated  oleic  acid,  and  that  this,  in  the  normal  nerve, 
is  oxidised  by  the  chrome  salt,  whereas,  in  the  degenerated 
nerve,  so  much  is  set  free  that  it  cannot  all  be  oxidised, 
and  therefore  stains  with  osmic  acid.  The  white  sub- 
stance gradually  disappears  ;  at  the  end  of  a  month  the 
phosphorus  has  all  gone,  and  by  the  end  of  about  forty-four 
days  the  fat  can  no  longer  be  detected.  At  this  stage 
Marchi's  method  is  useless,  and  the  degenerated  fibres  may 
be  demonstrated  by  the  fact  that  they  do  not  stain  with 
osmic  acid  or  with  Weigert's  hasmatoxyhn  method  which 
stains  the  white  sheaths  of  normal  fibres.  As  the  degenera- 
tion advances,  the  axis  cylinder  breaks  down,  and  the  nerve 


NERVE  77 

corpuscles  proliferate  and  absorb  the  remains  of  the  white 
sheath,  so  that  nothing  is  left  but  the  primitive  sheath  filled 
by  nucleated  protoplasm. 

Into  this,  axons  may  grow  downwards  from  the  central 
end  of  the  nerve,  and  regeneration  may  occur.  This 
generally  begins  after  about  forty  days  and  is  well  marked 
after  about  one  hundred  days. 

Some  investigators  have  maintained  that  regeneration 
occurs  by  the  development  of  new  fibrils  in  the  degenerated 
nerve  itself,  but  the  mass  of  evidence  indicates  that,  when 
an  apparent  peripheral  regeneration  has  occurred,  it  has 
been  due  to  the  ingrowth  of  axons  from  adjacent  cut 
nerves. 

The  neurolemma  with  its  nuclei  appears  to  be  essential  for 
regeneration.  It  is  not  present  in  the  white  fibres  of 
the  central  nervous  system,  and  hence  regeneration  does  not 
occur  there  after  the  nerve  fibres  have  been  severed  and 
have  degenerated. 

(6)  The  cell  is  dependent  for  its  proper  nutrition 
upon  the  condition  of  the  rest  of  the  neuron.  When  the 
axon  is  cut,  the  chromatin  of  the  cell  nucleus  slowly 
decreases,  and  the  nucleus  becomes  displaced  to  one  side, 
and  ultimately  the  whole  cell  may  degenerate.  This  is 
sometimes  called  Nissl's  degeneration  (see  fig.  19,  h). 


B.  ACTION  OF  NEURONS  IN  SERIES. 

So  far,  the  physiology  of  single  neurons,  or  of  neurons 
running  side  by  side  in  nerves,  has  been  considered.  But 
in  the  nervous  system,  as  already  indicated,  they  are 
arranged  in  chains  or  series,  the  activity  of  one  set  leading 
to  changes  in  other  sets  (fig.  17,  p.  53). 

The  apparently  inextricable  labyrinth  of  neurons  runnino- 
throughout  the  nervous  system  may  be  arranged  in  three 
groups  or  arcs,  according  to  their  distribution. 

Each    arc    consists    of    ingoing    neurons,    starting    from 


78 


VETERINARY  PHYSIOLOGY 


Receptors  on  the  one  side,  and  outgoing  neurons  ending  in 
Effectors  on  the  other  side ;  the  neurons  between  them 
constituting  the  Conductor  mechanism. 


THE   NEURAL   ARCS. 
A.  Arrangement. 

The  neural  arcs  will  later  have  to  be  studied  more  in 
detail.  At  present  a  mere  outline  of  their  distribution  will 
be  given. 


Fig.  33. — To  show  the  three  Arcs  in  the  Central  Nervous  System.  A,  Peri- 
pheral ingoing  neuron  giving  oif  collaterals  in  tlie  cord  and  some 
terminating  above  in  the  nuclei  of  the  posterior  columns  ;  B,  peripheral 
outgoing  neurons  ;  C,  ingoing  cerebral  neurons  ;  D,  outgoing  cerebral 
neurons,  crossing  to  the  opposite  side  //;  E,  ingoing  cerebellar 
neurons  ;  F,  outgoing  cerebellar  neurons. 


1.  Spinal  Arc. — A.  Ingoing  Neurons  (fig.  33,  A) — This 
set  of  neurons  is  developed  primarily  in  connection  with  the 
surface  of  the  body,  but  they  also  start  in  muscles,  joints, 
and  internal  organs.  They  start  in  dendritic  expansions  at 
the  periphery,  and  enter  the  cord  by  the  dorsal  roots  of 
the  spinal  nerves  in  the  swelling  or  ganglion  upon  which  the 
cell  of  each  neuron  is  situated  (see  p.  106).  In  the  cord 
they  divide  into  (a)  branches  running  for  a  short  distance 
down  the  cord;  (b)  branches  running  up  the  spinal  cord. 
From  these  branches,  processes  pass  forward  to  form 
synapses  with  the  neurons  in  the  ventral  part  of  the  spinal 
cord  from  which  the  outgoing  fibres  to  the  skeletal  muscles 


NERVE  79 

spring.  It  is  probable  that  several  series  of  intercalated 
neurons  exist  between  the  ingoing  and  outgoing  neurons. 

B.  Outgoing  Neurons  (fig.  33,  B). — (a)  From  the  nerve 
cells  in  the  ventral  part  of  the  grey  matter  of  the  cord,  fibres 
are  given  off  which  pass  out  in  the  anterior  or  ventral  roots  of 
the  spinal  nerves  to  skeletal  muscles,  and  (6)  from  cells 
situated  in  the  lateral  part  of  the  grey  matter  of  the  cord 
fibres  pass  out  to  be  connected  with  the  visceral  neurons 
which  pass  to  muscles  or  to  glands  (p.  54). 

The  fibres  entering  and  leaving  the  base  of  the  brain  by 
the  cranial  nerves  belong  to  this  spinal  arc. 

The  action  of  these  spinal  neurons  is  controlled  and 
modified  by  the  two  other  arcs. 

2.  Cerebral    Arc— A.  Ingoing  Neurons     (fig.     33,     C). 

Lower  Neurons. — The  ingoing  fibres  of  the  spinal  cord  and 
cranial  nerves  not  only  give  oft'  branches  to  form  the  spinal 
arcs,  but  they  also  form  other  synapses  in  or  just  above  the 
cord  from  which  fresh  neurons  run  upwards,  cross  the  middle 
line  to  the  opposite  side,  and  finally  form  synapses  higher  up 
and  chiefly  in  the  thalamus  (fig.  101).  From  these  fibres 
pass  (a)  to  other  basal  ganglia  such  as  the  red  nucleus,  and  (6) 
to  the  cortex  cerebri  to  form  synapses.  From  these  fibres 
pass  to  the  outgoing  neurons. 

B.  Outgoing  Neurons — The.se  cross  the  middle  line  and  run 
down  the  spinal  cord  to  act  upon  the  spmal  arcs  (fig.  33,  D). 

3.  Cerebellar  Arc— A.  Ingoing  (fig.  33,  E).—{a)  Some 
of  the  branches  of  the  spinal  ingoing  neurons  from  muscles 
and  joints  end  in  synapses  round  nerve  cells  at  the  side  of 
the  grey  matter  of  the  spinal  cord.  From  these,  fibres  run  up 
to  the  cerebellum  to  form,  directly  or  indirectly,  synapses 
round  the  cells  in  this  organ. 

(h)  Fibres  from  the  synapses,  formed  by  the  incoming 
fibres  from  the  vestibular  part  of  the  labyrinth  of  the  ear, 
also  course  to  the  cerebellum. 

B.  Outgoing  (fig.  33,  F). — From  the  cells  of  the  cortex  of 

the  cerebellum  (a)  fibres  pass  to  masses  of  grey  matter the 

roof  nuclei — and  to  a  mass  of  cells  Ij'ing  outside  the 
cerebellum  on  each  side — Deiters'  nucleus — where  they  form 


80 


VETERINARY   PHYSIOLOGY 


sj'napses  with  neurons  which  send  their  axons  down  the 
same  side  of  the  spinal  cord,  and  give  off  collaterals  which 
act  upon  the  spinal  arcs.  (6)  Fibres  also  pass  to  the 
red  nucleus  of  the  cerebrum,  from  which  fibres  pass  down 
the  cord  to  modify  its  action. 

B.  General  Action  of  the  Arcs. 

The  special  action  of  each  of  these  arcs  may  be  studied 
by  observing  the  effect  of  interruption  of  its  different 
parts. 

1.  Cerebral  Arc — (1)  If,  in  a  normal  frog,  the  attitude, 
movements,  responses  to  various  stimuli  and  power  of  balanc- 
ing be  studied,  aud  if  (2)  the  anterior  part  of  the  brain,  the 


Fig.  34. — A,  Pigeon  with  the  Cerebellum  destroyed  to  show  struggle  to 
maintain  the  balance  ;  B,  Pigeon  with  Cerebrum  removed  to  show 
balance  maintained,  but  the  animal  reduced  to  somnolent  condition. 


cerebrum,  be  removed,  it  will  be  found  that  the  animal 
still  sits  in  its  characteristic  posture.  When  touched  it 
jumps  ;  when  thrown  into  water  it  swims.  It  is  a  perfect 
reflex  machine,  with  the  power  of  balancing  itself  unimpaired. 
But  it  differs  from  a  normal  frog  in  moving  only  when 
directly  stimulated,  and  in  showing  no  signs  of  hunger  or  of 
thirst.  A  worm  crawling  in  front  of  it  does  not  cause  the 
characteristic  series  of  movements  for  its  capture  which  is 
seen  in  a  normal  frog.  Generally  a  condition  of  decere- 
bration  rigidity  appears  after  a  time,  and  the  frog,  when  held 
by  the  sides  and  placed  upon  the  table,  remains  with  its  legs 
rigidly  stretched,  supporting  the  body  well  above  the  surface, 
with  the  back  arched  like  that  of  an  angry  cat. 

In  the  pigeon  (fig.  34,  B),  removal  of  the  cerebral   hemi- 


NERVE  81 

spheres  reduces  the  animal  to  the  condition  of  a  somnolent 
reflex  machine.  The  bird  sits  on  its  perch,  generally  with  its 
head  turned  back,  as  if  sleeping.  If  a  sudden  noise  is  made, 
if  light  is  flashed  in  its  eye,  or  if  it  is  touched,  it  flies  off  its 
perch  and  lights  somewhere  else.  Clapping  the  hands  and 
letting  peas  fall  on  the  floor  both  produce  a  start,  but  the 
bird  makes  no  endeavour  to  secure  the  peas,  as  it  would  do 
in  the  normal  state. 

In  the  clog,  by  a  succession  of  operations,  Goltz 
removed  the  greater  part  of  the  cerebral  cortex  without 
causing  paralysis  of  the  muscles.  The  animal  became  dull 
and  listless,  and  did  not  take  food  unless  it  was  given  to  it. 
It  showed  no  sign  of  recognising  persons  or  other  dogs,  and 
did  not  respond  in  the  usual  way  when  petted  or  spoken  to. 
But  it  snapped  when  pinched,  shut  its  eyes  and  turned  its 
head  away  from  a  bright  light,  and  shook  its  ears  at  a  loud 
sound.  It  did  not  sit  still,  but  walked  constantly  to  and 
fro  when  awake.  It  slept  very  heavily.  In  fact,  all  the 
responses  of  the  animal  might  be  classed  as  reflex  responses 
to  immediate  excitation. 

In  monkeys,  removal  of  the  cerebral  cortex  leads  to 
such  loss  of  the  so-called  voluntary  movements  that  all  other 
symptoms  are  masked.  But  recent  experiments  have  shown 
that,  after  removal  of  considerable  parts  of  the  cortex, 
recovery  of  functions  may  occur. 

2.  Cerebellar  Arc.  — If  the  part  of  brain  behind  the  cerebrum,^ 
including  tlie  cerebellum,  be  removed  from  the  frog,  all  power 
of  balancing  is  lost  and  the  animal  lies  on  its  back  or  belly  as  it 
may  have  been  placed.  But  when  the  toes  are  pinched  the  legs 
are  drawn  up  into  their  characteristic  attitude  of  flexion 
alongside  the  body  and  are  maintained  there,  while  the 
muscles  feel  firm. 

3.  Spinal  Arc — When  the  spinal  cord  is  destroyed  the 
muscles  are  flaccid  and  soft,  and  the  limbs  remain  in  any 
position  they  may  be  placed,  and  no  response  to  pinching 
occurs. 


82  VETERINARY   PHYSIOLOGY 

I.   THE  SPINAL  ARC. 

Reflex  Action. 

When  the  cerebrum  and  cerebellum  are  destroyed,  the 
frog  is  under  the  influence  of  the  spinal  arcs  alone,  and  it 
may  be  used  for  studying  the  action  of  these  arcs  uncom- 
plicated by  the  disturbing  effects  of  the  cerebral  or  cerebellar 
arcs. 

For  the  study  of  the  action  of  the  spinal  arcs  in  higher 
animals,  Sherrington  has  used  a  dog  with  the  spinal  cord 
cut  below  the  level  of  the  phrenic  nerves,  which  supply  the 
diaphragm — the  great  muscle  of  respiration,  so  that  the  dog 
can  continue  to  breathe. 

In  such  an  animal  it  is  possible  to  record  the  movements 
of  a  limb  as  a  whole  by  attaching  it  to  a  reducing  lever, 
i.e.  a  lever  in  which  the  power  is  applied  to  the  long  limb, 
so  that  the  extent  of  movement  is  reduced  on  the  record. 

The  action  of  any  muscle  or  muscles  may  be  recorded  by 
cutting  their  tendons  and  attaching  them  to  the  lever. 

In  the  horse,  as  a  result  of  injury  or  disease,  the  lower  part 
of  the  cord  may  be  cut  off  from  the  higher  arcs  and  its 
uncomplicated  action  may  be  studied. 

Beginning  with  the  frog  with  the  whole  brain  destroyed, 
it  is  found  that  if  one  of  the  toes  is  pinched  the  leg  is  drawn 
up.      This  is  one  of  the  simplest  examples  of  reflex  action. 

Reflex  Action  may  be  defined  as  the  response  of  effectors 
to  the  stimulation  of  receptors  ivithout  volition  being  involved. 

Such  reflex  actions  are  definite  and  pmyoseful.  This 
may  be  demonstrated  by  placing  a  small  bit  of  blotting- 
paper  dipped  in  acetic  acid  on  the  thigh  of  the  frog,  when 
a  series  of  movements  will  be  made,  manifestly  with  the 
object  of  removing  the  paper  {Practical  Physiology). 
These  movements  involve  the  most  perfectly  co-ordinated 
action  oi  a  large  series  of  muscles,  some  contracting,  some 
relaxing,  the  contraction  and  relaxation  alternating  in  an 
orderly  manner  between  the  groups  of  muscles. 

The  mechanism  involved  consists  of  the  receptors  stimu- 
lated,   the    ingoing  nerves    to    the    spinal     cord,     the    great 


NEEVE  83 

labyrinth  of  intercalated  neurons  in  the  cord,  the  outgoing 
neurons  and  the  muscles  upon  which  these  act. 

The  marvel  is,  that  in  the  labyrinth  of  neurons  in  the 
cord  any  sufficiently  definite  course  can  be  taken  to  secure 
the  perfectly  harmonious  and  co-ordinated  response  which 
occurs,  and  it  is  not  surprising  that  the  definite  passage  of 
the  impulse  may  be  readily  interfered  with.  Thus,  the 
administration  of  strychnine  leads  to  the  abolition  of 
definite  reaction  and  to  a  generalised  contraction  of  the 
muscles. 

Only  by  a  long  process  of  evolution  can  such  definite 
paths  have  been  marked  out  through  this  labyrinth,  and 
only  on  the  supposition  that,  if  once  a  certain  response  has 
followed  a  given  stimulus,  the  same  response  will  tend  to 
follow  it  again,  can  we  understand  how  these  definite  reflex 
actions  have  been  gradually  established.  The  reflexes  are 
then  inherited  reactions  and  in  the  process  of  evolution  only 
those  of  benefit  to  the  species  have  survived. 

Primarily  reflex  action  was  confined  to  the  segment  of 
the  body  in  which  it  was  originated,  but  gradually  the  reflex 
association  of  all  segments,  of  different  levels  of  the  cord, 
has  been  established. 

In  order  that  the  appropriate  response  should  follow  a 
given  stimulus,  the  structural  and  functional  integrity  of 
all  the  structures  involved  is  of  course  essential,  and  any 
modification  in  the  condition  of  any  part  may  seriously 
alter  the  result. 

Above  all,  the  condition  of  the  spinal  cord  itself  is  of 
importance. 

1.  The  impulse  in  passing  across  the  synapses  in  the 
cord  takes  time,  and  the  period  between  the  application  of 
the  stimulus  and  the  resulting  action  is  much  longer  than 
the  time  which  the  impulse  would  take  to  travel  up  the 
ingoing  nerve  and  down  the  outgoing  nerve  (p.  70).  This 
is  called  the  latent  period  of  reflex  action. 

The  duration  of  the  latent  period  varies  with  the  strength 
of  the  stimulus,  and  with  the  condition  of  the  spinal  cord. 
It  also  probably  varies  with  the  number  of  synapses  which 
have   to   be   crossed   in    the  cord,   and   hence   some  simple 


84  VETERINARY  PHYSIOLOGY 

reflexes,  such   as    the  knee-jerk    (p.    89),   have  very  .much 
shorter  latent  periods  than  more  complex  reflexes. 

2.  As  a  result  of  the  impulse  having  to  traverse  these 
various  synapses,  there  is  not  the  same  strict  correspondence 
between  the  strength  of  the  stimulus  and  the  resulting  action 
as  is  found  in  stimulating  single  sets  of  neurons  in  a 
nerve  (p.  68).  The  result  does  vary  with  the  strength  of 
the  stimulus — a  weak  stimulus  producing  a  feeble  contraction 
of  only  a  few  muscles,  and  a  stronger  stimulus  a  more 
vigorous  response  in  a  larger  number  of  muscles.  But  it 
also  varies  with  the  condition  of  the  spinal  cord,  after 
the  administration  of  strychnine  the  least  touch  produces 
a  powerfid  generalised  contraction,  while  if  the  cord  is  cooled 
a  much  feebler  response  is  obtained. 

3.  When  a  nerve  is  stimulated,  the  excitation  stops  as 
soon  as  the  stimulus  ceases.  But  in  reflex  action  there 
is  apt  to  be  a  continuance,  an  after-discharge,  for  some  time 
after  the  stimulus  is  removed.  This  is  well  seen  when  the 
toes  of  the  spinal  frog  are  pinched  ;  the  leg,  when  drawn  up, 
remains  in  position  for  a  considerable  time.  The  duration  of 
this  after-discharge  varies  with  the  strength  of  the  stimulus, 
and  with  the  condition  of  the  synapses  in  the  cord  {Practical 
Physiology). 

4.  The  rhythmic  repetition  of  a  subminimal  stimulus  is 
more  capable  of  liberating  a  reflex  action  than  it  is  of 
causing  a  stimulation  of  a  neuron.  Apparently  the  resist- 
ance to  the  passage  of  the  impulse  over  the  synapses 
is  decreased  by  repetition  ;  just  as  while  one  knock  at  a 
door  may  not  secure  its  opening,  a  series  of  knocks  may 
do  so.  , 

5.  While  a  nerve  conducts  impulses  in  both  directions,  a 
reflex  arc  allows  its  passage  across  the  synapse  in  one 
direction  only,  from  the  receiving  to  the  reacting  neuron. 
There  is,  as  it  were,  a  valve  action.  This  was  demonstrated 
by  using  the  electric  variation  which  accompanies  the  passage 
of  a  nerve  impulse  (p.  60).  If  the  ingoing  neuron  A 
(fig.  35)  and  the  outgoing  neuron  B  are  connected  with 
galvanometers,  when  A  is  stimulated,  an  electric  variation 
occurs  in  A   and  in  B.     But  if  B  is   stimulated  while   an 


NERVE  85 

electric  variation  occurs  in  B,  it  does  not  occur  in  A.  The 
impulse  has  failed  to  cross  the  synapse. 

6.  A  nerve  may  be  stimulated  again  and  again  at  very  short 
intervals   of  time,  the    refractory  period    bein^r    about    -^ — 

.  ^J.  O  6000 

sec.  Renex  arcs  manifest  much  more  prolonged  refractory 
periods  during  which  it  is  impossible  to  elicit  another 
response.  This  is  of  importance  because  a  reflex  act  takes 
a  very  appreciable  time  to  be  completed,  and  unless  it  is 
completed  before  it  is  again  started  confusion  of  movements 
would  be  produced. 

By  the  use  of  the  spinal  dog  (p.  82)  Sherrington  has 
been  able  to  show  how  it  is  that  definite  reflex  actions 
are  possible,  and  how,  in  spite  of  the  number  of  stimuli 
which  are  constantly  falling  on  the  body,  there  is  no  confusion, 
no  mixture  of  reflexes. 


Fig.  35.— To  show  the  method  of  demonstrating  the  Valve  Action 
in  reflexes  by  means  of  the  electrical  response. 

He  finds  that  the  response  varies  with  the  character  and 
locality  of  the  stimulus. 

Thus,  if  the  hind  foot  of  the  dog  be  pinched,  a  flexor 
■withdrawal  of  the  leg  occurs  varying  in  extent  with  the 
strength  of  the  stimulus.  If  the  stimulus  is  strong,  this 
may  be  accompanied  by  an  extension  of  the  opposite  leg — a 
crossed  extensor  thrust. 

If,  on  the  other  hand,  a  finger  is  thrust  into  the  pad  of 
the  dog's  hind  foot,  an  extensor  thrust,  such  as  occurs  in 
the  act  of  walking,  is  produced. 

If,  while  this  extensor  thrust  is  being  produced,  the  foot 
is  pinched — i.e.  if  a  harmful  stimulus  is  applied,  the 
extensor  reflex  is  checked  and  is  replaced  by  the  flexor  with- 
drawal reflex.  As  Sherrington  puts  it,  nocuous  stimuli  are 
prepotent 


86 


VETERINARY  PHYSIOLOGY 


(1)  In  the  example  given,  the  harmful  pinch  caused  the 
replacement  of  one  reflex  by  another.  But  (2)  occasionally 
the  application  of  a  second  stimulus  may  simply  cause  the 
disappearance  of  the  reflex  in  progress. 

(3)  A  third  eff'ect  may  be  produced.  The  second  stimulus, 
if  it  is  of  a  somewhat  similar  nature  to  the  first,  and  in 
proximity  to  it,  may  cause  a  simple  augmentation  of  the 
action. 


Fig.  36. To  show  the  play  of  different  ingoing  neurons  upon  a  motor  neuron 

FC  to  the  vasto-crureus  muscle  e  of  dog  in  producing  reflex  action. 
Stimulation  of  the  ear,  tail,  forefoot,  and  pressure  on  the  pad  of  the 
hind  foot  of  the  same  side,  all  cause  excitation,  as  also  do  stimula- 
tion of  the  shoulder  of  the  opposite  side  and  nocuous  stimuli  of  the 
opposite  hind  foot.  On  the  other  hand,  stimulation  of  the  slioulder 
of  the  same  side,  as  in  the  scratch  reflex  and  nocuous  stimuli  of  the 
hind  foot  of  the  same  side  inhibit  it.     (SherrinCxTON.  ) 


In  both  the  flexor  withdrawal  and  the  extensor  thrust 
the  same  muscles  of  the  thigh  are  used — the  extensors  and 
flexors  ;  but  in  flexor  withdrawal  the  extensors  are  inhibited, 
while  in  the  extensor  thrust  they  are  contracting.  The 
nerve  to  the  extensors,  therefore,  in  the  first  action  carries 
checking  impulses,  in  the  second  stimulating  impulses.  It 
is  the  common  path  for  both  kinds  of  outgoing  impulses. 

The  ingoing  neurons  from  the  body  are  about  five  times 
as  numerous  as  the  outgoing.       Each    set   may  be   looked 


NERVE 


87 


upon  as  the  private  path  of  the  special  reflex  it  produces. 
In  acting  upon  the  muscles  the  special  incoming  impulse 
has  to  get  command  of  the  common  outgoing  path. 

Fig.  36  shows  how  one  set  of  outgoing  neurons  is  played 
upon  by  a  whole  series  of  ingoing  neurons  from  difterent 
parts  of  the  body. 

When  any  muscle  is  made  to  contract,  its  antagonist 
is  inhibited  or  relaxed.  This  has  been  demonstrated  by 
taking  a  simultaneous  record  from  flexors  and  extensors. 

Under  the  influence  of  strychnine  the  inhibitory  effects 


Fig.  37. — To  show  the  way  in  which  the  different  Reflex  Arcs  react  on  one 
another.  S.,  skin;  M.,  muscle;  V.,  viscus ;  C,  spinal  cord  with 
synapses.     (M'Dougall.  ) 


may  be  converted  into  excitor  effects,  and  when  this  occurs 
havoc  is  played  with  the  co-ordination  of  the  reflex. 

A  study  of  the  familiar  "scratch  reflex"  which  can 
often  be  produced  by  rubbiug  the  shoulder  of  a  normal 
dog,  and  which  can  much  more  inevitably  be  produced 
in  the  spinal  animal,  has  demonstrated  that,  whatever 
be  the  character  of  the  stimulus  producing  it,  the  scratch- 
ing movements  occur  at  a  perfectly  definite  rate  of  four 
or  five  per  second,  and  that  the  movements  consist  of  a 
perfectly  rhythmical  alternation  of  contraction  and  relaxation 
of  the  flexor  and  extensor  muscles  by  which  the  leg  is  first 


88 


VETERINARY  PHYSIOLOGY 


drawn  up,  then  extended,  again  drawn  up,  and  so  on.  The 
cause  of  this  is  that  the  first  movement  of  muscles  and 
joints  leads  to  the  stimulation  of  nerves  from  the  muscles 
and  joints  to  the  spinal  cord  (fig.  37),  and  it  is  these  which 
bring  about  the  reversal  of  action.  This  leads  to  a  fresh  set 
of  impulses  from  the  muscles  and  joints  which  re-establishes 
the  first  action.  So  the  rhythm  is  maintained  for  a  consider- 
able time  after  the  stimulus  which  started  it  has  stopped. 

These  impulses  from  the  muscles  and  joints  do  not  always, 
as  in  the  scratch  reflex,  bring  about  a  reversal  effect.  In 
many  cases  they  tend  to  keep  up  the  position  produced,  and 


Fig.  38. 


-Reflex  Figures  struck  in  Decerebrated  Cat  on  stimulating  a  fore 
and  a  hind  paw.     (Sherrington.) 


hence  to  fix  a  particular  posture.  This  is  best  seen  in 
animals  which  have  had  their  cerebrum  removed  above  the 
tectum  (fig.  38). 

Postural  Reflexes. — By  putting  such  an  animal  in 
certain  postures,  sustained  reflex  movements  may  be  caused, 
or  they  may  be  inhibited. 

Magnus  has  shown  that,  in  decerebrated  cats,  the  position 
of  the  head  and  the  cervical  vertebras  modifies  the  tonus  of 
the  muscles  of  the  limbs,  and  so  determines  the  position 
taken  by  the  animal.  If  the  head  is  bent  forward,  as  when 
a  cat  looks  under  an  object  in  pursuit  of  its  prey,  the  tonus 
of  the  fore  limbs  decreases,  that  of  the  hind   limbs  increases, 


NERVE  89 

and  the  cat  sinks  down  on  its  forequarters.  This  is  due,  not 
merely  to  stimuli  coming  from  the  joints  of  the  neck,  but 
also  to  stimuli  coming  from  the  labyrinth  of  the  internal  ear 
(p.  121). 

In  ducks  it  has  been  found  that  by  placing  the  head  and 
neck  in  various  positions — e.g.  in  that  taken  up  in  diving — 
a  postural  reflex  inhibition  of  the  movements  of  breathing  is 
induced. 

Fatigue  of  the  Reflex  Arc. — Nerve  fibres  do  not  manifest 
fatigue  (p. 6 9),  but  reflex  arcs  readily  do  so,  apparently  through 
a  change  in  the  synapses.  These  synapses  are  also  much 
more  susceptible  to  the  influence  of  poisons — e.g.  deficiency 
of  oxygen  or  the  action  of  such  drugs  as  chloroform — than 
are  nerve  fibres.  As  a  reflex  response  decreases  from  fatigue 
it  becomes  more  and  more  easily  replaced  by  other  reflexes. 
But  a  very  brief  cessation  of  the  stimulus,  and,  still  more, 
the  brief  substitution  of  another  reflex,  rapidly  restores  the 
activity  of  the  action. 


Visceral  Reflexes. 

Reflex  actions  in  connection  with  various  visceral  muscles 
are  also  connected  with  the  spinal  cord.  Many  of  these  are 
complex  reflexes  involving  inhibition  of  certain  muscles  and 
increased  action  of  others,  some  visceral,  some  skeletal.  The 
best  marked  of  these  are  the  reflex  acts  of  micturition 
(p.  581),  defsecation  (p.  335),  erection,  and  ejaculation  (p.  623). 
The  lumbar  enlargement  is  the  part  of  the  cord  involved. 

As  has  been  shown  by  the  study  of  postural  reflexes,  the 
nervous  arcs  in  the  cord  exercise  a  constant  reflex  tonic 
action,  due  to  the  constant  inflow  of  incoming  impressions. 
When  this  tonic  action  is  interfered  with  by  any  condition 
which  interferes  with  the  integrity  of  the  reflex  arc,  the 
response  from  the  muscle  may  be  diminished.  This  is 
exemplified  in  the  contraction  of  the  quadriceps  extensor 
femoris  which  occurs  when  the  ligamentum  patellae  is  struck 
sharply,   causing  a   kick   at   the   knee  joint — the   knee  jerk 


90 


VETERINARY   PHYSIOLOGY 


(fig.  39).  When  the  reflex  arc  at  the  level  of  the  3rd  and 
4th  lumbar  nerves  is  interfered  with,  the  knee  jerk  is 
diminished  or  is  absent,  and  when  the  activity  of  the  arc  is 
increased,  by  the  removal  of  the  influence  of  the  cerebrum, 
the  jerk  is  increased.  The  fact  that  the  latent  period  is  very 
much  shorter  than  that  of  most  reflex  actions  has  been  used 
as  an  argument  against  the  reaction  being  a  true  reflex.  But 
the  reflex  arc  is  certainly  necessary  for  its  production,  and, 
in  spite  of  the  short  latent  period,  it  may  be  regarded  as  a 
true  reflex  from  the   quadriceps  extensor  muscle   involving 


.AAA'WAAAAAAAAAA/V^     T. 


Fig.  39. — The  Neuro-rouscular  Mechanism  concerned  in  the  Knee  Jerk,  and 
the  time  of  the  knee  jerk  (A.J.)  compared  with  the  time  of  a  reflex 
action  {A.R.). 


only  one  or  two  synapses  and  set  up  by  the  sudden  develop- 
ment of  tension  in  the  muscle  which  is  being  maintained  in 
a  state  of  tonus  (fig.  39). 

Tonus  is  most  manifest  in  muscles  concerned  with  the 
maintenance  of  posture,  and  it  might  be  called  a  postural 
reflex  response. 

When,  as  the  result  of  disease  or  injury,  a  part  of 
the  spinal  cord  has  been  severed  from  the  brain,  a  marked 
increase  in  the  spinal  reflexes  is  manifest.  This  is  particu- 
larly well  seen  when  the  downcoming  flbres  from  the 
cerebrum  are  interrupted  (p.  194).  In  this  case  the 
condition  is  similar  to  that  of  the  decerebrated  dog  or  cat, 


NERVE  91 

and  the  tonic  condition  of  the  muscles  is  increased  because 
the  impulses  from  the  vestibulo-cerebellar  arc  (p.  121), 
which  increase  the  tonic  action  of  the  spinal  arcs,  are 
no  longer  held  in  check  by  impulses  from  the  cere- 
brum. 

When  the  cerebellar  arc  is  thrown  out  of  action  one  of 
the  most  marked  features  is  a  loss  of  muscular  tone. 


Trophic  Influence  of  the  Cord. 

The  spinal  cord  presides  over  the  nutrition  of  the 
muscles,  which  are  directly  supplied  by  fibres  coming  from 
cells  in  the  cord.  When  disease  destroys  the  cells  of  the 
anterior  horn,  the  muscles  atrophy  and  degenerate. 

Injury  of  the  cells  connected  with  the  outgoing  visceral 
fibres  does  not  lead  to  the  same  atrophy  and  degeneration 
of  the  visceral  muscles  supplied.  Apparently  the  post- 
ganglionic neurons  survive  degeneration  of  the  pre-ganglionic 
fibres  (p.  76),  and  are  able  to  maintain  the  nutrition  of  the 
structures  supplied.  Even  after  the  post-ganglionic  fibres 
have  degenerated,  it  is  probable  that  the  terminal  nerve 
plexuses,  which  in  the  intestine  at  least  act  as  local  reflex 
centres,  survive  and  are  capable  of  presiding  over  the 
nutrition  of  the  tissue. 

Certain  facts  seem  to  indicate  that  the  inofoinff  fibres  with 
their  cells  in  the  ganglion  upon  the  posterior  root  are 
connected  with  the  nutrition  of  the  structures  from  which 
they  pass.  Thus  shingles — herpes  zoster,  an  outbreak  of 
vesicles  along  the  distribution  of  a  cutaneous  nerve — has 
been  found  to  be  associated  with  inflammatory  conditions  of 
the  ganglia  (p.  93). 

Relationship  of  Nerve  Cells  to  Muscles. 

The  Nissl's  degeneration  (p.  77)  of  special  groups  of 
cells  in  the  anterior  horn  of  grey  matter  after  amputation  of 
the  leg  at  different  levels  seems  to  indicate  that  the  various 
groups  of  cells  have  definite  connections  with  individual 
muscles  (see  fig.  40). 


92 


VETERINARY   PHYSIOLOGY 


Peripheral  Reflexes. 

There  is  evidence  that  reflex  action  occurs  in  groups  of 
neurons  outside  the  spinal  cord  in  connection  with  the 
viscera. 


External  rotators  of  hip 

•  Hamstrings. 

Calf  muscl 
of  foot. 

Intrinsic  muscles  of  foot. 


d  extrinsic  muscles 


Muscles  of  bladder  and  urethra. 
Extrinsic  muscles  of  foot. 
Intrinsic  muscles  of  foot. 

Levator  and  sphincter  ani. 


Fig,  40.  —The  Groups  of  Cells  in  the  Anterior  Horn  of  grey  matter  at  the 
level  of  the  "ind,  3rd,  and  4th  sacral  nerves,  as  determined  by  the 
degenerations  which  follow  amputation.     (From  Bruce.) 

1.  The  Myenteric  Plexus. — The  terminal  plexus  in  the 
wall  of  the  alimentary  canal  acts  reflexly  to  produce  the 
characteristic  movements  by  which  the  contents  are  churned 
up  and  driven  down  the  gut  (p.  332). 


Fig.  41. To  show  the  possible  Axon  Reflex  in  connection  with  the  bladder. 

I.M.G.,    the    inferior    mesenteric   ganglion  ;    H.N.,    the   hypogastric 
nerve  ;  B.,  the  bladder. 


NERVE  93 

2.  Bladder  Reflex- — In  connection  with  some  of  the 
collateral  ganglia  apparent  reflex  action  has  been  described. 
The  bladder  is  supplied  by  the  hypogastric  nerves  which 
come  from  the  inferior  mesenteric  ganglion.  The  pre- 
ganglionic fibres  to  this  are  in  the  splanchnic  nerves. 
When  the  splanchnics  are  stimulated  the  bladder  contracts. 
But  if  they  are  cut  above  the  ganglion,  stimulation  of  the 
central  end  of  one  cut  hypogastric  will  cause  contraction  of 
the  bladder  if  the  other  hypogastric  is  intact  (fig.  41). 

Langley  and  Anderson  argue  that  this  is  not  a  true 
reflex,  but  that  it  is  due  to  the  fact  that  a  nerve  will  conduct 
in  both  directions,  so  that,  if  the  hypogastric  nerve  is 
stinmlated,    an    impulse    is    carried     up    in    a    fibre    which 


Fig.  42. — To  show  possible  Axon  Reflex  in  fibres  of  posterior  root. 

bifurcates  in  the  ganglion,  one  branch  passing  through,  one 
forming  a  synapse,  and  that,  through  the  synapse,  post- 
ganglionic fibres  in  the  other  nerve  are  set  in  action  (fig.  41). 
They  have  called  this  an  axon  reflex. 

It  seems  just  as  possible  that  true  ingoing  fibres  give  off 
side  branches  in  the  ganglion  and  produce  the  reflex. 

The  possibility  of  axon  reflexes  occurring  in  visceral 
nerves  requires  further  investigation. 

3.  Posterior  Ganglion  Reflex. — Bayliss  has  found  that 
stimulation  of  the  peripheral  end  of  the  posterior  root  of  a 
spinal  nerve  causes  vaso-dilatation  in  the  hind  leg  of  the  dog, 
and  Bruce  has  shown  that  this  may  be  reflexly  excited  by 
applying  irritants  to  the  skin,  even  if  the  posterior  root  is 
cut  centrally  to  the  ganglion.     The  conclusion  has  been  drawn 


94  VETERINARY   PHYSIOLOGY 

that  bifurcating  fibres  pass  from  the  ganglion,  one  limb 
going  to  the  skin,  one  limb  to  the  blood-vessels,  and  that 
thus  a  reflex  occurs  in  one  axon  without  passing  through  a 
synapse  (fig.  42).  It  is,  however,  not  proved  that  synapses 
do  not  exist  in  the  ganglion  on  the  posterior  root. 


II.  THE  CEREBRAL  AND  CEREBELLAR  ARCS. 

In  reflex  action  involving  the  spinal  arcs  the  ingoing 
fibres  from  the  receptors  in  the  skin  and  subjacent  parts  of 
the  body  when  stimulated  produce  inevitable  results  vary- 
ing only  with  the  condition  of  the  spinal  neurons  and 
synapses. 

When  the  cerebral  and  cerebellar  arcs  are  brought  into 
play,  the  resulting  actions  are  much  more  complex,  and  the 
possibility  of  differences  in  the  resulting  action  is  enormously 
increased. 

The  object  of  the  development  of  these  higher  arcs  is 
simply  to  secure  a  more  appropriate  response  to  external 
conditions,  to  enable  the  different  external  conditions  acting 
through  the  various  receptors  (p.  52)  not  only  to  produce 
each  its  own  special  effect,  but  also  to  ensure  that  the  effects 
may  be  brought  together  so  that  the  resulting  action  may  be 
determined  by  their  combined  effect. 

In  studying  the  action  of  these  arcs  it  will  be  convenient 
to  take  the  ingoing  side  first  and  the  outgoing  side  later,  and 
in  the  study  of  the  cerebral  arc  to  intercalate  between 
these  a  consideration  of  the  relationship  of  brain  action  to 
consciousness  and  to  mental  activity. 


A.  INGOING    SIDE    OF    THE    ARCS. 

Special  parts  of  the  central  nervous  system  are  connected 
and  associated  with  the  peripheral  structures,  so  that  a 
definite  reaction  to  each  of  the  various  kinds  of  stimuli  may 
occur.  These  reactions  may  be  accompanied  by  changes  in 
consciousness — by  sensations ;  and  since  the  lower  animals 
have  not  the  power  of  communicating  the  character  of  these 
sensations  by  speech,  the  possibility  of  studying  them  is 
restricted. 


96  VETERINARY   PHYSIOLOGY 

If  the  special  part  of  the  nervous  system  with  which  any- 
set  of  receptors  is  connected  be  destro^'ed  or  become  discon- 
nected from  the  receptors,  stimulation  will  produce  no  effect. 
The  condition  is  that  of  a  telephone  with  the  transmitter 
intact  but  with  the  receiver  out  of  action. 

The  study  of  the  physiology  of  the  peripheral  receptors 
must  therefore  be  taken  up  along  with  that  of  those  parts 
of  the  central  nervous  system  with  which  they  are  connected. 

These,  when  thrown  into  action  through  the  receptors,  may 
(1)  produce  certain  inevitable  reactions,  as  has  been  shown 
by  the  study  of  reflex  action  (p.  82),  and  these  may  or  may 
not  (2)  be  accompanied  by  changes  in  the  consciousness 
known  as  sensations,  which  cannot  be  investigated  in  lower 
animals. 


Receptor  Arrangements. 

The  Receptors  may  conveniently  be  grouped  in  three 
classes. 

1.  Those  connected  with  the  viscera  and  stimulated  by 
the  activity  of  these  organs — the  Intero-ceptive  Receptors. 

2.  Those  connected  with  the  surface  and  stimulated 
by  changes  in  the  surroundings — the  Extero-ceptive  Receptors. 

Some  are  acted  upon  by  changes  at  the  surface  of  the 
body,  such  as  the  contact  of  gross  matter,  others  by  changes 
originating  at  a  distance,  e.g.  light  waves.  The  former  may 
be  called  non-distance  recej^tors — the  latter  distance  receptors. 

3.  A  set  of  receptors  called  into  play  by  movements  of  the 
body,  i.e.  by  the  action  of  the  body  itself,  and  hence  called 
Proprioceptive  Receptors. 

We  have  already  seen  that  each  special  effect  is  brought 
about  by  the  development  of  special  receptors,  each 
responding  chiefly  to  one  kind  of  external  change  (p.  52). 

It  is  not  the  case  that  each  reacts  to  one  kind  of  change 
only,  but  rather  that  it  reacts  specially  to  one  kind,  that  it 
is  tuned  to  one  kind  and  responds  much  less  readily  to  all 
others.  As  Sherrington  puts  it,  it  has  a  "  low  threshold  " 
for  one  kind  of  stimulus,  a  "  high  threshold  "  for  all  others. 
Thus,  the  eye  is  generally  stimulated  by  the  ethereal  vibra- 


NERVE  97 

tion  of  light,  but  it  may  be  stimulated  by  sudden  pressure 
or  by  electrical  changes. 

In  whatever  way  any  special  variety  of  receptor  is  stimu- 
lated, the  sensation  which  results  is  of  the  same  kind.  Thus 
the  retina  of  the  eye  may  be  stimulated  by  ethereal  vibra- 
tions of  light,  or  by  mechanical  pressure,  or  by  an  electric 
current,  but  however  stimulated,  a  sensation  which  we  call 
visual  is  produced.  This  fact  was  formulated  by  Johannes 
Miiller  as  the  doctrine  of  Specific  Nerve  Energy,  The  con- 
verse holds  good  that  the  same  kind  of  stimulus  applied  to 
different  kinds  of  receptors  produces  different  kinds  of 
sensation. 

The  sets  of  Receptors  developed  in  the  body  wall  and 
viscera  may  first  be  considered.  They  may  be  called  the 
Body  Receptors. 


I.  BODY    RECEPTOR    MECHANISMS. 

A.  General  Arrangement  and  Physiology. 
I.  Visceral  (Intero-ceptive). 

A.  Structure. — Throughout  the  internal  organs  are  various 
peripheral  terminations  of  ingoing  nerves,  some  of  the 
nature  of  simple  dendritic  expansions,  some  of  dendritic 
expansions  enclosed  in  definite  fibrous  capsules  (Pacinian 
corpuscles). 

B.  Physiology. — These  are  called  into  action  by  different 
kinds  of  stimulation,  nocuous  and  innocuous,  to  produce 
reflex  adjustments  of  the  bodily  mechanism  either  without  or 
with  the  involvement  of  consciousness — that  is,  either  with- 
out or  with  the  production  of  sensation. 

(a)  Central  Reflex  Adjustment. — When  food  is  taken 
into  the  stomach,  it  stimulates  the  ends  of  the  afferent 
nerves,  and  these  carry  the  impulse  up  to  the  central  nervous 
system  to  produce  a  reflex  dilatation  of  the  gastric  blood- 
vessels. 

(6)  Peripheral  Reflex  Adjustment. — Many  of  these  visceral 
reflexes  occur  apart  from  the  central  nervous  system  in 
peripheral  plexuses  (p.  92).      This  is  specially  well   seen  in 


98  VETERINARY   PHYSIOLOGY 

the  wall  of  the  alimentary  canal  where  the  co-ordinated 
reflex  of  peristalsis  (p.  333),  involving  co-ordinated  con- 
traction behind  the  contents  of  the  gut  and  inhibition  in 
front  of  the  contraction,  is  dominated  by  the  myenteric 
plexus.  The  peripheral  mechanism  is  controlled  by  impulses 
from  the  central  nervous  system  passing  by  excitor  and  by 
inhibitory  nerves  which  are  reflexly  called  into  play  when 
any  modification  of  the  ordinary  peristalsis  is  required. 
The  importance  of  the  local  mechanism  is  indicated  by  the 
small  number  of  nerve  fibres  which  pass  to  the  spinal  cord 
from  the  viscera. 

When  the  visceral  receptors  are  abnormally  stimulated, 
abnormal  reflex  responses  may  result.  Thus,  when  the 
stomach  and  certain  other  parts  of  the  visceral  tract  are 
irritated,  the  act  of  vomiting  may  be  produced. 

Normally  the  visceral  reflexes,  peripheral  and  central,  are 
carried  out  without  consciousness  being  involved.  But,  in 
abnormal,  conditions,  consciousness  may  be  implicated  and 
sensations  may  be  produced  which  direct  the  attention  to  the 
condition  of  the  viscera.  This  is  sometimes  called  common 
sensibility. 

In  the  stomach  and  intestine  these  sensations  are  generally 
the  result  of  stretching  of  the  muscular  coats.  This  may 
lead  to  a  feeling  of  distension  or  to  actual  pain.  Later,  the 
existence  of  a  separate  set  of  pain  receptors  and  jDain  nerves 
in  the  skin  will  be  considered.  In  the  viscera  there  is  no 
evidence  of  their  existence,  and  the  unpleasant  sensation 
characterised  as  painful  must  be  considered  as  due  to  over- 
stimulation of  nerves  which,  when  normall}^  stimulated,  give 
rise  to  no  sensation.  But  there  is  evidence  that  abnormal 
stimulation  of  visceral  nerves  may  give  rise  to  pain,  which 
is  referred  by  the  sutferer  to  the  distribution  of  the  corre- 
sponding somatic  nerve,  such  referred  pain  is  seen  in  heart 
disease  (p.  423).  The  stomach  and  intestines  seem  to  be 
destitute  of  receptors  capable  of  stimulation  by  touching, 
pricking,  or  by  changes  of  temperature.  Hence,  after  the 
abdominal  wall  is  divided,  abdominal  operations  may 
frequently  be  performed  without  a  general  anaesthetic. 

The    gullet    and    anal    canal    are    provided    with    more 


NERVE  99 

specialised  receptors,  resembling  in  some  respects  those 
of  the  skin. 

The  sensation  of  hunger  is  associated  with  rhythmic 
periodic  contraction  of  the  empty  stomach,  and  it  also  is  due 
to  stimulation  of  the  ingoing  nerves  in  the  muscular  wall  of 
the  viscus. 

The  sensation  of  thirst,  although  it  is  due  to  deficient 
water  throughout  the  body  generally,  is  experienced  in 
the  mouth  and  throat,  which  are  not  moistened  by 
the  free  secretion  of  saliva.  In  these  regions  alone 
•does  the  deficiency  of  water  stimulate  any  receptive 
mechanism. 

The  outside  of  the  body  is  richly  supplied  with  a  variety 
of  receptors,  each  one  of  which  has  a  low  threshold  of 
stimulation  for  one  particular  kind  of  stimulus  and  a  very 
high  threshold  for  all  other  kinds. 

Those  developed  in  connection  with  the  skin  are  generally 
stimulated  by  changes  in  the  immediate  vicinity,  while 
those  developed  in  the  head,  such  as  the  eye,  ear,  and 
■olfactory  receptors,  are  stimulated  by  changes  at  a 
•distance,  thus  warning  the  animal  of  conditions  it  is 
Approaching. 

II.  Cutaneous. 

A.  Structure. — The  cutaneous  receptors  are  of  three 
kinds  : — 

1.  Dendritic  terminations  of  nerves  between  the  cells  of 
the  deep  layer  of  epidermis. 

2.  Tactile  corpuscles,  consisting  of  a  naked  branching 
varicose  dendrites  enclosed  in  a  fibrous  capsule  (fig.  43). 

3.  A  plexus-like  arrangement  of  the  dendrites  of  nerve 
fibres  round  the  roots  of  the  hairs. 

B.  Physiology. — When  the  sole  of  the  foot  is  stroked, 
the  leg  is  involuntarily,  i.e.  reflexly,  drawn  up.  If  the  skin 
•of  the  foot  is  subjected  to  a  high  temperature,  it  is 
withdrawn.  These  are  examples  of  simple  responses  to 
stimulation  of  the  receptors  in  the  skin,  and  these  responses 
are  frequently  accompanied  by  sensations  of  a  definite  kind. 


100 


VETERINARY  PHYSIOLOGY 


Much  of  our  knowledge  of  the  mode  of  action  of  these 
cutaneous   mechanisms  has  been  gained  by  a  study  of  the 
rehition  of  the  changes  of  con- 
sciousness to  the  nature  of  the 
stimuli  producing  them. 

In  studying  reflex  action 
it  has  been  seen  that  the 
stimuli  acting  upon  the  skin 
may  be  divided  into  harmful, 
or  nocuous,  and  non-harmful, 
and  that  these  tend  to  produce 
different  reactions  and  are 
accompanied  by  different  kinds 
of  sensation — the  former  char- 
acterised as  unpleasant  or  pain- 
ful; the  latter  by  other  characters, 
such  as  contact,  warmth,  or  cold. 
The  mechanisms  for  tlie 
reception  and  transmission  of 
nocuous  or  painful  stimuli  and  of  ordinary  cutaneous  stimuli 
seem  to  be  independent. 


Fig.  43. — Simple  form  of  sensory 
nerve  termination.  In  the  tac- 
tile corpuscle  the  nerve  fibre 
coils  round  the  capsule  before 
entering.      (Dogiel.) 


1.  Reaction  to  Nocuous  Stimuli— Pain. 

By  exploring  the  surface  of  any  small  area  of  skin  with 
the  point  of  a  very  sharp  needle  it  will  be  found  that  prick- 
ing certain  parts  gives  rise  to  a  more  painful  sensation  than 
pricking  other  parts.  The  first  may  be  called  pain  spots. 
By  gentle  pressure  by  a  bristle  with  a  rounded  end  other 
spots  may  be  discovered,  which,  when  touched,  give  a 
sensation  of  contact.  These  are  the  touch  spots.  Pieces  of 
skin  cut  out  from  the  pain  spots  show  no  special  nerve  end- 
ings, but  pieces  cut  out  from  the  touch  spots  show  the 
presence  of  a  tactile  corpuscle  in  a  papilla  under  the 
epidermis. 

It  must  be  recognised  that  the  mechanism  of  the  pain 
spots  has  been  developed  to  lead  to  appropriate  responses  to 
nocuous  stimuli,  and  that  the  implication  of  consciousness 


NERVE  101 

In  fact  pain  is  purely  a  relative  term,  and  conditions 
which  in  one  animal  will  cause  pain  will  not  cause  it  in 
another,  while  stimuli  which  will  produce  what  are  called 
painful  sensations  when  the  nervous  system  is  debilitated 
may  give  rise  to  sensations  not  considered  as  painful  when 
the  nervous  system  is  normal. 

All  pain,  since  it  means  a  change  in  consciousness, 
is  metaphysical.  There  is  no  such  thing  as  "  physical  pain." 
The  fatigue  and  the  other  sequences  to  any  kind  of  pain  are 
frequently  cited  as  proofs  of  the  influence  of  the  mind  on 
the  body.  But  we  have  no  right  to  assume  that  they  are 
caused  by  the  "  pain  "  rather  than  by  the  physical  disturb- 
ances in  the  nervous  system  of  which  the  pain  is  an 
accompaniment. 

Separate  nerve  fibres  appear  to  carry  pain-producing 
impulses  up  the  spinal  cord,  and  in  some  diseases  the  sense 
of  touch  may  be  lost  without  loss  of  the  sense  of  pain,  and 
vice  versa  (p.  112). 

2.  Reactions  to  Contact. 
The  Tactile  Sense. 

The  tactile  sense  may  best  be  studied  under  three 
heads  : — 

1.  The  Power  of  Distinguishing  Differences  of  Pressure. — 
Variations  of  pressure  in  time  and  space  are  alone  distin- 
guished. We  live  under  an  atmospheric  pressure  of  760 
mm.  of  mercury,  but  this  gives  rise  to  no  sensation.  Any 
sudden  increase  or  diminution  of  pressure,  however,  leads  to 
a  marked  change  of  sensation,  but  a  slow  change  causes  a 
lesser  modification  of  consciousness.  When  a  finger  is  im- 
mersed in  mercury,  the  sensation  of  pressure  is  felt  as  a 
ring  at  the  surface  of  the  mercury,  where  the  greater  pres- 
sure of  the  mercury  joins  the  lesser  pressure  of  the  air 
(Practical  Physiology). 

The  acuteness  of  the  pressure  sense  varies  in  different 
parts  of  the  body,  being  greatest  where  the  nerve  terminations 
are  most  abundant,  as  in  the  lips  of  the  horse. 

The  acuteness  of  the  sense  is  influenced  by  the  condition 


102  VETERINARY  PHYSIOLOGY 

of  the  mechanisin,  peripheral  or  central,  (a)  Peripheral — 
When  the  skin  is  cold,  the  sense  is  much  less  acute  than 
when  it  is  warm.  (6)  Central — The  nerve  centres  are  readily 
fatigued,  and  the  power  of  appreciating  sensation  is  thus 
decreased. 

2.    The  Power  of  Localising  the  Place  of  Contact. — Where 
the  tactile  organs  are  abundant,  the  power  of  distinguishing 
accurately    the     point    touched 
l^^  i^^  k~^  V~\  k^  is    more    acute   than   in    places 

where  they  are  more  scat- 
tered. For  this  reason,  if  two 
contacts  are  made  at  the  same 
time,   they    may   be   very  close 


r^rKKh 


Fig.  44. — Relationship  of  Sensa-      together  in  the  former  situation, 

tion  to  Stimulus,  with  weak  and  ^^^    y         ^^^^j    ^^^^^    ^£   ^^^^ 

strong  stimuli.      Stimuli  repre-  -'  t      j 

sented  by  vertical  lines— the     may  be    localised    and    felt    as 
strength    being    indicated    by      (jigtinct  from  the  Other,  whereas 

their  height.    Sensations  repre-       •        ,       ,  •  •         xi 

sented  by  the  curves.  m  the  latter  Situation  they  may 

be  felt  as  a  single  contact. 

3.  The  Power  of  Distinguishing  Contacts  in  Time. — If  the 
finger  be  brought  against  a  toothed  wheel  rotated  slowly, 
the  contacts  of  the  individual  teeth  will  be  felt  separately. 
But,  if  the  wheel  is  made  to  rotate  more  and  more  rapidly, 
the  separate  sensations  are  no  longer  felt,  and  a  continuous 
sense  of  contact  is  experienced  {Practical  Physiology).  This 
indicates  that,  if  stimuli  follow  one  another  sufficiently  rapidly, 
the  sensations  produced  are  fused.  From  this  it  is  obvious 
that  the  sensation  lasts  longer  than  the  stimulus — the 
contact  (fig.  44). 

The  duration  of  the  sensation  depends  upon  the  degree 
of  stimulation  of  the  peripheral  tactile  organs. 

Probably  two  sets  of  receptors  are  involved  in  the  tactile 
sense. 

\st.  The  tactile  corpuscles,  which  are  stimulated  by  direct 
pressure  on  the  surface  of  the  skin.  It  is  these  which  form  the 
centres  of  the  touch  spots  already  mentioned.  They  are  best 
investigated  by  von  Frey's  bristles — bristles  of  varying  thick- 
ness fixed  in  suitable  handles  and  with  the  pressure  required 
to  bend  them  determined  by  pressing  on  a  spring  balance. 


NERVE  103 

2nd.  The  plexuses  round  the  roots  of  hairs,  and  possibly  in 
the  epidermis,  which  are  stimulated  when  a  small  piece  of  soft 
cotton- wool  is  drawn  over  the  skin.  This  might  be  called 
the  kinetic  stimulation  of  the  tactile  mechanism,  and  the 
other  the  static  stimulation. 


3.    Reactions  to  Changes  of  Temperature. 
Thermal  Sense. 

Heat,  like  light,  is  physically  a  form  of  vibration  of  the 
ether. 

1.  The  temperature  sense  depends  upon  the  fact,  that  when 
heat  is  withdrawn  from  the  body,  a  sensation  of  coolness  or 
cold,  and,  when  heat  is  added  to  the  body,  a  sensation  of 
warmth  or  heat  is  produced.  This  depends  upon  the  tempera- 
ture of  the  body  in  relation  to  the  surroundings,  and  not 
merely  on  the  temperature  of  the  surroundings.  If  three 
basins  of  water  are  taken,  one  very  hot,  one  very  cold,  and  one 
of  medium  temperature,  and  if  a  hand  be  pLaced,  one  in  the 
very  hot  and  one  in  the  very  cold  water  for  a  short  time, 
and  then  transferred  to  the  basin  with  water  at  a  medium 
temperature,  the  water  will  feel  hot  to  the  hand  that  has 
been  in  the  cold  water  and  cold  to  the  hand  that  has  been 
in  the  hot  water  (Practical  Physiology). 

2.  The  rate  at  which  heat  is  abstracted  or  added  is  the 
governing  factor  in  causing  the  sensation  ;  a  sudden  change 
of  temperature  stimulates  far  more  powerfully  than  a  slow 
change.  For  this  reason  the  thermal  conductivity  of  sub- 
stances in  contact  with  the  skin  has  an  influence  upon  the 
sensation.  If  a  piece  of  iron  and  a  piece  of  flannel  laid  side 
by  side  be  touched,  the  first  will  feel  cold,  the  second  will 
not.  This  is  because  the  former  has  high  thermal  con- 
ductivity, the  latter  has  not,  and  thus  the  former  abstracts 
heat  more  rapidly  than  the  latter.  When  the  skin  of  a  horse 
is  covered  by  sweat,  heat  is  rapidly  abstracted  and  shivering 
may  be  produced  (p.  2C7). 

3.  Certain  parts  of  the  skin  are  stimulated  by  the  with- 
drawal of  heat,  and  their  stimulation  is  accompanied  by 
sensations  of  cold,  while  others  are  stimulated  by  the  addition 


104  VETERINARY  PHYSIOLOGY 

of  heat  and  give  rise  to  a  sense  of  warmth.  This  may  be 
demonstrated  by  taking  the  cold  point  of  a  pencil  and  passing 
it  over  the  back  of  the  hand,  when  it  will  be  felt  as  cold  only 
at  certain  points  ;  such  points  have  been  called  cold  spots, 
while  similar  spots,  stimulated  by  the  addition  of  heat, 
are  called  hot  spots.  Such  cold  and  hot  spots  have 
been  excised  and  examined  microscopically,  but  no 
special  receptor  mechanisms  have  been  found.  In  all 
probability,  special  developments  of  the  intercellular  plexus  in 
the  epidermis  form  the  receptors  (Practical  Physiology). 

4.  The  temperature  sense  is  independent  of  the  tactile 
sense,  and  the  changes  set  up  travel  in  separate  fibres  in  the 
cord  (p.  112).  The  one  sense  may  be  lost  and  the  other 
retained. 

4.    Special  Cutaneous  Receptors. 

In  certain  regions  of  the  skin  special  receptor  organs 
exist,  stimulation  of  which  excites  reflexes  in  connection  with 
the  sexual  and  other  special  functions.  In  the  anal  canal 
tactile  and  thermal  receptors  exist,  but  distension  gives  rise 
to  a  special  sensation — the  desire  to  defsecate. 

Von  Frey's  observations  on  man  indicate  that  in  the  skin 
there  are  at  least  four  receptor  mechanisms,  stimulated  separ- 
ately by  pain,  by  touch,  by  the  withdrawal  of  heat,  and  by  the 
addition  of  heat.  He  finds  that  certain  parts  of  the 
surface  do  not  respond  to  all  three  kinds  of  stimuli,  but 
only  to  one  or  two.  Thus,  pain  alone  can  be  produced 
from  the  cornea,  tooth  pulp  and  dentine,  and  the  parietal 
pleura  ;  pain  and  temperature  sensations,  but  not  sense  of 
touch,  from  the  edge  of  the  cornea  and  conjunctiva  ;  and 
touch  and  temperature  sensations  alone  from  certain  patches 
inside   the  mouth. 

When  a  severed  nerve  begins  to  unite  and  regenerate  (p.  7  7), 
an  imperfect  return  of  all  these  types  of  sensation  occurs,  but 
only  after  complete  regeneration  are  the  senses  completely 
restored. 

In  addition  to  the  interoceptive  and  exteroceptive 
receptors  of  the  body,  other  receptors  are  found   throughout 


NERVE 


105 


the  muscles,  tendons,  and  joints  which  iire  stimulated  by 
movements  generally  originated  by  stimuli  from  without,  and 
which  we  have  already  seen  (p.  88),  may  have  an  important 
influence  on  the  course  of  the  movements  produced.  These 
are  one  kind  of  the  group  of  receptors  which  Sherrington  has 
called  Proprioceptors.  They  might  be  called  the  muscle- 
joint  receptors. 

III.  Muscle    and   Joint    (Proprioceptive). 

A  double  mechanism  is  involved — 1st.  A  mechanism 
stimulated  by  the  contraction  of  the  muscles  ;  and  2nd,  a 
mechanism  acted  on  by  movements  at  the  joints. 

A.  Structure — (1st)  Muscle  Spindles. — Among  the  fibres 
of  the  muscles  are  found  long  fusiform  structures  containing 
modified  muscle  fibres.  Into  each  spindle 
a  medullated  nerve  passes  and  breaks 
up  into  a  non-medullated  plexus  round 
tliese  fibres  (fig.  45).  That  these  nerves 
are  ingoing  and  not  ordinary  motor  fibres 
is  shown  by  the  fact  that  they  do  not 
degenerate  when  the  anterior  root  of  a 
spinal  nerve  which  carries  the  motor 
fibres  is  cut,  but  that  they  do  degenerate 
when  the  posterior  root  is  cut  outside  the 

ganghon.  Fig.   45.  —  Structure 

(2nd)    Organs    of     Golgi    are    small      ^^  "^^^s^^®  spindle 

fibrous  capsules  in  the  tendons  near  the 

muscle  fibres,  and   into    each    a    jnedul- 

lated   fibre    enters,    and,  losing  its  white 

sheath,    forms    a  plexus    of    fibrils   with 

varicosities  upon  them. 

(3rd)   Varicose  terminations  of  axons 

surrounded  by  fibrous   tissues  are  found 

in  the  synovial  membranes  and  round  joints. 

B.  Physiology. — Through  these  mechanisms  information 
is  transmitted  to  the  central  nervous  system  as  to  the  position 
and  movements  of  the  various  parts,  and  this  is  of  the 
utmost  importance  in  modifying  and  guiding  the  movements 
without  consciousness  being  necessarily  implicated  (p.  83). 


-only  one  fibre  re- 
presented, h,  cap- 
sular space ;  c,  cap- 
sule;  p,  motor 
termination;  T.S., 
sensory  termina- 
tion on  the  spindle 
fibre.  (From  Re- 
gaud  and  Fayre.) 


106 


VETERINARY   PHYSIOLOGY 


When  the  consciousness  is  affected,  vakiable  information 
as  to  the  conditions  of  the  surroundings  may  be  afforded  by 
this  sense  in  conjunction  with  the  sense  of  touch.  In 
estimating  the  weight  of  bodies,  these  sensations  are  much 
used.  The  body  to  be  weighed  is  taken  in  the  hand,  and  by 
determining  the  amount  of  muscular  contraction  required 
to  support  or  raise  it,  the  weight  is  judged.  The  shape  and 
size  of  objects  are  also  determined  by  this  sense  in  con- 
junction with  the  sense  of  touch.  In  the  dark,  the  distance 
of  objects  is  also  judged  by  estimating  the  extent  of  move- 
ment of  the  limb  necessary  to  touch  them. 

The  sensibility  to  deep  pressure  which  exists  after  section 
of  the  cutaneous  nerves  must  be  due  to  a  stimulation  of 
these  receptors. 

B.    Connection  of  Body  Receptors  with  the 
Central  Nervous  System. 

1.    Ingoing  Nerves. 

The  ingoing  nerves  from  these  various  receptors  pass 
towards  the  spinal  cord  in  regular  segmental  series  (fig.  46), 


Fig.  46. — Structure  of  a  Typical  Spinal  Nerve.  P.R.,  posterior  root  with 
ganglion;  A. JR.,  anterior  root;  S.I.,  ganglion  of  sj'mpathetic  chain; 
W.R.,  its  white  ramus;  G.B.,  its  grey  ramus;  V.N.,  visceral  nerve 
with  collateral  ganglion  ending  in  terminal  plexus;  S.X.,  somatic 
nerve. 

except  where  the  symmetry  is  interrupted  by  the  outgrowth 
of  a  limb.  There  the  segmental  arrangement  is  disturbed 
into  a  pre-axial  and  post-axial  arrangement. 


NERVE  107 

The  fibres  enter  the  spinal  cord  by  the  posterior  roots  of 
the  spinal  nerves  (fig.  46). 

(i.)  Section  of  a  series  of  posterior  roots  leads  («)  to  loss  of 
sensation  in  the  structures  from  which  the  fibres  come,  and 
(6)  to  a  loss  of  muscular  co-ordination,  as  a  result  of  cutting 
off  the  afferent  impulses  connected  with  the  muscle-joint 
mechanism  (p.  105),  and  (c)  to  loss  of  tone  in  these  muscles. 

As  a  result  of  this  section,  the  parts  of  the  fibres  cut  off 
from  the  cells  of  the  ganglia  on  the  posterior  root  die  and 
degenerate  (p.  76).  Therefore,  if  the  root  is  cut  between 
the  ganglion  and  the  cord,  the  degeneration  extends  inwards 
and  up  the  posterior  columns  of  the  cord,  and,  if  it  is  cut 
outside  the  ganglion,  the  degeneration  passes  outwards  to  the 
periphery. 

Stimulation  (a)  of  the  central  end  may  cause  reflex 
movements  and  pain  ;  (6)  of  the  peripheral  end  may,  in 
certain  situations,  cause  dilatation  of  blood-vessels  (p.  93). 

(ii.)  Section  of  the  anterior  root  causes  paralysis  of  the 
muscles  and  other  structures  supplied  by  the  outgoing 
fibres,  and  the  fibres  die  and  degenerate  (p.  76). 

Stimulation  of  the  peripheral  part  causes  contraction  of 
the  muscles  supplied. 

2.   Course  of  the  upgoing  Fibres  in  the  Spinal  Cord. 

In  the  spinal  cord  the  course  of  the  upgoing  fibres  has 
been  traced  by  experiment,  by  clinical  observation,  and  by 
pathological  investigation,  chiefly  in  man  and  in  the  ape. 

(1)  Section  of  Posterior  Roots. — As  already  pointed  out, 
when  the  posterior  roots  are  cut  reflex  action  is  abolished  and 
all  sensation  in  the  part  of  the  body  supplied  by  the  nerve 
likewise  disappears. 

(2)  Section  of  Spinal  Cord. — If  the  spinal  cord  is  cut  across, 
after  a  certain  period  of  spinal  shock,  reflex  action 
occurs  and  may  be  increased,  but  all  sensation  is  absent 
below  the  level  of  the  section. 

It  must  therefore  be  concluded  (1)  that  these  ingoing 
nerves   act  upon   the    effector  mechanism   in   the    cord    by 


108  VETERINARY   PHYSIOLOGY 

which  reflex  action  is  produced  (p.  82);  (2)  that  other  parts 
of  the  ingoing  nerves  run  up  the  spinal  cord  to  act  upon 
some  part  of  the  brain  the  activity  of  which  is  related 
to  those  changes  in  consciousness  which  we  call  sensa- 
tions. 

(o)  Hemisection  of  Spinal  Cord. — If  only  one  half  of  the 
spinal  cord  is  cut  across  all  sensation  of  changes  of  temperature, 
and  all  sensation  of  pain  are  lost  on  the  opposite  side  below 
the  level  of  section,  but  tactile  sense  is  partially  lost  for  some 
distance  below  on  the  same  side,  and  further  down  on  the 
opposite  side.  The  proprioceptive  sensations  are  lost  on  the 
same  side. 

The  fibres  connected  with  pain  and  with  heat  and  cold 


Fig.  47. — To  show  the  Ascending  Degeneration  in  the  Spinal  Cord.     1,  after 
section  of  the  posterior  roots.      2,  after  hemisection  of  the  cord. 

receptors  must  cross  the  middle  line  at  once  and  run  up  the 
opposite  side  of  the  cord,  while  those  connected  with  touch 
must  run  for  some  distance,  probably  in  part,  right  up 
the  same  side  of  the  cord.  Those  connected  with  the 
muscle-joint  sense  must  run  up  the  same  side  of  the 
cord.      (For  outgoing  fibres  see  p.  195.) 

(4)  Ascending  Degenerations  of  Cord. — Taking  advantage  of 
the  fact  that  nerve  fibres  when  separated  from  their  cells  die 
and  degenerate  (p.  76),  light  has  been  thrown  upon  the  course 
of  these  fibres  in  man  and  apes  by  studying  the  degeneration 
which  follows:  — 

1.  Section  of  a  series  of  posterior  roots  inside  the  ganglia. 

2.  Section  of  one  half  of  the  cord. 


NERVE 


109 


It  must  be  remembered  that  the  nerve  ribres  of  the 
spinal  cord  do  not  regenerate,  since  they  have  no  neuro- 
lemma! sheath. 

Fig.  47  shows  where  the  degenerated  fibres  are  found 
in  each  case. 

1.  Section  of  the  posterior  roots  leads  to  degeneration 
extending  up  in  the  posterior  columns. 


To  Ceroillum 


Fig.  48. — To  show  Redistribution  of  Impulses  in  the  Cord  and  the  general 
course  of  impulses  of  different  kinds. 

2,  Section  of  the  cord  leads  to  this,  and,  in  addition,  to 
a  zone  of  degeneration  round  the  lateral  and  anterior 
columns. 

The  conclusions  to  be  drawn  are — (1)  that  the  fibres  of 
the    posterior    columns    are    a    direct    continuation    of    the 


110 


VETERINARY   PHYSIOLOGY 


posterior  root  fibres  which  have  their  cells  in  the  ganglion 
upon  the  posterior  root. 

(2)  That  the  fibres  which  degenerate  only  after  section  of 
the  cord  must  have  their  cells  in  the  cord  itself,  i.e.  that 
the  incoming  fibres  of  the  posterior  roots  have  formed 
synapses  with  fresh  neurons  (fig.  48). 


B  — /.; 


B 

Fig.  49. — To  show  Redistribution  of  Impulses  in  the  Cord.  1  to  5,  incom- 
ing fibres  of  posterior  root  ;  ^-l,  tract  of  kina?sthetic  sense  and  touch 
for  some  distance  before  crossing  ;  B,  tract  of  unconscious  impulses 
for  muscular  co-ordination  and  tone;  0,  same  as  B,  but  on  opposite 
side  ;  D,  tract  of  impulses  of  pain,  heat  and  cold  ;  £J,  tract  of  impulses 
of  touch  and  pressure.     (Page  May.  ) 


The  degenerated  fibres  may  be  traced  across  the  an- 
terior white  commissure  to  the  opposite  side  of  the 
cord  from  cells  situated  in  the  posterior  horn  of  grey 
matter. 


NERVE  111 

The  ingoing  fibres  thus  enter  the  cord,  some  pass  straight 
up  the  posterior  columns,  some  enter  the  grey  matter  to 
form  synapses  with  fresh  neurons  (fig.  49)  from  which 
fibres  pass  up  on  the  lateral  and  anterior  aspects  of  the  cord. 

By  tracing  these  degenerated  fibres  upwards,  it  is  found 
that  they  end  as  follows  : — 

(a)  Fibres  of  the  Posterior  Columns. — These  end  in  the 
nuclei  of  the  postero-internal  and  postero-external  columns  in 
the  lower  part  of  the  medulla  oblongata,  where  they  form 
synapses  with  fresh  neurons.  From  these  nuclei  (1)  fibres 
cross  to  the  opposite  side  and  run  up  in  the  fillet  through 
the  medulla,  through  the  pons  Varolii,  through  the  mesial 
fillet,  till  they  reach  (a)  the  tectum,  where  they  form 
synapses  ;  (6)  the  thalamus  opticus,  where  synapses  are 
also  formed  ;  (2)  fibres  pass  to  the  cerebellum  of  the  opposite 
side. 

(6)  Fibres  in  the  Lateral  and  Anterior  Columns. — These  pass 
up  till  the  medulla  is  reached,  and  here  they  separate  into 
two  distinct  sets. 

i.  Those  going  to  the  Cerebellum. — These  are  the  more 
marginal  set  of  fibres  in  the  cord. 

(a)  Those  on  the  postero-lateral  aspect  enter  the  cere- 
bellum by  its  inferior  peduncle. 

(h)  Those  on  the  more  anterior  aspect  enter  by  the 
superior  peduncle  (p.  127). 

ii.  Those  going  to  the  Tectum  and  Cerebrum. — These  pass  up 
with  the  mesial  fillet,  and  (a)  on  reaching  the  tectum  some 
form  synapses  there,  (6)  some  run  on  and  end  in  the 
thalamus  opticus,  where  they  form  synapses  (fig. 
50,  p.  113). 

(5)  Pathological  conditions  and  injuries  of  the  cord  in  man  show 
that,  when  the  lesion  is  towards  the  anterior  margin,  tactile 
sensations  are  abolished,  and  if  it  is  more  in  the  anterior  part 
of  the  lateral  column  painful  and  thermal  sensations  are  lost. 
In  gunshot  injuries  to  the  cord  the  anterior  part  is  more  apt 
to  escape,  so  that  the  sensations  of  touch  may  persist 
while  painful   and    thermal   sensations  are  lost.      The   sense 


112  VETERINARY   PHYSIOLOGY 

of  tickling  is  lost,  not  with  tactile  sense,  but  with  the  sense 
of  pain. 

The  upgoing  fibres  in  the  lateral  columns  may  thus  be 
sorted  out  into,  five  sets  as  is  shown  in  fig.  49. 

They  may  be  called — 

1.  The  direct  cerebellar  tract,  B. 

2.  The  anterior  cerebellar  tract,  C. 

3.  The  lateral  spino-thalamic,  D. 

4.  The  spino-tectal. 

5.  The  anterior  spino-thalamic,  E. 

Injury  to  the  posterior  columns  causes  a  marked  loss  of 
the  muscle-joint  sense  and  of  tactile  sense  for  some  distance 
below  the  lesion. 

The  fibres  from  each  of  the  several  kinds  of  receptors  in 
the  body  which  run  side  by  side  in  the  nerves  are  thus 
sorted  out  ^^^y biologically  in  the  spinal  cord  by  passing 
through  synapses — (1)  Those  from  pain  receptors  being  sent 
up  a  definite  tract  in  the  lateral  column  (fig.  49).  (2) 
Those  from  temperature  receptors  passing  along  with  the  last. 
(3)  Those  from  tactile  receptors  passing  up  the  posterior 
columns  for  a  considerable  distance  before  forming  synapses 
and  after  crossing  the  middle  line  (fig.  48)  being  grouped 
at  the  anterior  margin  of  the  cord.  (4)  Those  from  the 
proprioceptive  receptors  of  muscles  and  joints  (a)  passing 
up  the  posterior  columns  to  the  top  of  the  cord  before  form- 
ing synapses  from  which  fibres  lead  up  the  brain-stem  and 
finally  to  the  cortex  cerebri ;  (6)  forming  synapses  in  the 
grey  matter  of  the  cord  in  Clark's  vesicular  column,  from 
which  fibres  run  up  in  the  two  cerebellar  tracts  to  the 
cerebellum  (p.  127). 


3.  Connections  with  the  Brain-Stem  and  Cerebrum. 

These  various  fibres  from  the  synapses  in  or  above  the 
cord  which  pass  up  the  brain-stem  end  by  synapses — 

A,  in  the  tectum  (T,  fig.  58), 

B,  in  the  thalamus  {Tli,  fig.  58), 


NERVE 


113 


while  from  the  thalamus  fresh  fibres  pass  on  to  the 
cortex  to  end  in  the  area  situated  round  the  central 
fissure. 

Each  of  these — tectum,  thalamus,  and  cortex — is  a 
shunting  station,  from  which  impulses  are  sent  down  the 
cord  to  act  upon  the  spinal  reflex  arcs. 

1.  Spino-tectal  Synapses. — In  the  tectum  these  incoming 
impulses  from  the  body  are  associated  with  incoming 
impulses  from  the  eye  and  ear  (fig.  50,  C.Q). 

If  the  brain-stem  be  cut  above  the  tectum,  as  it  is  in  the 


Fig.  50. — To  show  the  endings  in  the  thalamus  of  the  ingoing  fibres  from 
all  the  receptor  mechanisms.  S.,  ingoing  fibres  from  the  body 
generally  ;  VIII.,  ingoing  fibres  from  the  cochlea;  V.,  ingoing  fibres 
of  the  fifth  cranial  nerve  ;  //.,  ingoing  fibres  of  the  optic  nerve  ;  C.Q., 
tectum.     (Elliot  Smith.) 

process    of    decerebration,    the     tectal     synapses     and     the 
cerebellar  connections  are  left  intact. 

The  action  of  these,  uncontrolled  by  the  upper  cerebral 
synapses,  is  to  set  up  an  extensor  tone  of  the  muscles 
known  as  decerebration  rigidity.  In  man  the  disconnection 
of  the  cerebral  cortex  from  the  cord  by  the  interrup- 
tion of  the  pyramidal  tracts  brings  about  this  condi- 
tion with  a  characteristic  increase  in  the  spinal  reflexes 
(p.  82). 

2.   Spino-thalamic  Synapses. — It    was    from   the   thalamus 


114 


VETERINARY   PHYSIOLOGY 


and  other  basal  ganglia  that  the  cortex  cerebri  originally 
developed,  and  in  lower  vertebrates  the  separation  is  incom- 
plete except  as  regards  the  rhinencephalon,  which  originally 
developed  independently  as  the  centre  for  the  olfactory 
organs  (p.  133)  ;  (fig.  50,  V.,  S.,  VIIL). 


A^/  ._. 


Fig.  51. — To  show  the  mapping  out  of  the  Cerebral  Cortex  in  man  on  its 
outer  and  inner  aspects  into  areas  by  the  cliaracter  and  distribution  of 
the  cells,  and  fibres  to  show  the  convolution  round  the  central  fissure 
connected  with  the  reception  of  stimuli  from  the  body-receptors. 
(Campbell.) 


In  the  thalamus  the  ingoing  fibres  from  the  body 
receptors  are  brought  into  close  association  with  those  from 
the  distance  receptors  of  the  head,  the  eye,  and  the 
ear,  and  in   this  region  there  is  some  evidence  in  man  that 


NERVE 


115 


stimulation   may  be   associated   with    crude   modification  of 
consciousness. 

As  will  be  seen  later  there  is  good  evidence 
that  stimulation  of  the  thalamus  may  lead  to  muscular 
movements  without  implication  of  the  cortex  through  the 
corpus  striatum  and  red  nucleus  (p.  189). 

3.  Synapses  in  the  Cortex  Cerebri. — From  the  nuclei  of  the 
thalamus  fibres,  which  early  get  their  white  sheath,  extend 


-^^X 


ECTOSYLVIAN 
TOSYLVIAN   6  .. 


Parietal 

VISUAL 


S  OrVltiUs. 

SCruciatiLS. 
•SCorona-lis 
-Homologuc       '"^^'^'"- 
oj  RoliiuLo 

S  Literalis. 
S  fctoUteralis. 

Post-ca-Tcari-rve 


S  PosUitevAli; 


TUSfRCULUM   OLFACT. 


OLFACTORY.  A.. 


JLFACTOfiV  B 


rXTRA-RHINie. 


Fio.  52. — Superior  and  inferior  aspects  of  the  brain  of  the  dog  to  show  the  various 
sulci  and  the  distribution  of  the  chief  receiving  and  reacting  mechanisms. 

outwards  to  that  part  of  the  cortex  cerebri  which  lies 
round  the  central  fissure  (figs.  50  and  53).  That  this  part 
of  the  cortex  is  closely  associated  with  the  changes  in 
consciousness  has  been  proved  both  by  studying  the  effects  of 
(a)  stimulation,  and  of  (h)  removal,  and  (c)  by  careful  observa- 
tion of  the  symptoms  following  disease  or  injury. 

Mott  found  that,  when  the  cortex  round  the  central 
fissure  on  one  side  of  the  brain  of  a  monkey  is  removed, 
clips  may  be  attached  to  the  skin  on  the  opposite  side  of  the 
body  without  attracting  attention,  while  if  they  are  placed 
on  the  same  side  they  are  at  once  removed.  He  therefore 
regards  this  region  of  the  brain  as  connected  with  the 
reception  of  tactile  impressions. 

These  conclusions  have  been  supported  and  amplified  by  ex- 
perimental observations  during  operations  on  the  human  brain. 


IK 


VETERINARY    PHYSIOLOGY 


Harvey  Gushing  has  described  a  case  in  which,  for  rehef  of 
epileptic  attacks  beginning  in  the  right  hand,  the  brain  was 
exposed  and  stimulated.  When  certain  parts  of  the  post- 
central convolution  were  stimulated,  sensations,  not  painful 
in  character,  were  experienced  in  the  right  hand,  with  also 
a  vague  sensation  of  warmth. 

Horsley  has  described  a  case  in  which,  for  severe  spasmodic 
contraction  of  the  hand  and  arm,  he  removed  a  consider- 
able part  of  the  pre- central  convolution  from  the  brain 
of  a  boy.     This  was  followed   by  temporary  loss  of  tactile 


Fig.  53. — The  passage  of  fibres  from  the  nuclei  of  the  thalamus  to  the 
cortex  cerebri  of  a  primitive  mammal.  Th.Op.,  thalamus  ;  L.P.,  lobus 
pyriformis  ; /.r. ,  fissura  rhinica  ;  T///'",  auditory  area;  //'",  visual 
area  ;  H.,  hippocampus.     (Elliot  Smith.) 


and  thermal  sensibility  and  of  knowledge  of  the  position 
of  the  limb  of  the  opposite  side — stereognostic  sense 
(p.  106). 

The  evidence  thus  seems  conclusive  that  the  area  of  the 
cortex  round  the  central  fissure  is  receptive  for  what  may 
be  called  the  various  bodily  sensations  of  touch,  tem- 
perature, and  muscle-joint  sense.  But,  so  far,  no  indica- 
tion that  it  is  connected  with  painful  sensations  is  forth- 
coming. 

Very  probably,  as  is  indicated  by  the  observations 
of    Gushing,    in    man    and    in    apes    the    post-central    is 


NERVE  117 

mainly  receiving,  just  as  we  shall  afterwards  find  the 
pre-central  is  mainly  discharging.  The  arrangement  and 
character  of  the  cells  in  these  two  regions  are  distinctly 
different. 

This  area  must  be  a  sort  of  chart  of  the  opposite  side  of 
the  body  in  which  each  part  is  represented. 


4.  Connections  with  the  Cerebellum. 

The  ingoing  fibres  from  the  muscles  and  joints  have  been 
seen  to  play  an  important  part  in  guiding  ordinary  spinal 
reflex  action. 

It  has  also  been  shown  that  by  their  connection  with  the 
cerebral  cortex  consciousness  is  implicated  and  special  sen- 
sations produced  (p.  115). 

Through  their  connection  with  the  superior  vermis 
of  the  cerebellum  they  become  associated  with  incoming 
impressions  from  the  skin  and  from  special  receptor 
mechanism  in  the  head,  through  the  action  of  which  the 
position  and  movements  of  the  head  in  space  bring  about 
adjustments  of  the  balance  of  the  body. 

The  eyes  play  a  certain  part  in  such  adjustments,  but 
the  special  mechanism  developed  is  the  labyrinth  of  the 
internal  ear  for  which  the  cerebellum  has  developed  as  the 
great  receiving  and  reacting  centre.  The  mechanism  may 
be  studied  as  the  labyrintho-cerebellar  mechanism. 

Labyrintho-Cerebellar  Mechanism 
I.  Labyrinth. 

1.  structure. 

In  the  petrous  part  of  each  temporal  body  there  is 
a  somewhat  complex  space,  the  bony  labyrinth,  consist- 
ing of  three  separate  parts.  (1)  The  vestibule;  (2)  the 
cochlea  connected  with  hearing  which  will  be  considered 
later  ;  and  (8)  three  semi- circular  canals  opening  from  the 
superior  and  posterior  aspect  of  the  vestibule  (see  p.  170)  of 
the  internal  ear.      One  lies  in  the  horizontal  plane  and  has  a 


118 


VETERINARY  PHYSIOLOGY 


C 


a^ 


swelling  or  ampulla  anteriorly.  The  other  two  lie  in  vertical 
planes  each  placed  diagonally  to  the  mesial  plane  of  the  body  as 
indicated  in  fig.  54.  The  superior 
of  these  has  a  swelling  or  ampulla 
in  front;  the  posterior  has  an 
ampulla  behind. 

The  horizontal  canals  may  be 
considered  as  forming  the  arc  of  a 
circle  with  an  ampulla  at  each 
end.  The  superior  canal  of  one 
side  has  its  ampulla  in  front,  while 
its  twin — the  posterior  of  the 
opposite  side  —  has  its  ampulla 
behind,  and  they  together  form 
with     an     ampulla     at     each     end 


p.c 
Fig.  54.— The  Relationship  of 
the  Semicircular  Canals  to 
one  another,  /i.e.,  horizontal 
canal;  «.c.,  superior  canal; 
p.c,  posterior  canal. 


a     circle 


the     arc     of 

(fig.  54).  _ 

The  bony  labyrinth  contains  a  clear  lymph-like  fluid,  and 

in   this    a    membranous    labyrinth   lies.       

(a)  In  the  vestibule  are  two  small  vesicles, 
the  saccule  and  utricle,  connected  by  a 
narrow  duct. 

(6)  From  the  latter  of  these  the 
membranous  semicircular  canals  run 
into  the  bony  canals.  Each  of  them 
has  an  ampulla  which  almost  com- 
pletely fills  the  bony  ampulla  in 
which  it  lies,  while  the  part  lying 
in  the  bony  canal  is  very  narrow 
and  occupies  only  a  small  part 
(fig.  55). 

On     the      convex      aspect     of     each 
thickened    ridge    covered    with    columnar 
hair-like    processes,   and    among    these    cells    the   dendritic 
terminations     of     the     vestibular     nerves     run.       Similar 

the  utricle 
endolymph 
over  these 
of    lime — 


Fig.  55. — Bony  and 
membranous  canal 
and  ampulla  to 
illustrate  their 
mode  of  action. 


of     the      lumen 

ampulla     is     a 
cells    with    stiff 


patches     exist     on     the     inner     aspects     of 
and    of    the    saccule.        A    somewhat     viscous 
fills     the    membranous    labyrinth,    and     in    it 
swellings    lie    small    concretions     of    carbonate 
the  otoliths. 


NERVE  119 

2.  Connections  with  the  Central  Nervous  System. 

The  fibres  of  the  vestibular  root  take  origin  in  dendrites 
between  the  cells  of  the  macula?  in  the  ampullse  of  the 
semicircular  canals  and  of  the  saccule,  and  have  their  nerve 
cells  upon  then'  course  (fig.  56). 

They  enter  the  medulla  ventrally  to  the  auditory  nerve, 
along  with  which  they  are  described  as  the  eighth  cranial 
nerve. 

They  have  wide  and  important  central  connections  which 
may  be  divided  into  four  arcs  : — 

(1)  Labyrintho-Spinal  Arc. — As  the  fibres  enter  the  medulla 
they  divide  and  run  upwards  and  downwards.  Those  pass- 
ing down  form  synapses  witli  the  higher  spinal  arcs  which 
take  part  in  spinal  reflex  actions.  From  the  upward  and 
down-going  branches,  collaterals  enter  the  nucleus  of  Deiters 
lying  in  the  side  of  the  pons  Varolii  and  there  form  synapses. 
From  these  fresh  neurons  send  fibres  down  the  cord  on  the 
same  side  and  on  the  opposite  side  to  act  upon  the  spinal 
arcs. 

(2)  Labyrintho-Oculo-Motor  Arc. — This  is  essentially  a  part 
of  the  spinal  arc,  but  it  is  so  important  that  it  may  be  dealt 
with  separately.  The  ingoing  fibres  form  synapses  in  Deiters' 
nucleus,  from  which  fibres  pass  to  act  upon  the  oculo-motor 
nuclei,  and  thus  to  influence  the  movements  of  the  muscles 
of  the  eyes  (p.  161). 

(3)  Labyrintho-Cerebellar  Arc. — Some  of  the  upgoing  fibres 
pass  to  the  deep  nuclei  of  the  cerebellum,  from  which  fibres 
pass  down  the  spinal  cord  to  act  upon  the  spinal  reflex 
arcs, 

(4)  Labyrintho-Cerebral  Arc — Other  upgoing  fibres  pass  to 
the  cerebrum  forming  synapses  in  the  optic  thalamus,  from 
which  fibres  pass  on  to  the  cortex.  From  this,  fibres  extend 
down  the  cord  to  act  upon  the  spinal  reflex  arcs  and  upon 
the  oculo-motor  mechanism. 


120 


VETERINARY   PHYSIOLOGY 


1  Sl^rKcttpdvc  G.K. 


56. — Connections  of  Semicircular  Canals  M-ith  Central  Nervous  Svstem 
in  Four  Arcs.  Ves.R.,  Vestibular  root  of  eighth  nerve  sending  fibre 
upwards  to  the  cerebrum  and  cerebellum,  downwards  to  the  centre  in 
medulla  oblongata  (Afecl.)  and  to  Deiters'  nucleus  {X.Deit.),  from  whicli 
fibres  pass  to  the  oculo-motor  mechanism  (X.  17.)  and  to  the  centres 
in  the  anterior  horn  of  the  spinal  cord. 


NERVE  121 

3.  Physiology. 
A.     Lab  ijrintho- Spinal     and      Cerebellar      Arcs.  —  This 
mechanism  acts  in  two  ways  : — 

1st.  As  a  great  tonic  dominator  of  the  muscular  system. 

2nd.   As  a  great  proprioceptive  reflex   adjuster  of  move- 
ments. 

B.  Labyrintho- Cerebral  Arc. — This  acts  by  modifying 
consciousness. 

A.  Labyrintho-Spinal-Cerebellar. — 1.  Tonic  Action.— One 
effect  of  removal  of  the  labyrinth  on  one  side  is  to  cause 
a  loss  of  tone  in  the  muscles  of  the  same  side.  If 
the  labyrinths  on  both  sides  are  removed,  a  general  loss 
of  tone  occurs.  The  result  of  this  is  that  a  very  small 
force  is  capable  of  preventing  the  muscles  from  adjust- 
ing the  position  of  the  head.  In  a  pigeon  deprived  of 
its  labyrinths,  if  a  small  weight  is  attached  to  the  head  and 
the  head  is  bent  over  the  back,  it  remains  in  this  abnormal 
position.  It  is  the  cerebellar  arc  which  is  involved  in  the 
maintenance  of  tonus,  and  after  removal  of  the  deep  nuclei 
of  the  cerebellum  and  the  paracerebellar  nuclei  the  tonus  is 
lost.  The  cerebellar  arc  acts  upon  the  spinal  arcs  (a) 
through  its  connections  with  the  red  nucleus  (p.  127),  under 
the  tectum  ;  (6)  probably  directly  through  descending  fibres 
of  the  vestibulo-spinal  tract.  If  the  spinal  arcs  are  interfered 
with  by  section  of  a  series  of  posterior  roots,  the  influence  of 
the  labyrintho-cerebellar  arc  is  lost. 

The  importance  of  this  tonic  action  is  illustrated  by  the 
pugihst's  "knock-out"  blow  on  the  chin,  which  drives  the 
condyles  of  the  lower  jaw  against  the  petrous  part  of  the 
temporal  bone  which  contains  the  labyrinths,  and  throws 
them  out  of  action.  Instantly  there  is  a  complete  loss  of 
muscular  tone,  and  the  man  falls  in  a  heap  on  the  ground. 

The  cerebral  arc  is  not  involved.  In  fact,  an  increased 
tonus  appears  after  removal  of  the  cerebrum. 

2.  Proprioceptive  Reflex  Adjustment  of  Movements. — (1) 
This  action  of  the  labyrinth  is  shown  by  the  effects  of  injury 
and  removal.  When  one  of  the  canals  is  opened,  an 
operation    which     can    be     performed    in    the    pigeon    with 


122  VETERINARY   PHYSIOLOGY 

comparative  ease,  the  head  is  rotated  backwards  and  forwards 
in  the  plane  of  the  canal.  If  the  labyrinth  on  one  side  is 
removed,  rotation  to  that  side  from  overaction  of  the 
labyrinthine  mechanism  on  the  opposite  side  occurs.  It  is, 
however,  soon  recovered  from.  The  same  result  follows 
section  of  the  vestibular  nerve. 

(2)  Stimulation  of  one  of  the  canals  by  electricity  in  the 
cartilaginous  fishes  causes  movements  of  the  eyes  and  fins 
as  if  the  animal  were  being  rotated  in  the  plane  of  the  canal 
stimulated. 

Injury  to  a  canal  causes  loss  of  control  of  the  group  of 
muscles  which  govern  the  movements  of  the  head  round  the 
axis  of  that  canal.  Overaction,  on  the  other  hand,  causes 
muscular  adjustments  to  counteract  movements  round  the 
same  axis. 

Such  a  mechanism  might  be  called  into  play  either — 

(1)  By  the  position  of  the  head  in  space. 

(2)  By  the  movements  of  the  head. 

(1)  That  the  position  of  the  head  in  space  acts  is  in- 
dicated by  ((/,)  the  experiments  of  Kriedl  upon  the  shrimp 
Palinurus.  When  this  creature  casts  its  shell  it  also  casts 
its  otic  vesicle  with  the  contained  grains  of  sand  that  act  as 
otoliths,  and,  when  its  new  shell  is  grown,  it  inserts  particles 
of  sand  into  the  vesicles.  By  supplying  it  with  particles  of 
iron,  Kriedl  compelled  it  to  insert  these,  and  when,  by 
means  of  a  magnet,  they  were  brought  into  contact  with 
different  parts  of  the  vesicle,  the  animal  took  up  different 
positions. 

(h)  The  static  action  of  the  labyrinths  is  further  shown 
by  the  production  of  definite  postures  in  decerebrated  cats, 
by  placing  the  head  in  different  positions  (p.  88).  It  is  also 
shown  by  the  production  of  apnrea,  absence  of  breathing,  in 
ducks,  by  placing  the  head  in  certain  positions,  e.g.  directed 
straight  down  in  the  diving  position.  Another  element — a 
proprioceptive  reflex  from  the  joints  of  the  cervical  vertebree 
— plays  a  secondary  part  in  these  adjustments. 

These  static  actions  probably  depend  upon  the  part  of 
the  nerve  terminations  in    the    utricles    and    saccule    upon 


NERVE  123 

which  the  otoliths  press  at  any  time  as  a  result  of  the  force 
of  gravity. 

Such  a  static  mechanism  accounts  for  the  adjustment 
of  the  optic  axis  in  the  blind  according  to  the  position  of  the 
head. 

(2)  That  movements  of  the  head  play  an  important  part 
is  shoAvn  by  the  fact  that,  when  an  animal,  deprived  of  its 
labyrinths,  attempts  to  move,  the  most  marked  disturbances 
of  muscular  action  occur.  It  is  still  more  clearly  indicated 
by  a  consideration  of  how  the  system  of  semicircular  canals 
is  influenced  by  any  acceleration  or  retardation  of  move- 
ments of  the  head. 

If  the  head  is  moved  in  any  plane,  certain  changes  are 
set  up  in  the  ampulla  or  ampullfe  towards  which  the  head  is 
moving,  and  converse  changes  in  the  ampulla  or  ampullae  at 
the  other  end  of  the  arc  of  the  circle. 

If,  for  example,  the  head  is  suddenly  turned  to  the  right, 
the  inertia  of  the  endolymph  and  perilymph  tends  to  make 
them  lag  behind.  Thus  the  endolymph  in  the  ampulla  of 
the  left  horizontal  canal  will  tend  to  flow  into  the  canal, 
but  the  canal  is  so  small  that  the  fluid  will  merely  accumulate 
in  the  ampulla,  and  thus  a  high  pressure  will  be  produced 
(fig.  55++).  The  perilymph  will  tend  to  lag  behind, 
and  a  low  pressure  will  result  outside  (fig.  55  —  ).  The 
converse  will  take  place  in  the  opposite  horizontal  canal. 

When  the  movement  is  continued  the  pressures  will  be 
readjusted,  and,  on  stopping  the  movement,  the  opposite 
conditions  will  be  induced. 

In  forward  nodding  movement  of  the  head,  the  two 
superior  canals  have  the  pressure  of  endolymph  increased  in 
their  ampulise — in  backward  movement  this  occurs  in  the 
two  posterior  canals.  In  nodding  to  the  right  the  superior 
and  posterior  canals  of  the  right  ear  undergo  this  change. 

B.  Lahyrintho- Cerebral  Arcs. — Modifications  of  Conscious- 
ness.—These  changes,  which  can  be  studied  only  in  man,  act, 
not  only  in  bringing  about  an  adjustment  of  the  balance 
through  the  cerebellar  arc,  but  also  on  account  of 
the  connections  with  the  cerebrum,  in  modifying  con- 
sciousness,     giving     a     sensation     of     movement    in     the 


124 


VETERINARY  PHYSIOLOGY 


direction  of  the  auipullse  in  which  the  pressure  is 
increased.  The  mode  of  action  may  be  analysed  by  a 
study  of  the  sensations  which  accompany  acceleration  or 
retardation  of  movements.  Acceleration  produces  a  sensa- 
tion ;  when  the  motion  is  uniform  there  is  no  sensation, 
but  when  it  is  suddenly  stopped  a  sensation  of  rota- 
tion in  the  opposite  direction  is  produced.  The  fact  that 
only   fairly  rapid   acceleration  or  retardation  of  movements 


Fio.  57. — Diagram  of  the  Arrangement  of  Fibres  and  Cells  in  the  Cortex  of 
the  Cerebellum.  G.L.,  molecular  layer;  N.L.,  nuclear  laver ; 
P.,  Purkinje's  cells  sending  out  axons  to  the  deeper  ganglia.  (After 
Ramon  y  Cajal.) 


produces  an  effect  explains  why  it  is  that  the  action  of 
the  canals  may  fail  to  make  an  aviator  when  flying  in 
cloud  aware  of  his  position  in  space,  so  that  he  may  actually 
be  upside  down  without  knowing  it. 

The  information  conveyed  from  the  labyrinth  must 
concord  with  that  derived  from  other  channels,  e.g.  from 
vision.  When  they  do  not  accord,  a  sensation  of  giddiness 
and  an  inability  to  maintain  the  balance  are  induced.  If 
one  sets  a  poker  point  down  upon  the  ground,  then  places 
the  forehead  on  the  top  and  rapidly  circles  the  poker  three 
times,    when   one    stands    up    one    experiences    a   sense    of 


NERVE  125 

giddiness  and  an  inability  to  walk  steadily  This  is  due 
to  the  fact  that  the  sudden  stoppage  of  rotation  induces 
the  sensation  of  rotation  in  the  opposite  direction,  while 
the  visual  impressions  convey  the  information  that  this 
rotation  is  not  actually  taking  place. 

B.  The  Cerebellum. 

The  cerebellum  is  developed  primarily  as  the  ganglion 
or  receiving  mass  of  cells  for  the  vestibular  mechanism,  and  in 
it  stimuli  from  that  mechanism  are  associated  with  those 
from  other  receptors,  so  that  they  may  be  harmonised  and 
all  the  movements  of  balancing  adjusted  without  implication 
of  consciousness. 

1.  Structure. 

The  cerebellum  lies  above  the  fourth  ventricle,  and  is 
joined  to  the  cerebro-spinal  axis  by  three  peduncles  on 
each  side  (tig.  5  8).  It  consists  of  a  central  lobe,  the  upper  part 
of  which  is  the  superior  vermis,  and  two  lateral  lobes,  each  with 
a  secondary  small  lobe,  the  flocculus.  Its  surface  is  raised 
into  long  ridge-like  folds  running  in  the  horizontal  plane, 
and  is  covered  over  with  grey  matter,  the  cortex. 

In  the  substance  of  the  white  matter,  forming  the  centre 
of  the  organ,  are  several  masses  of  grey  matter  on  each  side, 
the  most  important  of  which  are — (i.)  the  group  of  roof 
nuclei;  and  (ii.)  the  dentate  nucleus  (fig.  58). 

The  cortex  may  be  divided  into  an  outer  somewhat  homo- 
geneous layer  (the  molecular  layer,  fig.  57,  G.L.)  and  an 
inner  layer  studded  with  cells  (the  nuclear  layer,  N.L.). 
Between  these  is  a  layer  of  large  cells  —  the  cells  of 
Purkinje  (P.). 

By  Golgi's  method  the  arrangement  of  fibres  and 
cells  in  the  cerebellar  cortex  has  been  shown  to  be  as 
follows  : — 

(i.)  Fibres  coming  into  the  cortex  from  the  white  matter 
either  end  in  synapses  round  cells  in  the  nuclear  layer,  or 
proceed  at  once  to  the  outer  layer  (fig.  57).  (ii.)  From  the 
cells  in  the  nuclear  layer,  processes  pass  to  the  outer  layer 


126  VETERINARY    PHYSIOLOGY 

and  there  form  synapses  with  other  cells,  (iii.)  From  these, 
processes  pass  to  the  cells  of  Purkinje,  round  which  they 
arborise  ;  and  (iv.)  from  Purkinje's  cells  the  outgoing  fibres 
of  the  cerebellum  pass  into  the  white  matter  and  so  to  the 
deep  nuclei,  to  Deiters'  nuclei,  and  to  the  red  nuclei  (fig.  58). 

2.  Connections. 

The  cerebellum  is  connected  (fig.  58) — 

1.  With  the  Spinal  Cord. 

(a)  hi  coining  Fibres. — 1.  The  direct  cerebellar  tract 
(p.  112)  passes  up  in  the  restiform  body  to  end  chiefly  in  the 
superior  vermis  on  both  sides.  2.  The  ventro-spinal  tract 
(p.  112)  passes  to  the  cerebellum  in  the  superior  peduncle 
and  ends  in  the  superior  vermis.  3.  Fibres  from  the  nuclei 
of  the  posterior  columns  of  the  same  side,  and  also  from  the 
opposite  side  (p.  112),  pass  in  the  restiform  body  to  the 
cerebellum.  4.  Fibres  from  the  vestibular  root  of  the  eighth 
nerve  also  pass  to  the  cerebellum  (fig.  58,  p.  127). 

(6)  Commissural  Fibres. — Strong  bands  of  fibres  connect 
the  dentate  nucleus  and  other  parts  of  the  cerebellum  with 
the  inferior  olivary  nucleus  of  the  opposite  side. 

(c)  Outgoing  Fibres. — Fibres  pass  from  the  superior 
vermis  to  the  deep  nuclei,  and  to  Deiters'  nuclei  (fig.  58), 
from  which  others  pass  down  in  the  descending  antero-lateral 
tract  of  the  cord. 

2.  With  the  Cerebrum. — 1.  Fibres  run  down  in  the  crusta 
of  each  crus  from  the  frontal  and  occipital  parts  of  the  cortex 
cerebri.  They  form  synapses  with  cells  in  the  pons,  and  from 
these,  fibres  pass  in  the  middle  peduncle  to  the  cerebellum, 
2.  The  fibres  of  the  superior  peduncle,  coming  chiefly  from 
the  dentate  nucleus  and  superior  vermis,  cross  in  the  middle 
line  and  end — (a)  partly  in  the  red  nucleus  of  the  opposite 
side,  from  which  the  rubro-spinal,  or  pre-pyramidal  tract, 
extends  downward  into  the  spinal  cord  ;  (b)  partly  in  the 
thalamus  opticus.  From  the  thalamus  fibres  pass,  some  to 
the  cortex,  some  to  the  oculo-motor  nuclei. 

Functionally  these  connections  are  : — 

A.   Ingoing. — 1.   From  the  spinal  cord— chiefly  proprio- 


NERVE  127 

ceptive,  but  also  tactile.     2.  From  the  labyrinth  of  the  internal 

ear proprioceptive.      3.  From  the  cerebrum — chiefly  visual. 

B.  Outgoing. — 1.  To  the  spinal  cord  to  act  on  the  reflex 
arcs.  2.  To  the  oculo- motor  mechanism.  3.  To  the 
cerebrum,  through   the  thalamus  and   red  nucleus.      These 

CO. 


i 


Fig.  58.— The  more  important  connections  of  the  Cerebellum.  Sk,  skin  ;  M,  muscle  ; 
F,  vermis;  D.N.,  Deiters'  nucleus;  L,  labyrinth;  P,  pons;  T,  tectum; 
R.N.,  red  nucleus  ;  Th,  thalamus  ;  E,  eye  ;  C.C.,  cortex  cerebri  ;  E,  eye. 

influence    the    spinal    arcs    through    the    rubro- spinal    and 
through  the  cortico-spinal  arcs  (p.  79). 

The  cerebellum  thus  constitutes  the  central  part  of  one  of 
the  great  nervous  arcs  (p.  79). 

3.   Physiology. 
1.   Removal  of  Cerebrum. — Something  may   be  learned  of 
the  functions  of  the  cerebellum  by  a  study  of  animals  from 


12S  VETERINARY    PHYSIOLOGY 

which  the  cerebrum  above  the  tectum  has  been  removed. 
Decerebration  rigidity  in  the  position  of  extension  manifests 
itself.  This  rigidity  is  removed  on  one  side,  if  that  side  of 
the  spinal  cord  in  the  neck  be  divided,  showing  that  it  is  a 
cerebellar  action.  The  hypertonus  is  not  removed  on  slicing 
away  the  cerebellum  until  the  basal  nuclei  and  nucleus  of 
Deiters  are  destroyed.  Apparently  the  cerebellum  is  con- 
trolled by  the  cerebrum,  and  acts  to  excess  after  its  removal, 
either  (a)  directly  upon  the  spinal  arcs,  or  (6)  indirectly 
through  its  superior  peduncles  and  the  red  nucleus. 

While  absinthe  leads  to  clonic  spasms  in  the  intact 
animal,  when  one  cerebral  hemisphere  is  removed  the  clonus 
is  re})laced  by  tonus  on  the  opposite  side,  the  absinthe  now 
stimulating  the  cerebellum  alone. 

2.  Removal  of  the  Cerebellum. — This  operation  is  easily 
performed  in  the  pigeon  (fig.  34,  p.  80),  and  the  animal, 
for  a  time  at  least,  loses  the  power  of  balancing  itself,  and, 
when  disturbed,  makes  violent  movements  to  recover  its 
equilibrium. 

In  the  dog  three  stages  are  seen — 

(1)  Irritative  Stage. — Immediately  after  removal,  there 
is  marked  extension  of  the  spine — opisthotonus — extension 
of  the  fore  limbs,  and  alternating  clonus  of  the  hind  limbs. 
In  the  monkey  the  same  symptoms  appear,  but  there  is  tonic 
flexion  at  the  elbows. 

(2)  Stoige  of  Inadequacy. — These  symptoms  gradually 
pass  off,  and  the  animal  then  shows  general  muscular 
weakness. 

(3)  Stage  of  Compensation. — Later  still,  this  condition  may 
in  large  measure  be  recovered  from. 

3.  Partial  Removal. — If  one  side  of  the  cerebellum  be 
removed  the  symptoms  are — (1)  At  once  a  tonic  contraction 
of  the  muscles  of  the  limbs  of  the  same  side  by  which  the 
fore  limb  in  the  dog  may  be  powerfully  extended.  The  head 
is  twisted  with  the  ear  to  the  shoulder  of  the  side  of  the 
removal,  and  the  chin  to  the  opposite  shoulder,  and  the 
animal  may  be  driven  round  its  long  axis  to  the  opposite 
side.      The  eyes  show  a  coarse  lateral  nystagmus — a  jerking 


NERVE  129 

from  side  to  side.  (2)  These  irritative  symptoms  soon  pass 
off,  and  the  animal  then  manifests  inadequacy  or  weakness 
in  the  Hmbs  of  the  affected  side,  so  that  it  droops  to  that  side, 
and,  if  a  quadruped,  may  circle  to  that  side.  (3)  After  some 
weeks  these  symptoms  disappear,  compensation  for  the  loss 
of  one  side  of  the  cerebellum  being  established. 

When  compensation  is  completed  in  the  dog,  destruc- 
tion of  the  cerebral  cortex  of  the  opposite  side  leads  to  a 
reappearance  of  the  muscular  inadequacy. 

In  some  cases  in  man,  when  slowly  progressing  disease 
has  destroyed  the  organ,  no  loss  of  equiUbration  appears. 
In  other  cases  the  cerebellum  has  been  congenitally 
almost  absent,  and  yet  the  individual  has  not  shown  any 
sign  of  want  of  power  of  maintaining  his  balance.  Evidently, 
therefore,  in  such  cases  the  cerebrum  compensates  for  the 
absence  of  the  cerebellum. 

4.  Stimulation  of  the  cerebellum  has  yielded  results  some- 
what difficult  of  interpretation,  but  the  most  recent  investi- 
gations seem  to  show  that  stimulation  of  the  cortex  with 
currents  strong  enough  to  produce  movements  when  applied 
to  the  discharging  part  of  the  cerebral  cortex  in  the  monkey 
(see  p.  192),  does  not  produce  manifest  effects.  On  the 
other  hand,  comparatively  weak  currents  applied  to  the 
basal  nuclei  do  produce  movements,  the  most  manifest  of 
which  are  conjugate  movements  of  the  eyes,  and  of  the 
eyes  and  head  to  the  side  stimulated.  If  the  nucleus  of 
Deiters  is  stimulated,  the  movements  are  rather  in  the 
muscles  of  the  limbs  of  the  same  side. 

It  has  been  further  found  that  powerful  stimulation  may 
also  cause  flexion  of  the  elbow  of  the  same  side  and 
extension  of  the  opposite  elbow  with  extension  of  the  trunk 
and  lower  limbs.  This  may  be  associated  with  the  main- 
tenance of  the  body  in  the  erect  position  and  with  the 
alternate  movements  of  the  legs  in  the  act  of  progression. 

Although  stimulation  of  the  cerebellar  cortex  has  failed 
to  reveal  any  localisation  of  function,  a  study  of  the  relative 
development  of  different  parts  in  different  groups  of  animals, 
and  a  study  of  the  effects  of  local  removal,  seem  to  indicate 
that  the  median  part  is  connected  with  the  movements  of 
9 


130  VETERINARY   PHYSIOLOGY 

paired  muscles — the  anterior  part  with  the  movements  of 
the  eyes,  tongue,  and  muscles  of  the  face  ;  the  middle  part 
with  the  muscles  of  the  neck  ;  and  the  posterior  part  with 
the  synergic  movements  of  the  hind  limbs  in  walking,  etc. 

The  lateral  lobes  seem  to  be  connected  with  the  inde- 
pendent movements  of  the  limbs  of  the  same  side.  These 
various  parts  are  concerned,  not  with  special  anatomical 
groups  of  muscles,  but  with  definite  co-ordinated  movements. 
Removal  of  a  part  concerned  with  the  movements  of  a  limb 
in  one  direction  is  accompanied  by  a  spontaneous  deviation 
in  the  opposite  direction. 

4.   General  Conclusions — 

1.  The  cerebellum,  through  the  influence  of  the  in- 
coming fibres  from  the  labyrinths  and  probably  also  trom 
the  spinal  proprioceptive  mechanism  keeps  up  a  con- 
stant tonic  influence  on  certain  of  the  musclefi  of  the 
body. 

2.  It  is  a  great  central  mechanism  of  reflex  adjust- 
ment of  the  balance  when  disturbed  in  any  way.  In  per- 
forming this  function,  its  action  is  guided  by  incoming 
impressions  (1)  from  the  labyrinths  (p.  121)  ;  (2)  from  the 
spinal  proprioceptive  mechanism  (p.  105)  ;  (3)  from  visual 
impressions  received  through  the  cerebrum  (p.  165)  ;  and 
(4)  from  tactile  impressions.  It  acts  upon  the  spinal 
reflex  arcs  to  modify  and  regulate  their  action  upon  the 
muscles. 

It  may  thus  be  described  as  a  ganglion  superimposed 
upon  the  brain-stem  which  associates  incoming  impulses  to 
secure  tonic  maintenance  of  the  balance  and  appropriate 
readjustment  of  the  balance  without  consciousness  being 
involved. 


III.  DISTANCE    RECEPTORS    OF    THE    HEAD. 

We  have  now  studied — 

1.  The  ordinary  reflex  response  through  the  spinal  arcs 
to  stimulation  of  receptors  (p.  82). 

2.  The  way  in  which  this  is  modified  by  the  stimulation 
of  muscle-joint  receptors  (p.  105). 


NERVE  131 

3.  The  inward  course  of  the  fibres  from  the  receptors  of 
the  body  to  their  termination  in  different  parts  of  the  brain 
(p.  106  et  seq.). 

4.  The  mode  of  action  of  these  various  parts  of  the 
brain  (p.  116  et  seq.). 

5.  The  adjustment  of  the  head  and  body  brought  about 
through  the  vestibulo-cerebellar  arrangement  (p.  121). 

We  must  next  consider  a  series  of  receptors  developed  in 
the  head  which  are  stimulated  by  changes  at  a  distance,  and 
which  thus  warn  tlie  animal,  so  that  it  may  prepare  to  adapt 
itself  to  these  conditions. 


I.    For  Chemical  Substance. 

1.    BUCCAL   MECHANISM. 

Taste. 

This  is  really  a  modification  of  cutaneous  sensibility,  and 
it  might  have  been  studied  along  with  it ;  but  its  close 
association  with  the  sense  of  smell  makes  it  more  convenient 
to  deal  with  it  here. 

The  mouth  is  richly  supplied  with  receptors  for  tactile, 
painful  and  thermal  stimuli,  and  the  receptors  for  thermal 
stimuli  extend  dowm  to  the  lower  end  of  the  gullet. 

In  the  mouth  special  receptors  have  been  developed  with 
the  object  of  determining  whether  any  particular  material 
should  be  swallowed  or  rejected,  according  to  whether  it  is 
beneficial  or  nocuous.  Pavlov  found  that,  in  dogs,  the  flow  of 
saliva  varies  with  the  material  put  in  the  mouth.  Flesh 
calls  forth  a  flow  of  viscous  saliva  which  lubricates  the 
mass  and  facilitates  the  act  of  swallowing,  while  sand  placed 
in  the  mouth  causes  a  free  flow  of  very  watery  saliva  to 
wash  out  the  nocuous  substance.  Pavlov  used  the  reflex 
flow  of  saliva  to  study  what  he  has  called  Conditioned  Reflexes 
— i.e.  reflexes  associated  with  changes  in  consciousness. 
Two  different  notes  were  sounded  near  a  dog,  and  it  was 
habitually  fed  after  one  of  these  but  not  after  the  other. 
It  was  found  that  sounding  the  former  produced  a  flow  of 
saliva. 


132  VETERINARY   PHYSIOLOGY 

A.  Receptors. — The  most  important  receptors  consist  of 
groups  of  spindle-shaped  cells  with  which  the  dendritic  ter- 
minations of  the  nerves  from  the  mouth  are  connected. 
Each  group  of  cells  is  surrounded  by  a  series  of  flat  epithelial 
cells  like  the  staves  of  a  barrel  to  form  a  taste  bulb.  These 
taste  bulbs  are  most  abundant  on  the  sides  of  the  large 
circumvallate  papillse  which  form  the  prominent  V-shaped 
line  on  the  posterior  part  of  the  dorsum  of  the 
tongue. 

B.  Connection  with  the  Central  Nervous  System. — The 
posterior  third  of  the  tongue  is  supplied  by  the  glosso- 
pharyngeal nerve.  The  anterior  two-thirds  are  supplied  by 
the  lingual  of  the  fifth  and  the  chorda  tympani  of  the 
seventh.  It  has  been  maintained  that  all  the  taste 
fibres  enter  the  medulla  by  way  of  the  Gasserian  ganglion 
and  the  root  of  the  fifth  nerve  ;  but  the  study  of  cases  in  man 
in  which  the  ganglion  has  been  removed  does  not  support  this 
view,  and  the  evidence  seems  to  indicate  that  the  fibres 
enter  the  medulla  by  the  roots  of  the  nerve  in  which  they 
run. 

The  first  synapses  are  in  the  medulla  and  pons,  and  from 
this  the  lemniscus  quinti — the  fillet  of  the  V. — passes  up, 
crosses,  and  forms  synapses  in  a  special  nucleus  of  the 
thalamus  (fig.  50),  from  which  fibres  must  pass  to  the 
cortex,  but  to  what  part  is  not  clearly  determined. 

The  close  association  of  taste  and  smell,  and  the  results 
of  Ferrier's  experiments  (p.  136),  make  it  possible  that  the 
cortical  representation  is  in  the  hippocampal  region.  On 
this  point  evidence  is  by  no  means  conclusive. 

C.  Physiology. — As  to  the  way  in  which  this  mechanism 
is  stinmlated  our  knowledge  is  very  imperfect.  In  order  to 
act,  the  substance  must  be  in  solution.  The  strength  of  the 
sensation  depends  (i.)  on  the  concentration  of  the  solution, 
(ii.)  upon  the  extent  of  the  surface  of  the  tongue  acted  upon, 
(iii.)  upon  the  duration  of  the  action,  (iv.)  and  upon  the 
temperature  of  the  solution.  If  the  temperature  is  very 
high  or  very  low,  the  taste  sensation  is  impaired  by  the 
sensations  of  heat  or  cold. 

It   is   most   difiicult   to   classify  the  many  various  taste 


NERVE 


133 


sensations  which  may  be  experienced,  but  they  may  roughly 
be  divided  into  four  main  groups  : — 

1.  Sweet.  3.  Acid. 

2.  Bitter.  4.  Saline. 


2.   NASAL  MECHANISM- 
Sense  of  Smell. 


The  olfactory  organs    are    the   most   fundamental  of  all 
distance  receptors,  and  they  play  a  most  important  part  in 


Fig.  59. — The  Connections  of  the  Olfactory  Fibres.  A,  olfactory  cells  ;  B, 
synapses  in  the  olfactory  bulb  I.  ;  II.,  olfactory  tracts  ;  III.,  olfactory 
centre  ;  IV.,  decussation  of  fibres.     (Howell.) 

the  life  of  the  lower  animals  in  guiding  them  to  their  food 
and  repelling  them  from  danger,  in  causing  positive  and 
negative  chemiotaxis. 

Smell,  as  Sherrington  puts  it,  is  taste  at  a  distance. 
Just  as  the  taste  organs  are  stimulated  by  substances  taken 
into  the  mouth,  so   the  olfactory  organs  are    stimulated    in 


134 


VETERINARY   PHYSIOLOGY 


terrestrial  animals  by  volatile  substances  inhaled  through 
the  nose,  and  in  fishes  by  substances  dissolved  in  water. 
Aquatic  mammals,  such  as  the  Cetacea,  possess  this  mechanism 
in  an  imperfectly  developed  condition,  and  seem  to  rely  on 
the  sense  of  taste.     They  have  been  called  anosmatic. 

In  the  dog,  and  in  many  other  mammals,  this  mechanism 
is  enormously  more  developed  than  in  man,  and  it  plays  a 
much  more  important  part  in  the  lives  of  these  animals. 

A.  Receptors. — Over  the  upper  part  of  the  nasal  cavity 
the  cokunnar  epithelial  cells  are  devoid  of  cilia,  and  between 


U.  D. 


T.N. 


Fig.  60. — Some  of  the  more  important  Central  Connections  of  the  Olfactory 
Receptors.  O.F.,  olfactory  cell;  O.B.,  olfactorj^  bulb;  O.T.,  olfactory- 
tubercle;  U.,  uncus;  He,  hippocampus;  Af.,  corpus  mammillare ; 
T.A.,  thalamus;  Jib. ,  hahenula, ;  T.N.,  tegmental  nucleus;  T.,  fibres 
to  the  tegmentum.      (Bryce.) 


them  are  placed  spindle-shaped  cells  (fig.  59,  A),  which 
send  processes  through  the  mucous  membrane,  and  through 
the  cribriform  plate  of  the  ethmoid  into  the  olfactory  bulb  I. 

In  the  bulb  these  neurons  form  synapses,  B,  with  other 
neurons,  C,  the  axons  of  which  pass  to  the  base  of  the 
olfactory  tracts  (fig.  60). 

B.  Connections  with  the  Central  Nervous  System. 
— The  olfactory  tract  connects  with  {a)  the  olfactory 
tubercle  which  is  rudimentary  in  the  human  brain,  and  with 
(6)  the  pyriform  area  (fig.  61),  which  forms  a  prominent 
feature  on  the  base  of  the  brain  of  animals  in  which  smell 
plays  an  important  part,  but  which  is  less  developed  in  the 


NERVE 


135 


human  brain.  To  this  the  fibres  from  the  olfactory  bulb,  the 
secondary  olfactory  neurons,  pass.  These  form  synapses  from 
which  the  tertiary  olfactory  neurons  pass  to  the  fascia  clentata 
running  along  the  edge  of  the  hippocampus,  and  to  tlie  hippo- 
campus  (He).  From  these  structures  the  fornix  (F.)  arises 
which  carries  fibres  across  to  the  opposite  hippocampus 
and  backwards  on  the  same  side  to  the  thalamus  (T.A.).  The 
olfactory  fibres  from  the  pyriform  lobe  have  also  extensive 
sub-thalamic  connections.  They  pass  to  form  synapses  in 
each  corpus  mammillare  (M.),  from  which  fibres  extend  down 
in  the  tegmentum,  probably  to  act  on  the  spinal  arcs. 


I 

a\ 

- 

L 

.P. 

■""""■"ci 

Fig.  61. — Side  view  of  the  Cerebrum  of  one  of  the  Lowest  Mammals.  B.O., 
olfactory  bull) ;  7'.  O.L.,  olfactory  tract;  Tuherc,  olfactory  tubercle; 
L.P.,  lobus  pyriformis  ;  f.r.,  fissura  rhinica,  above  which  is  the  neo- 
pallium, divided  into  receiving  areas  ;  //'",  visual ;  VIII'",  auditory  ; 
*S"',  sensations  from  body;  V",  sensations  from  face  ;  L.A.H.,  motor 
areas  for  leg,  arm,  and  head.     (Elliot  Smith.) 

The  evidence  points  to  the  hippocampus,  including  the 
fascia  dentata,  being  the  receiving  area,  the  area  which 
when  stimulated,  gives  rise  to  sensations  of  smell,  while  the 
fibres  of  the  fornix  serve  to  connect  these  impressions  with 
those  from  the  other  sense  organs. 

The  cortex  in  this  olfactory  region,  the  rhinencephalon, 
contains  cells  arranged  in  two  layers — (1)  outside  a  layer  of 
small  cells,  often  in  clusters,  the  granular  layer;  (2)  below 
this  layers  of  larger  cells,  the  suh-granular  layer.  The  fibres 
passing  to  it  get  their  medullary  sheaths  at  an  early  stage 
of  development. 


136  VETERINARY   PHYSIOLOGY 

Ferrier  states  that  removal  of  the  hippocampal  convolu- 
tion, including  the  lobus  pyriformis,  leads  in  monkeys  to 
loss  of  taste  and  smell,  and  that  stimulation  causes  torsion 
of  the  nostrils  and  lips,  as  if  sensations  of  smell  or  taste 
were  being  experienced. 

Other  experimenters  have  observed  interference  with  the 
sense  of  smell  in  destructive  lesions  of  the  hippocampal  lobe, 
and  one  case  at  least  has  been  described  in  which  a  tumour 
of  the  right  gyrus  hippocampus  was  associated  with  sensa- 
tions of  smell. 

C.  Physiology. — To  act  upon  the  olfactory  mechanism 
of  terrestrial  animals  the  substance  must  be  volatile,  and 
must  be  suspended  in  the  air.  In  this  condition  infinitesimal 
quantities  of  such  substances  as  musk  are  capable  of 
producing  powerful  sensations.  The  mucous  membrane 
must  be  moist,  and  this  is  secured  by  the  activity  of 
Bowman's  glands,  situated  in  it.  These  are  under  the 
control  of  the  fifth  cranial  nerve,  and  section  of  this  leads 
indirectly  to  loss  of  the  sense  of  smell  through  dryness  of 
the  membrane. 


II.   FOR  VIBRATION  OF   ETHER. 

Vision. 

A.  General  Considerations. 

While  the  addition  to  and  withdrawal  from  the  surface  of 
the  body  of  the  slower  ethereal  waves  which  are  the  basis  of 
heat  act  upon  the  special  nerve  terminations  in  the  skin  to 
give  rise  to  sensations  of  heat  and  cold,  a  certain  range  of 
more  rapid  vibrations  act  specially  upon  the  nerve-endings 
in  the  eye.  These  produce  molecular  changes  which  in 
turn  affect  the  centres  in  the  brain,  and  play  a  most 
important  part  in  the  adjustment  of  movements  for  the 
benefit  of  the  body,  and  which  give  rise  to  changes  in 
consciousness  which  we  call  sight.  The  range  of  vibrations 
which  can  act  in  this  way  is  comparatively  limited,  the 
slowest  being  about  435  billions  per  second,  the  most  rapid 
about  764  billions.      Vibrations  more  rapid  than  this,  while 


NERVE  137 

capable  of  setting  up  chemical  changes,  as  in  photography, 
do  not  produce  visual  sensations. 

1.  The  action  of  light  upon  the  protoplasm  of  lower 
organisms  has  been  already  considered  (p.  26),  and  it  has 
been  seen  that  it  may  be  either  general  or  unilateral,  pro- 
ducing the  phenomena  of  positive  or  negative  phototaxis. 

2.  In  more  complex  animals,  special  sets  of  cells  are  set 
apart  to  be  acted  on  by  light,  and  these  are  generally 
imbedded  in  pigmented  cells  to  prevent  the  passage  of  light 
through  the  protoplasm.  Such  an  accumulation  of  cells 
constitutes  an  eye,  and,  in  the  simpler  organisms,  an  eye  can 
have  no  further  function  than  to  enable  the  presence  or 
absence  of  light  or  various  degrees  of  illumination  to  produce 
their  effects  through  the  impulses  which  are  sent  to  the 
nervous  system. 

3.  In  the  higher  animals  these  cells  are  so  arranged  that 
certain  of  them  are  stimulated  by  light  coming  in  one 
direction,  others  are  stimulated  by  light  coming  in  another, 
and  while  the  former  are  connected  with  one  set  of  synapses 
in  the  brain,  the  latter  are  connected  with  another.  Thus, 
light  from  one  point  will  stimulate  one  set  of  cells  which, 
through  the  nerve  fibres  passing  to  the  central  nervous 
system,  will  excite  one  part  of  the  brain,  and  light  from 
another  point  will  act  upon  other  cells  which  will  excite 
another  part  of  the  brain,  and  thus  not  merely  the  degree  of 
illumination  but  also  the  source  of  illumination  becomes 
distinguishable. 

By  this  arrangement  it  becomes  possible  to  gain 
knowledge  of  the  shape  of  external  objects.  One  directs  the 
eye  to  the  corner  of  the  ceiling,  and  the  idea  that  it  is  a 
corner  is  due  to  the  fact  that  three  different  degrees  of 
illumination  are  appreciated,  and  that  these  can  be 
localised — one  above,  one  to  the  right,  and  one  to  the  left. 
One  set  of  cells  is  stimulated  to  one  degree,  another  set  of 
cells  to  another  degree,  and  a  third  set  of  cells  to  a  third 
degree  ;  and  the  different  stimulation  of  these  different  sets 
of  cells  leads  to  a  different  excitation  of  separate  sets  of 
neurons  in  the  brain.  These  changes  in  the  brain  are 
accompanied  by  the  perception  of  the  three  parts  differently 


138  VETERINARY   PHYSIOLOGY 

illuminated.  From  the  previous  training  of  the  nervous 
system  we  have  been  taught  to  interpret  this  as  due  to  a 
"  corner."  But  this  interpretation  is  simply  a  judgment 
based  upon  the  sensations,  and  it  may  or  may  not  be  right. 
Instead  of  actually  looking  at  a  corner  we  may  be  looking 
at  the  picture  of  one. 

From  the  very  first  it  must  be  remembered  that  the 
modification  of  our  consciousness  which  we  call  vision  is  not 
directly  due  to  external  conditions,  but  is  a  result  of  changes 
set  up  in  our  brain.  Our  sensation  is  associated  with  changes 
in  our  brain  produced  by  changes  in  the  eye  set  up  by  rays  of 
light  coming  from  the  object. 

Usually  such  changes  are  set  up  by  a  certain  range  of 
vibrations  of  the  ether,  but  they  may  be  set  up  in  other 
ways — e.g.  by  the  mechanical  stimulation  of  a  blow  on  the 
eye  ;  but  however  set  up,  they  give  rise  to  the  same  kind  of 
changes  in  consciousness — visual  sensation.  It  was  in 
connection  with  vision  that  Johannes  Miiller  formulated  the 
doctrine  of  specific  nerve  energy,  that  different  varieties  of 
stimuli  applied  to  the  same  organ  of  sense  always  produce 
the  same  kind  of  sensation.  The  converse  also  holds  good, 
that  the  same  stimulus  applied  to  diff'erent  organs  of  sense 
2)roduces  a  different  hind  of  sensation  for  each. 

4.  The  visual  mechanism  gives  the  power  not  only  of  appre- 
ciating the  degree  and  source  of  illumination,  but  also  of 
appreciating  colour.  Physically  the  different  colours  are 
simply  different  rates  of  vibration  of  the  ether ;  physio- 
logically they  are  different  kinds  of  changes  set  up  in  the 
retina ;  psychologically  they  are  different  kinds  of  sensa- 
tions. The  slowest  perceptible  vibrations  produce  changes 
accompanied  by  a  sensation  which  we  call  red,  the  most 
rapid  vibrations  produce  a  different  set  of  changes  which  we 
call  violet.  But,  as  will  be  afterwards  shown,  these  sensa- 
tions may  be  produced  by  other  modes  of  stimulating  the  eye. 

A  flat  picture  of  the  outer  world  is  formed,  and,  from  this 
flat  picture,  we  have  to  make  judgments  of  the  size,  distance, 
and  thickness  of  the  bodies  looked  at. 

The  idea  of  size  is  based  upon  the  extent  of  the  eye-cells 


NERVE  139 

stimulated  by  the  light  coming  from  the  object.  If  a  large 
surface  is  acted  upon,  the  object  seems  large  ;  if  a  small 
surface,  the  object  seems  small.  But  the  extent  of  eye-cells 
acted  on  depends  not  merely  upon  the  size  of  the  object,  but 
also  upon  its  distance  from  the  eye,  since  the  further  the 
object  is  from  the  eye  the  smaller  is  the  image  formed. 
Hence,  ideas  of  size  are  judgments  based  upon  the  size  of 
the  picture  in  the  eye,  and  the  estimation  of  the  distance 
of  the  object  and  past  experience  plays  an  important  part. 

The  idea  of  thickness  or  contour  of  an  object  is  also  largely 
a  judgment  based  upon  colour  and  shading.  AVhen  a  cube 
is  looked  at,  it  is  judged  to  be  a  cube  because  of  the 
different  degrees  of  illumination  of  the  different  sides — 
degrees  of  illumination  which  may  be  reproduced  in  a  flat 
picture  of  such  a  cube. 


B.  Anatomy  of  the  Eye. 

Before  attevipting  to  study  the  physiology  of  the  eye,  the 
student  must  dissect  an  ox's  or  a  pig's  eye,  and  then  make 
himself  familiar  with  the  microscopic  structure  of  the 
various  parts. 

The  eye  may  be  described  as  a  hollow  sphere  of  librous 
tissue  (fig.  62),  the  posterior  part,  the  sclerotic  (ScL),  being 
opaque  ;  the  anterior  part,  the  cornea  (Cor.),  being  trans- 
parent and  forming  in  most  animals  part  of  a  sphere  of 
smaller  diameter  than  the  sclerotic.  In  the  horse  the  curva- 
ture of  the  cornea  is  less  in  the  horizontal  plane  than  in 
the  vertical.  Inside  the  sclerotic  coat  is  a  loose  fibrous 
layer,  the  choroid  (Chor.),  the  connective  tissue  cells  of 
wdiich  are  loaded  with  melanin,  a  black  pigment.  This  is 
the  vascular  coat  of  the  eye — the  larger  vessels  running  in 
its  outer  part,  and  the  capillaries  in  its  inner  layer. 
Anteriorly,  just  behind  the  junction  of  the  cornea  and 
sclerotic,  it  is  thickened  and  raised  in  a  number  of  ridges, 
the  ciliary  processes  {Cil.M.),  running  from  behind  forward 
and  terminating  abruptly  in  front.  The  ciliary  muscle  is 
situated  in  these.  It  consists  of  two  sets  of  non-striped 
muscular    fibres — first,   radiating    fibres     which    take  origin 


140 


VETERINARY   PHYSIOLOGY 


from  the  sclerotic  just  behind  the  corneo- sclerotic  junction, 
and  run  backwards  and  outwards  to  be  inserted  into  the 
bases  of  the  ciliary  processes  ;  second,  circular  fibres  which 
run  round  the  processes  just  inside  the  radiating  fibres.  The 
choroid  is  continued  forward  in  front  of  the  ciliary  processes 
to  the  pupil  as  the  ii'is,  and  in  it  are  also  two  sets  of  non- 
striped  muscular  fibres — first, 
•  the     circular     fibres,    a    well- 

marked  band  running  round 
the  pupil,  and  called  the 
sphincter  papilka  {Sph.P.) 
muscle ;  second,  a  less  well- 
marked  set  of  radiating  fibres, 
which  are  absent  in  some 
animals,  and  which  constitute 
the  dilator  pujnllce  muscle 
(D.P.).  In  the  horse  the 
pupil  is  elliptical,  the  long 
axis  being  in  the  horizontal 
plane,  and  from  the  edge  of 
the  iris  a  process  like  a  small 
bunch  of  grapes  projects,  and, 
when  the  pupil  is  contracted, 
nearly  occludes  it. 

The  memhrana  nictltans, 

lying  on  the  inner  aspect  of 

the  orbit,  consists  of  a  flexible 

plate  of  elastic    fibro-cartilage 

with       conjunctiva. 

When    the    eye    is    retracted 

the  post- orbital  fat  is  pushed 

forwards  and  thrusts  the  membrane  over  the  inner  half  of 

the  eyeball  (p.  160). 

The  part  of  the  eye  in  front  of  the  iris  is  filled  by  a 
lymph-like  fluid,  the  aqueous  humour,  while  the  part  behind 
is  occupied  by  a  fine  jelly-like  mucoid  tissue,  the  vitreous 
humour.  The  vitreous  humour  is  enclosed  in  a  delicate 
fibrous  capsule,  the  hyaloid  membrane,  and,  just  behind  the 
ciliary  processes,  this  membrane   becomes  tougher,  and  is  so 


Fig.  62. — Horizontal  section  through 
the  Left  Eye.  Cor.,  cornea  ;  Scl., 
sclerotic;  Opt.N.,  optic  nerve; 
Ghor.,  choroid;  Cil.M.,  ciliary 
processes  with  ciliary  muscle  ; 
D.P.,  dilator  pupilhe  muscle  ; 
Sph.P.,  sphincter  pupill*  muscle  ; 
L.,  crystalline  lens;  S.L.,  hyaloid 
membrane  forming  suspensory 
ligament  and  capsule  of  lens  ; 
Bet.,  retina.  The  vertical  line 
passing  through  the  axis  of  the 
eye  falls  upon  the  central  spot  covered 
of  the  retina. 


NERVE 


141 


iirmly  adherent  to  the  processes  that  it  is  difficult  to  strip 
it  off.  It  passes  forward  from  the  processes  as  the  suspensory 
ligament  (S.L.),  and  then  splits  to  form  the  lens  capsule. 
In  this  is  held  the  crystalline  lens  (L.),  a  biconvex  lens, 
with  its  greater  curvature  on  its  posterior  aspect.  It  is  an 
elastic  structure,  and  it  is  normally  kept  somewhat  pressed 
out  and  flattened  between  the  layers  of  the  capsule  ;  but, 
if  the  suspensory  ligament  is  relaxed, 
its  natural  elasticity  causes  it  to 
bulge  forward.  This  happens  when 
the  ciliary  muscle  contracts  and  pulls 
forward  the  ciliary  jjrocesses  with 
the  hyaloid  membrane. 

Between  the  hyaloid  membrane 
and  the  choroid  is  the  retina  (Ret.). 
This  is  an  expansion  of  the  optic 
nerve,  which  enters  the  eye  to  the 
inner  side  of  the  posterior  optic 
axis  (fig.  62).  The  white  nerve 
fibres  pass  through  the  sclerotic, 
through  the  choroid,  and  through 
the  retina,  to  form  the  white  ojytic 
disc.  This  is  about  1-5  mm. 
in  diameter  in  the  human  subject. 
The  fibres  lose  their  white  sheath,  and 
spread  out  in  all  directions  over  the 
front  of  the  retina,  to  form  its  first 
layer — the  layer  of  nerve  ^fibres  (1) 
(fig.  63).  These  nerve  fibres  take 
origin  from  a  layer  of  nerve  cells  (2) 
behind  them,  forming  the  second 
layer.  The  dendrites  of  these  cells  arborise  with  the 
dendrites  for  the  next  set  of  neurons  in  the  third 
layer,  the  internal  molecular  layer  (3).  The  cells  of 
these  neurons  are  placed  in  the  next  or  fourth  layer,  the 
inner  nuclear  layer  (4),  and  from  these  cells,  processes  pass 
backwards  to  form  synapses  in  the  fifth,  or  outer  molecular 
layer  (5),  with  the  dendrites  of  the  terminal  neurons.  These 
terminal  neurons  have  their  cells  in  the  sixth  or  outer  nuclear 


Fig.  63.  ^Diagram  of  a  Sec- 
tion through  the  Retina 
stained  by  Golgi's  method. 
For  description,  see  text. 
(From  Van  Gehuchten.) 


142 


VETERINARY   PHYSIOLOGY 


layer  (6)  of  the  retina,  and  they  pass  backwards  and  end  m 
two  special  kinds  of  terminations  in  the  seventh  layer  of  the 
retina — the  rods  and  cones  (7).  These  structures  are  com- 
posed of  two  segments — a  somewhat  barrel-shaped  basal 
piece,  and  a  transparent  terminal  part  which  in  the  rods 
is  cylindrical  and  in  the  cones  is  pointed.      Over  the  central 


Fig.  64. — The  use  of  the  Ophthalmoscope,  A,  by  the  direct  method.  B, 
by  the  indirect  method.  E,  eye  observed.  E^,  observer's  eye.  M, 
mirror.  .S",  source  of  light.  L,  convex  lens.  The  direction  of  the 
ingoing  rays  is  shown  in  strong  lines,  of  the  rays  from  the  observed 
eye  in  thin  lines. 


spot  of  the  eye  there  are  no  rods,  but  the  cones  lie  side 
by  side,  and  the  other  layers  of  the  retina  are  thinned  out. 
The  rods  and  cones  are  imbedded  in  the  last  or  eighth  layer 
of  the  retina — the  layer  of  pigment  cells,  or  tapetuni  nigrum. 
The  retina  stops  abruptly  in  front  at  the  ora  serrata,  but  the 
tapetum  nigrum,  along  with  another  layer  of  epithelial  cells 
representing  the  rest  of  the  retinal  structures,  is  continued 


NERVE  143 

forwards  over  the  ciliary  processes  and  over  the  back  of 
the  iris. 

The  blood-vessels  of  the  retina  enter  in  the  middle  of  the 
optic  nerve,  and  run  out  and  branch  in  the  anterior  layer  of 
the  retina. 

The  interior  of  the  eye  may  be  examined  by  the 
Ophthalmoscope,  either  by  the  direct  or  by  the  indirect  method 
(fig.  64).  By  the  direct  method  light  is  reflected  from  a 
small  mirror  into  the  eyes,  and  the  observer  examines  the 
fundus  directly  through  a  hole  in  the  middle  of  the  mirror 
(fig.  64,  A).  By  the  indirect  method  he  reflects  light  from 
a  slightly  concave  mirror  into  the  eye,  and  then  inserts 
between  the  eye  and  the  mirror  a  biconvex  lens  so  that  an 
image  of  the  fundus  is  formed,  and  this  is  examined  (fig. 
64,  B)  {Practical  Physiology). 

The  fluid  which  distends  the  eyeball  may  be  considered  as 
a  form  of  lymph. 

It  is  derived  by  exudation  from  the  vessels  in  the  ciliary 
processes.  Some  passes  back  into  the  vitreous  humour,  but 
the  greater  quantity  passes  forward  into  the  anterior  chamber, 
from  which  it  is  drained  away  in  the  lymph  spaces  in  the 
anterior  part  of  the  sclerotic  called  the  spaces  of  Fontana, 
which  open  into  the  canal  of  Schlemm.  When  these  spaces 
become  obstructed,  the  fluid  tends  to  accumulate,  and  the 
pressure  in  the  eyeball  rises  and  it  feels  hard  to  the  touch. 
This  condition  of  glaucoma  leads  to  disturbances  in  the 
nutrition  of  the  eyeball. 

The  pressure  in  the  eyeball  varies  with  the  arterial 
pressure,  and  it  is  also  influenced  by  any  obstruction  to  the 
flow  of  blood  in  the  veins  from  the  head  so  that  in  cases  of 
intracranial  tumour  the  vessels  of  the  retina  are  apt  to 
become  congested. 


C.  Physiology. 

The  study  of  vision  may  be  taken  up  in  the  following 
order  : — 

1.  The  mode  of  formation  of  pictures  on  the  nerve  structures 
(retina)  of  the  eye. 


144  VETERINARY   PHYSIOLOGY 

(1)  One  eye  (monocular  vision). 

A.  The    method    in    which    rays    of   light    are 

fociissed  (dioptric  mechanism). 

B.  The  stimulation  of  the  retina. 

(2)  Two  eyes  (binocular  vision). 

2.  The  conduction  of  the  nerve  impulses  from  the  retina  to 
the  brain. 

3.  The  mode  of  action  of  the  parts  of  the  brain  in  which 
the  changes  accompanying  visual  sensations  are  set  up 
(the  visual  centre). 


1.   THE  MODE  OF  FORMATION  OF  PICTURES  UPON 
THE  RETINA. 


(I.)  Monocular  Vision. 
7.    Tlie  Dioptric  Mechanism. 


1.  Distant  Vision. — The  eye  may  be  compared  to  a  photo- 
graphic camera,  having  in  front  a  lens,  or  lenses,  to  focus 
the  light  upon  the  sensitive  screen  behind.  The  picture 
is  formed  on  the  screen  by  the  luminous  rays  from  each 
point  outside  being  concentrated  to  a  point  upon  the  screen. 
This  is  brought  about  by  refraction  of  light  as  it  passes  through 
the  various  media  of  the  eye — the  cornea,  aqueous,  crystalline 
lens,  and  vitreous.  The  refractive  indices  of  these,  compared 
with  air  as  unity,  may  be  expressed  as  follows : — 

Cornea  .         .     1'33  Lens    .         .     145 

Aqueous         .     133  Vitreous       .     1"33 

When  a  ray  of  light  passes  from  a  medium  of  lower  into 
one  of  higher  refractive  index,  it  is  bent  towards  a  line 
drawn  at  right  angles  to  the  surface  (fig.  65),  and,  conversely, 
in  passing  from  a  medium  of  higher  into  one  of  lower  re- 
fractive index,  it  is  bent  away  from  a  line  at  right  angles  to 
the  surface. 

Light  therefore  passes  from  a  medium  of  one  refractive 
index  into  a  medium  of  another  refractive  index  (fig.  65) — 

1.   At  the  anterior  surface  of  the  cornea,  from  a  medium 


NERVE  145 

of  lower  to  one  of  higher  refractive  index,  so  that  the 
peripheral  rays  are  bent  towards  the  perpendicular.  Since 
in  the  horse  the  curvature  of  the  cornea  is  less  in  the 
horizontal  plane,  rays  of  light  in  this  plane  are  less  refracted. 

2.  At  the  anterior  surface  of  the  lens,  from  lower  to 
higher,  so  that  the  rays  are  again  bent  in  the  same  way. 

3.  At  the  posterior  surface  of  the  lens,  from  higher  to 
lower  refractive  index,  so  that  the  rays  are  bent  away  from 
the  perpendicular,  that  is,  again  towards  the  axial  ray.V 

The  degree  of  bending  depends  upon — 1st,  The  difference 
of  refractive  index  ;   2nd,  The  obliquity  with  which  the  light 


Fig.  65. — To  show  how  parallel  raj's  are  brought  to  a  focus  on  the  retina 
by  refraction  at  the  three  surfaces  (a),  anterior  surface  of  the  corneaj; 
(b),  anterior  surface  of  the  lens  ;  and  (c),  posterior  surface  of  the  lens. 

hits  the  surface.  This  will  vary  with  the  convexity  of  the 
surface. 

The  posterior  surface  of  the  lens  has  the  greatest  convexity 
with  a  radius  of  6  mm.  The  anterior  surface  of  the  cornea 
has  the  next  greatest,  with  a  radius  of  8  mm.  The  anterior 
surface  of  the  lens  has  the  least,  with  a  radius  of  10  mm. 

A  ray  of  light  passing  obliquely  through  these  media  will 
be  bent  at  the  three  surfaces  proportionately  to  the  curvature 
of  each. 

These  media,  in  fact,  form  the  physiological  lens,  a  com- 
pound lens  composed  of  a  convexo-concave  part  in  front. 
10 


146  VETERINARY   PHYSIOLOGY 

the  cornea  and  aqueous,  and  a  biconvex  part,  the  crystalline 
lens,  behind.  In  the  resting  normal  eye  (emmetropic  eye)  the 
principal  focus  is  exactly  the  distance  behind  the  lens  at 
which  the  layer  of  rods  and  cones  in  the  retina  is  situated, 
and  thus  it  is  upon  these  that  light,  coming  from  luminous 
points  at  a  distance,  is  focussed. 

2.   Near  Vision— Positive  Accommodation. — If  an  object  is 
brought  nearer  and  nearer  to  the  eye,  the  rays  of  light  enter- 
ing the  eye  become  more  and  more 
_^:^^^^^^^/\  divergent,  and  if  the  eye  be  set  so 

"'"■-    \)S^<^-'-""""  ^^^^    ^'^y^    from    a    distance — i.e. 

-^^H^^  "y/  parallel    rays — are    focussed,    then 

Fig.  66.-T0  show  that  rays  I'ays  from  a  nearer  object  will  be 

from    distant    and    near  focussed  behind  the  retina,   and  a 

objects  are  not  focussed  on         ^  -^  ^^-^^     ^^^     ^^      ^^^^^^ 

the  retina  at  the  same  time.  o 

fiig.  C)6).  This  means  that  near 
and  far  objects  cannot  be  distinctly  seen  at  tbe  same  time, 
a  fact  which  can  be  readily  demonstrated  by  Scheiners 
Experiment  (Practical  Physiology). 

Make  two  pinholes  in  a  card  so  near  that  they  fall 
within  the  diameter  of  the  pupil.  Close  one  eye  and  place 
the  holes  in  front  of  the  other.  Get  someone  to  hold  a 
needle    against    a    sheet    of   white    paper    at    about    three 


Fig.  67.— Scheiner's  Experiment represents  rays  from  the  near  needle 

when  it  is  looked  at,  and  -  -  -  -  rays  from  the  far  needle. 

yards  from  the  eye,  and  hold  another  needle  in  the  same 
line  at  about  a  foot  from  the  eye.  When  the  near  needle 
is  looked  at,  the  far  needle  becomes  double  (fig.  67). 

In  man  it  is  found  that  objects  at  a  greater  distance  than 
6  metres  may  practically  be  considered  as  "  distant,"  and  that 
they  are  focussed  on  the  retina. 

Objects  may  be  brought  nearer  and  nearer  to  the  eye,  and 
yet  be  seen  distinctly  up  to  a  certain  point,  the  near  point  of 
accommodation  within  which  they  cannot  be  sharply  focussed 


NERVE 


147 


upon  the  retina.  This,  however,  requires  a  change  in  the  lens 
arrangement  of  the  eye,  and  this  change,  beginning  in  man  when 
the  object  comes  within  about  6  metres  (the  far  point  of  accom- 
modation), becomes  greater  and  greater,  till  it  can  increase  no 
further  when  the  near  point  is  reached.  The  change  is  called 
positive  accommodation,  and  it  consists  in  an  increased 
curvature  of  the  anterior  surface  of  the  lens.  This  may  be 
prQved  by  examining,  in  a  dark  room,  the  images  of  a  candle 
formed  from  the  three  refracting  surfaces  (Sanson's  images), 
when  it  will  be  found  that  the  image  from  the  anterior  surface 
of  the  lens  becomes  smaller  and  brighter  when  the  eye  is 
directed  to  a  near  object.  The  examination  of  these  images 
is  facilitated  by  the  use  of  the  Phacoscope  {Practical 
Physiology). 

Positive  accommodation  is  brought  about  by  contraction 
of  the  ciliary  muscle  (seep.  139),  which  pulls  forward  the 
ciliary  processes  to 
which  the  hyaloid 
membrane  is  attached, 
and  thus  relaxes  the 
suspensory  ligament  of 
the  lens  and  the  front 
of  the  lens  capsule, 
and  allows  the  natural 
elasticity  of  the  lens  to 
bulge  it  forward  (fig.  68). 

This  change  of  posi- 
tive accommodation  is  accompanied  by  a  contraction  of  the 
pupil  due  to  contraction  of  the  sphincter  pupilhie  muscle. 
By  this  means,  the  more  divergent  peripheral  rays  which  would 
have  been  focussed  behind  the  central  rays  to  produce  a 
blurred  image  are  cut  off,  and  spherical  aberration  is  prevented. 

The  muscles  acting  in  positive  accommodation  —  the 
ciliary  and  sphincter  pupillas — (fig.  69,  G.M.  and  S.P.) 
are  supplied  by  the  third  cranial  nerve  (///.),  while  the 
dilator  pupillee  is  supplied  by  fibres  passing  up  the 
sympathetic  of  the  neck.  The  centre  for  the  third  nerve  is 
situated  under  the  aqueduct  of  Sylvius,  and  separate 
parts  preside  over  the  ciliary  muscle  and  the  sphincter  pupillse. 


Fig.  68.  — Mechanism  of  Positive  Accommoda- 
tion. The  continuous  lines  show  the  parts 
in  negative  accommodation,  the  dotted 
lines  in  positive  accommodation. 


148 


VETERINARY   PHYSIOLOGY 


The  sphincter  centre  is  reflexly  called  into  action,  and  the 
pupil  contracted — 

1st.  When  strong  light  falls  on  the  retina  and  stimu- 
lates the  optic  nerve.  In  this  way  the  retina  is  pro- 
tected against  over-stimulation.  The  two  eyes  act  together 
in  this  reflex  ;  if  one  be  covered  the  pupils  of  both  eyes 
dilate.     In  the  horse  the  light-reflex  is  sluggish. 

2nd.  When  the  image  upon  the  retina  becomes  blurred 
as  the  object  approaches  the  eye  and  the  centre  for  the 
ciliary  muscle  is  called  into  play  to  produce  accommoda- 
tion. 


Fig.  69.— Nerve  Supply  of  the  Intrinsic  Muscles  of  the  Eye  (see  text). 

3rd.  The  sphincter  centre  is  also  stimulated  by 
morphine  and  other  drugs,  and  in  the  first  stages  of 
asphyxia.  In  chloroform  anaesthesia  the  pupil  at  first 
responds  to  light,  later,  when  the  anaesthesia  is  full,  it 
is  contracted.  If  the  chloroform  is  pressed  further,  it 
dilates  and  does  not  respond  to  light.  This  is  a  danger 
signal. 

4th.   In  sleep  the  pupil  is  contracted. 

The  centre  for  dilatation  of  the  pupil  is  situated  in  the 
medulla  oblongata,  Like  the  centre  of  the  sphincter  it  may 
be  reflexly    excited     stimulation  of    ingoing  nerves  causing 


NERVE  149 

a  dilatation  of  the  pupil  when  the  medulla  is  intact 
(fig-  69). 

The  dilator  fibres  pass  down  the  lateral  columns  of  the 
spinal  cord  to  the  lower  cervical  and  upper  dorsal  region, 
where  they  arborise  round  cells  in  the  anterior  horn  (cilio- 
spinal  region).  From  these,  fibres  pass  by  the  anterior  root 
of  the  second  (Fig.  69,  2  D.X.),  possibly  also  of  the  first 
and  third  dorsal  nerves,  and,  passing  up  through  the 
inferior  cervical  ganglion,  run  on  to  the  superior  ganglion, 
where  they  arborise  round  cells  which  send  axons  to  the 
Gasserian  ganglion  of  the  fifth  cranial  nerve  (V.),  which 
course  through  this  and  pass  along  the  ophthalmic  division 
and  its  long  ciliary  branches  to  the  dilator  fibres  of  the 
iris  (D.P.). 

There  is  evidence  of  the  existence  of  a  peripheral 
mechanism  in  the  iris.  The  pupil  may  be  seen  to  contract 
and  dilate  in  the  eye  of  a  cat  after  decapitation,  and  various 
drugs  act  directly  upon  it.  Atropin  causes  a  dilatation  and 
physostigmin  and  pilocarpin  cause  a  contraction.  Adrenalin 
causes  dilatation  when  placed  in  the  eye  of  a  mammal,  but 
only  after  removal  of  the  superior  cervical  ganglion  of  the 
same  side,  which  seems  to  render  the  peripheral  mechanism 
more  sensitive.  A  nerve  plexus  exists  in  the  iris,  and  this 
probably  acts  upon  the  muscular  fibres. 

3.  Range  of  Accommodation. — The  power  of  positive  accom- 
modation varies  at  different  ages,  being  greatest  in  young 
animals,  because  in  early  life  the  lens  is  most  convex  and 
more  elastic. 

Presbyopia. — The  "  range  of  accommodation,"  i.e.  the 
difference  between  the  "  near  point "  and  the  "  far  point," 
steadily  decreases  as  age  advances  till  the  condition  of 
Presbyopia — old-sigh tedness — is  produced. 

Imperfections  of  the  Dioptric  Mechanism. 

(1)  Hypermetropia. — The  eye  may  be  too  short  from  before 
backwards,  and  thus,  in  the  resting  state,  parallel  rays  are 
focussed  behind  tiie  retina,  and,  in  order  to  see  even  a  distant 


150 


VETERINARY   PHYSIOLOGY 


object,  positive  accommodation  is  required.      As  the  object  is 
approached  to  the  eye,  it  is  focussed  with  greater  and  greater 


6  METRES. 

'  0-1   METRE. 

EMMETROPiC 

PRESBYOPIC 

HYPERMETROPIC 

MYOPIC 

Fig.  70. — To  illustrate  Presbyopia,  Hj-permetropia,  and  Myopia.  A,  emme- 
tropic eye ;  B,  presbyopic  eye ;  C,  hypermetropic  eye ;  D,  mj^opic  eye ; 
N.P.o,  the  near  point,  and  F.P.x,  the  far  point  of  accommodation. 

difficulty,    and   the   near    point   is    further   otf    than    in    the 

emmetropic  eye  (fig.  70,  C). 

This  long-sighted  eye  differs  from  the  slightly  presbyopic 
in  the  fact  that  not  merely 
divergent,  but  also  parallel 
rays,  are  unfocussed  in  the 
resting  state. 

(2)  Myopia. — In  certain  in- 
dividuals the  antero-posterior 
diameter  of  the  eye  is  too 
long,  and,  as  a  result,  parallel 
rays  —  rays  from  distant 
objects — are  focussed  in  front 
of  the  retina,  and  it  is  only 
when  the  object  is  brought 
near  to  the  eye  that  a 
perfect  image  can  be  formed. 
In  myopia  no  positive  accom- 
object    is     well    within     the 


Fig.  71. — To  show  the  cause  of  Astig- 
matism. A,  a  slight  curvature  of 
the  cornea  in  the  vertical  plane ;  B, 
more  marked  curvature  in  the  hori- 
zontal plane,  leading  to  rays  from  h 
— a  horizontal  line — being  focussed 
in  front  of  the  retina  when  a — a 
vertical  line — is  looked  at. 

modation    is    needed    till    the 


NERVE 


151 


normal  far  point.  The  near  point  is  approximated  to  the 
eye  (fig.  70,  D). 

(3)  Astigmatism  is  a  defect  due  to  unequal  curvature  of 
one  or  more  of  the  refracting  surfaces  in  different  planes. 
If  the  vertical  curvature  of  the  cornea  is  greater  than  the 
horizontal  when  a  vertical  line  is  looked  at,  horizontal  lines 
cannot  be  sharply  focussed  at  the  same  time  (fig.  71). 

Errors  of  refraction  are  common  in  the  horse,  astigmatism 
being  present  in  something  like  fifty  per  cent. 


//.    Stimulation  of  the  Retina. 
1.   Reaction  to  Varying  Illuminations. 
(1)  The  Blind  Spot. — At  the  entrance  of  the  optic  nerve 


the  retina  cannot  be  stimulated  because  there  are  no  end- 


FiG.  72. — Methods  of  demonstrating  the  Blind  Spot  :  a,  by  Mariotte's  experi- 
ment ;  b,  Area  subtending  the  Blind  Spot  in  which  objects  cannot  be  seen. 

organs  in  that  situation.      The  existence  of  such  a  blind  spot 
may  be  demonstrated  in  man — 

1st.   By  Mariotte's  experiment,  which  consists  in  making 
two  marks  in  a  horizontal  line  on  a  piece  of  paper,  closing 


152  VETERINARY   PHYSIOLOGY 

the  left  eye,  fixing  the  right  eye  on  the  left-hand  mark  with 
the  paper  held  at  such  a  distance  from  the  eye  that  both 
marks  are  visible,  then  bringing  the  paper  nearer  to  the  eye, 
when  the  right-hand  mark  will  first  disappear.  When  the 
paper  is  brought  still  nearer  it  will  reappear  (fig.  72,  a) 
(Practical  Physiology). 

2nd.  By  making  a  mark  on  a  sheet  of  paper,  and,  with 
the  head  about  a  foot  from  the  paper,  moving  the  point  of  a 
pencil  to  the  right  for  the  right  eye,  or  to  the  left  for  the  left 
eye,  when  the  point  will  disappear  and  again  reappear. 
The  eye  is  blind  for  all  objects  in  the  shaded  region 
(fig.  72,  a,  h)  {Practical  Physiology).  By  resolving  the 
various  triangles,  the  distance  of  the  blind  spot  from  the 
central  spot  of  the  eye  may  be  determined,  and  the 
diameter  of  the  blind  spot  may  also  be  ascertained. 
Hence  subtending  this  blind  spot  is  a  region  in  which  objects 
are  not  seen  (fig.  72,  a,  h). 

The  shape  of  the  blind  spot  may  be  mapped  out  by 
fixing  the  head  about  a  foot  from  the  paper,  moving  the 
point  of  the  pencil  out  till  it  disappears,  and  then  moving  it 
in  different  directions  and  marking  when  it  reappears  (fig.  72,  6). 
It  is  never  quite  circular,  and  often  shows  rays  extending 
from  its  edge  which  are  due  to  the  blood-vessels  (Practical 
Physiology). 

(2)  The  Field  of  Vision.— The  rest  of  the  retina,  forward  tc 
the  ora  serrata,  is  capable  of  stimulation. 

(3)  The  layer  of  the  retina  capable  of  stimulation  is  the 
layer  of  rods  and  cones.  This  is  proved  by  the  experiment 
of  Purkinje's  images,  which  depends  upon  the  fact  that,  if  a 
ray  of  light  is  thrown  through  the  sclerotic  coat  of  the  eye, 
the  shadow  of  the  blood-vessels  stimulates  a  subjacent  layer 
(fig.  73,  c),  and  the  vessels  appear  as  a  series  of  wriggling 
lines  on  the  surface  looked  at.  If  the  light  be  moved,  the 
lines  seem  to  move,  and,  by  resolving  the  triangles,  it  is 
possible  to  calculate  the  distance  behind  the  vessels  of  the 
part  stimulated,  and  this  distance  is  found  to  correspond  to 
the  thickness  of  the  retina.  The  shadows  of  the  blood- 
vessels are  not  seen  in  ordinary  vision,  because  they  then  fall 


NERVE 


153 


upon    parts   of  the   retina   which   are  insensitive    {Practical 
Physiology). 

(4)  The  Central  Spot.— Of    all    parts    of    the    retina    the 
central  spot  is  the  most  sensitive  to  differences  of  illumina- 


tion in  brigfht  light. 


The  cones,  which  entirely  replace  the 
rods  on  the  central  spot  of  the  eye,  are 
the  more  specialised  elements  of  the 
retina,  and  they  react  more  particularly 
to  bright  light,  which  soon  exhausts  the 
rods.  The  rods,  again,  react  to  faint 
illumination,  and  are  readily  fatigued  by 
bright  light.  This  explains  why  it  is 
that,  when  we  go  out  into  a  dark  night 
from  a  brightly-hghted  room,  we  can 
see  nothing  at  first,  but  after  a  time, 
when  the  rods  have  recovered,  we  begin 
to  see  objects  more  distinctly.  The  eye  yig.  73.— To  show  that 
must  become  adapted  to  the  dark,  and  the  hindmost  layer 
the  adaptation  is  better  in  some  people 
than  in  others. 

The  rods  seem  incapable  of  giving  rise 
to  colour  sensation,  and  when  the  solar 
spectrum  is  looked  at  in  a  very  dim  light, 
it  appears  as  a  greyish  band  of  illumina- 
tion with  the  red  end  wanting,  because 
the  slow  red  vibrations  fail  to  stimulate 
the  rods.  It  is  because  the  blue  end  of  the  spectrum  is  the 
more  active  in  faint  illumination  that  the  illusion  of  a 
moonlight  scene  may  be  got  by  looking  through  a  blue  glass, 
while  looking  through  a  yellow  glass  gives  the  idea  of 
sunlight  and  brilliant  illumination  (Practical  Physio- 
logy). 

(5)  Modes  of  Stimulation. 
ally  stimulated  by  the  ethereal  light  vibration,  but  they 
may  be  stimulated  by  mechanical  violence  or  by  sudden 
changes  in  an  electric  current.  But,  however  stimulated, 
the  kind  of  sensation  is  always  of  the  same  kind — a  visual 
sensation  (see  p.  138). 


of  the  retina  is  stim- 
ulated {Purkinje's 
Images).  a,  source 
of  light ;  b,  blood- 
vessel of  retina  ;  c, 
shadow  of  vessel  on 
rods  and  cones ;  d, 
image  of  shadow 
mentally  projected 
on  to  the  wall. 


-The  rods  and  cones  are  gener- 


154 


VETERINARY   PHYSIOLOGY 


(6)  Of  the  nature  of  the  changes  in  the  retina  when 
stimulated  we  know  little.     But  we  know  that — 

1st.  Under  the  influence  of  light  the  cells  of  the  tapetum 
nigrum  expand  forward  between  the  rods  and  cones. 

2nd.  A  purple  pigment,  which  has  been  called  rhodopsin, 
exists  in  the  outer  segment  of  the  rods,  and  this  is  bleached 
by  the  action  of  light.  Even  although  there  is  no  purple  in 
the  cones,  which  alone  occupy  the  sensitive  central  spot  of 
the  eye,  this  change  iu  colour  suggests  that  a  chemical 
decomposition  accompanies  stimulation. 

Srd.  Electrical  changes  occur  (p.  61j. 

(7)  Fatigue, 

time,  the  parts   of  the  optic  mechanism,  retina,  and  nerve 
centre  acted  upon  is  temporarily  blinded,  and  hence,  when 


If  a  brio'ht   light   be  looked    at  for  some 


Fig.  74. — The  Power  of  Localising  the  Source  of  Illumination  on  different 
parts  of  the  retina.  The  two  points,  a-b,  subtended  b}'  the  small 
angle,  fall  close  together  at  a-h  near  the  centre  of  the  retina,  and  still 
give  rise  to  a  double  sensation  ;  but  if  two  points,  c-d,  have  their 
images  formed  on  the  periphery  of  tlie  retina,  c-d,  these  images  must 
be  far  apart  to  cause  a  double  sensation. 


the  eye  is  directed  away  from  the  bright  light,  a  dark  spot  is 
seen.  This  is  sometimes  called  a  negative  after-image. 
When  coloured  lights  are  used,  the  phenomena  of  comple- 
mental  colours  are  produced  {Practical  Physiology). 

Sometimes  the  stimulation  of  the  retina  or  of  the  brain 
neurons  connected  with  it  may  last  after  the  withdrawal  of 
the  stimulus,  when  a  continuance  of  the  sensation — a  positive 
after-image — is  seen.  This  may  be  observed  if,  on  opening 
the  eyes  in  the  morning,  a  well-illuminated  window  is  looked 


NERVE  155 

at  and  the    eyes    closed  ;  a   persisting  image  of  the  window 
may  be  seen. 

2.  The  Power  of  Localising  the  Source  or  Direction  of 
Illumination. 

This  may  be  determined  by  finding  how  close  together 
two  separate  stimuli  may  fall  and  still  give  rise  to  a  double 
sensation.  Over  the  central  spot  two  points  of  illumination 
mav  be  very  near,  and  still  two  sensations  be  experienced 
(fig-  "4). 

On  passing  to  the  more  peripheral  part  of  the  retina, 
where  the  cones  are  more  scattered,  the  power  of  locaHsing 
decreases. 

3.  Colour  Sensation. 

1.  Physics  of  Light  Vibration. — Physically  the  various 
colours  are  essentially  ditierent  rates  of  vibration  of  the 
ether,  and  only  a  comparatively  small  range  of  these  vibra- 
tions stimulates  the  retina.  The  slowest  acting  vibrations  are 
at  the  rate  of  about  435  billions  per  second,  while  the  fastest 
are  not  more  than  764  billions — the  relationship  of  the 
slowest  to  the  fastest  is  something  like  four  to  seven.  The 
apparent  colour  of  objects  is  due  to  the  fact  that  they  absorb 
certain  parts  of  the  spectrum,  and  either  transmit  onwards 
or  reflect  other  parts. 

The  vast  varietv  of  colours  which  are  perceived  in  nature 
is  due  to  the  fact  that  the  pure  spectral  colours  are  modified 
by  the  brightness  of  illumination,  and  by  admixture  with 
other  parts  of  the  spectrum  {saturation).  Thus,  a  surface 
which,  in  bright  sunlight,  appears  of  a  brilliant  red,  becomes 
maroon,  and  finally  brown  and  black,  as  the  light  fades. 
Again,  a  pure  red  when  diluted  with  all  the  spectrum — i.e. 
with  white  light — becomes  pinker  as  it  becomes  less  and  less 
saturated  {Practical  Physiology). 

2.  Physiology  of  Colour  Sensation. — (1)  The  peripheral  part 
of  the  retina  in  man  is  colour  blind — is  incapable  of  acting^  so 
as  to  produce  colour  sensations.  Yellow  and  blue  can  be 
distinguished  further  out  upon  the  retina  than  can  red  and 
green.     There  is  a  zone  of  retina  which  is  blind  to  red  and 


156 


VETERINARY   PHYSIOLOGY 


green,  but  which  can  react  to  blue  and  yellow.  Only  the 
more  central  part  of  the  retina  is  capable  of  being  stimulated 
by  all  colours.  These  zones  are  not  sharply  defined,  and 
vary  in  extent  with  the  size  and  brightness  of  the  coloured 
image  (fig.  75)  {Practical  Physiology). 

(2)  While  the  various  sensations  which  we  call  colour  are 
generally  produced  by  vibrations  of  different  lengths  falling 
on  the  retina,  colour  sensations  are  also  produced  in  various 
other  ways. 


Fig.  75.  — Distribution  of  Colour 
Sensation  in  relationship  to  the 
surface  of  the  retina  [Colour  Peri- 
meter). A  indicates  the  extent 
of  retina  stimulated  by  white  and 
black  ;  B,  the  part  also  capable  of 
stimulation  by  blue  and  j^ellow  ; 
and  C,  the  central  part  capable 
also  of  stimulation  by  red  and 
green. 


Fig.  76. — Disc  which,  when 
rotated  in  a  bright  light, 
gives  the  impression  of 
colours. 


(a)  By  mechanical  stimulation  of  the  retina.  By  press- 
ing on  the  eyeball  as  far  back  as  possible  a  yellow 
ring,  or  part  of  a  ring,  may  often  be  seen  {Practical  Physi- 
ology). 

(6)  Simple  alternation  of  white  and  black  upon 
the  retina  may  produce  colour  sensation,  as  when 
a  disc  of  paper  marked  with  lines,  as  shown  in  fig. 
76,  is  rotated  rapidly  before  the  eye  {Practical  Physi- 
ology). 


NERVE  157 

(3)  By  allowing  ditterent  parts  of  the  spectrum  to  fall 
upon  the  eye  at  the  same  time,  it  is  possible  to  pro- 
duce either  a  sensation  of  white  or  of  some  other 
part  of  the  spectrum  {Practical  Physiology).  To  pro- 
duce a  sensation  of  white  from  two  or  three  different 
parts  of  the  spectrum  a  due  proportion  of  each  part  must 
be  taken,  since  different  parts  have  different  sensational 
activity. 

This  effect  of  mixing  different  parts  of  the  spectrum 
means  that  hy  different  'modes  of  stimidation  of  the  retina 
the  same  sensation  may  he  produced.  The  sensation  of 
orange  may  be  produced,  either  when  vibrations  at  about 
5  80  billions  per  second  fall  on  the  eye,  or  when  two 
sets  of  vibrations,  one  about  640  and  one  about  560 
billions,  reach  it.  By  no  possible  physical  combination  of 
the  two  is  it  possible  to  produce  the  intermediate  rate  of 
vibration. 

The  sensation  of  colour,  therefore,  depends  upon  the  nature 
of  the  change  set  up  in  the  retina,  and  not  upon  the  condition 
producing  that  change. 

What  we  call  colours  are  particular  changes  in  conscious- 
ness accompanying  particular  changes  in  brain  neurons  pro- 
duced by  particular  changes  set  up  in  the  retina,  in  whatever 
way  these  changes  may  have  been  produced. 

Colour-blindness. — While  every  one  is  colour-blind  at  the 
periphery  of  the  retina,  a  certain  proportion  of  people — 
about  5  per  cent. — are  unable  to  distinguish  reds  and  greens, 
even  at  the  centre  of  the  retina.  Colour-blindness  for  yellow 
and  blue  is  very  rare. 

In  complete  loss  of  colour  vision — monochromatic  vision 
— everything  appears  grey,  and  usually  there  is  blindness  in 
the  middle  of  the  field  over  the  central  spot.  Possibly  it  is 
produced  by  a  loss  of  function  in  the  cones. 

It  is  not  known  whether  a  condition  of  colour-blindness 
exists  in  lower  animals. 

When  white  light  passes  through  a  lens  it  is  partly 
decomposed  into  its  component  parts,  the  blue  rays  being 
more  refracted  than  the  red,  and  chromatic  aberration,  the 
unequal  focussing  of  the  different  parts  of  the  spectrum  on 


158  VETERINARY   PHYSIOLOGY 

the  retina,  is  thus  made  possible.  The  way  in  which  the 
more  divergent  peripheral  rays  are  cut  off  by  the  iris  prevents 
this  from  manifesting  itself  in  vision  (p.  148). 


(II.)  Binocular  Vision. 
A.  Advantages. 

In  most  of  the  lower  animals,  the  field  of  vision  of  each 
eye  is  separate  and  distinct  at  all  times,  but  in  the  horse  and 
dog  the  two  eyes  can  be  directed  forwards  so  that  the  fields 
of  vision  overlap  as  they  always    do  in  man  and   in  apes. 


Fig.  77. — Corresponding  Areas  of  the  two  Retinte  in  Binocular  Vision.  The 
upper  and  outer  area  of  the  right  retina  corresponds  to  the  upper  and 
inner  area  of  the  left  retina,  and  the  other  areas  correspond  as  shown 
by  the  shading.  In  each  pair  of  areas  definite  points  correspond  with 
one  another,  a — a. 

When  in  this  position,  the  combined  action  of  the  eyes 
affords  a  means  of  determining  the  distance  and  solidity 
of  near  objects. 

1.  Distance  of  Near  Objects. — As  an  object  is  approached, 
the  two  eyes  have  to  be  turned  forwards  by  the  internal 
recti  muscles,  and  by  the  degree  of  contraction  of  these,  an 
estimation  of  the  distance  is  made. 

2.  Solidity  of  an  Object. — If  the  object  is  near,  a  slightly 
different  picture  is  given  on  each  retina,  and  experience  has 
taught  that  this  stereoscopic  vision  indicates  solidity, 

B.  Single  Vision  with  Two  Eyes, 

1.  Corresponding  Areas  of  the  Two  Retinae. — In  order  that 
single  vision  may  occur  with  the  two  eyes,  the  eyes  must 
be  directed  to  the  same  place,  as  can  be  done  in  the  horse  and 


NERVE 


159 


dog  in  certain  position  of  the  eyes,  so  that  the  image  of  that 
place  falls  on  each  central  spot  {Practical  Physiology).  If 
this  does  not  occur,  double  vision  results.  The  central 
spot  of  one  eye  thus  corresponds  to  the  central  spot  of 
the  other,,  and  definite  points  in  each  retina  have  corre- 
sponding points  in  the  other,  which,  when  the  eyes  are 
working  together,  are  stimulated  by  the  same  part  of  the 
picture  (fig.  77). 

2.  Movements  of  Eyeballs. — To  secure  harmonious  action 
of  the  two  retinae,  it  is  necessary  that  the  eyes  should  be 


Fig.  78.— The  left  Eyeball  in  the 
Orbit  from  above,  with  the 
muscles  acting  upon  it.  The 
axis  of  the  orbit,  b  ;  the  axis 
of  the  eyeball,  a. 


In  01 


£xR.. 


s.oi_ 


InR. 


InR. 


Fig.  79. — The  Movements  of  the  Eyeball 
caused  by  the  various  Muscles  of  the 
Eye.     (Right  Eye.) 


freely  movable.  Each  eye  in  its  orbit  forms  a  ball  and  socket 
joint  in  which  the  eyeball  moves  round  every  axis  (fig.  78). 
The  orbit  looks  forward  and  outwards,  but  the  axis  of  the  eye 
(a)  is  directed  straight  forward  and  is  set  obliquely  to  the 
axis  of  the  orbit  (6).  The  centre  of  rotation  is  behind  the 
centre  of  the  ball.  The  movements  are  produced  by  three 
pairs  of  muscles. 

(1)  The  internal  and  external  recti  {I.U.  and  Ex.R.). 

(2)  The  superior  and  inferior  recti  acting  along  the  liues 
indicated  (S.R.). 

(3)  The  superior  and  inferior  obliques  acting  in  the   line 
(S.oh.). 


160  VETERINARY   PHYSIOLOGY 

The  internal  rectus  rotates  the  pupil  inwards. 

external      ,,  ,,  ,,  outwards. 

f  upwards      and 
superior     ,,  ,,  ■' 


inferior 

superior  oblique 
inferior 


\       wards. 

(  downwards   and  in- 

(       wards. 

{downwards  and  out- 
wards. 
r  upwards     and    out- 
I       wards. 


In  directing  the  eyes  to  the  right,  the  external  rectus  of 
the  right  eye  acts  along  with  the  internal  rectus  of  the  left. 
In  directing  the  eyes  straight  upwards,  the  superior  rectus 
and  inferior  oblique  of  each  eye  act  together  ;  and  in  looking 
downwards,  the  inferior  rectus  and  superior  oblique  come  into 
play  (fig.  79).  Every  movement  of  the  eye  involves  the 
co-ordinated  excitation  and  inhibition  of  muscles  (see  p.  193). 

In  the  horse,  dog,  and  other  similar  animals  the  eye  is  set 
more  nearly  in  the  axis  of  the  orbit,  and  the  obliques  do  not 
pass  backwards  upon  the  ball,  but  act  more  purely  as  rotators; 
the  superior  oblique  swinging  the  outer  angle  of  the  pupil 
upwards  and  inwards,  the  inferior  oblique  downwards  and 
inwards.  The  superior  and  inferior  recti  move  the  pupil 
more  directly  upwards  and  downwards. 

In  the  horse  and  other  herbivora  a  retractor  oculi  muscle 
is  inserted  all  round  the  ball  inside  these  muscles  just 
described,  and  it  can  retract  the  eye  in  the  orbit,  and  at 
the  same  time  pushes  forward  the  fatty  tissue  to  which  the 
nictitating  membrane  is  attached  and  thus  thrusts  this  over 
the  front  of  the  eye. 

4.  "Glance"  Movements  of  the  Eyes. — When  the  eyes  are 
allowed  to  sweep  over  a  landscape  or  any  series  of  objects, 
or  when  these  move  rapidly  past  the  eyes,  or  the  eyes  rapidly 
past  them,  as  in  travelling  by  train,  the  axes  are  directed  in 
a  series  of  glances  to  different  points,  and  the  succession  of 
pictures  thus  formed  gives  the  idea  of  a  continuous  series  of 
objects.  The  jerking  movement  of  the  eyes  may  be  well  seen  in 
a  passenger  looking  out  of  a  railway  carriage  in  motion.  This 
"  glance  vision  "  is  taken  advantage  of  in  the  cinematograph. 


NERVE 


161 


5.  Nervous  Mechanism. — A  somewhat  complex  nervous 
mechanism  presides  over  these  various  movements  cf  the  eyes. 
All  the  muscles  are  supplied  by  the  third  cranial  nerve,  except 
the  superior  oblique,  which  is  supplied  by  the  fourth  nerve, 
and  the  external  rectus,  which  is  supplied  by  the  sixth  nerve 
(fig.  80). 

The  centres  for  the  third  and  fourth  nerves  are  situated 
in  the  floor  of  the  aqueduct  of 
Sylvius  under  the  corpora  quadri- 
gemina,  while  the  centre  for  the 
sixth  is  in  the  pons  Varolii  (fig. 
80). 

The  various  centres  are  joined 
by  bands  of  nerve  fibres  which 
pass  between  the  sixth  and 
fourth  and  third  centres,  and,  in 
part  at  least,  cross  the  middle 
line. 

A  combined  mechanism,  each 
part  of  which  acts  harmoniously 
with  the  other  parts,  thus  pre- 
sides over  the  ocular  movements, 
and  this  mechanism  is  controlled 
by  impulses  constantly  received 
(a)  from  the  two  retinae  ;  (6)  from 
the  ears  ;  and  (c)  from  the  brain. 
Thus,  in  convergence  of  the 
optic  axes,  the  parts  of  the 
nuclei  of  the  third  nerves  which 
supply  the  internal  recti  muscles 
must  act  harmoniously  together, 
and  hence  a  mechanism  to  direct 

this  convergence  may  be  postulated.  In  lateral  deviation  of 
the  eyes,  such  as  is  reflexly  produced  by  sudden  auditory  or 
visual  stimulation  on  one  side,  that  part  of  the  nucleus  of 
the  third  nerve  which  presides  over  the  internal  rectus  of 
one  side  acts  harmoniously  with  the  sixth  nerve  supplying 
the  external  rectus  of  the  other  side,  and  hence  it  may  be 
supposed  that  a  directing  mechanism  for  lateral  deviation 
11 


Fig.  80.— The  Nervous  Mechan- 
ism presiding  over  thecombined 
movements  of  the  two  Eyes. 
I.R.,  internal  rectus;  E.R., 
external  rectus;  G.O.,  con- 
vergent centre  acting  on  the 
internal  recti  through  the 
nuclei  of  the  third  nerve  ;  S.O., 
superior  olive  (centre  for 
lateral  divergence)  acting  on 
the  external  rectus  of  the  same 
side  through  the  nucleus  of  the 
sixth,  and  on  the  internal  rectus 
of  the  opposite  side  through  the 
nucleus  of  the  third  ;  E.,  ear. 


162  VETERINARY   PHYSIOLOGY 

exists,  possibly  in  the  superior  olive  (fig.  80).  Similarly  a 
centre  or  centres  presiding  over  the  movements  of  the  eyes 
in  a  vertical  plane  may  be  supposed  to  exist. 

6.  Disturbances  of  the  Neuro-Muscular  Mechanism.— Paralysis 
of  these  muscles  and  of  the  nerves  supplying  them  leads 
to  a  loss  of  the  co-ordinated  movements  of  the  two  eyes, 
with  the  result  that  the  optic  axes  are  no  longer  parallel 
and  squint  is  produced.  As  a  result  of  this,  corresponding 
points  on  the  two  retinae  are  not  stimulated  by  rays  from  the 
same  object,  and  double  vision,  diplopia,  results  in  men  and 
apes. 

III.  CONNECTIONS  OF  THE  EYES  WITH  THE  CENTRAL 
NERVOUS  SYSTEM. 

1.  THE  OPTIC  NERVES  AND  OPTIC  TRACTS. 

From  each  eye  the  optic  nerve  extends  backwards  and 
inwards  to  join  the  other  optic  nerve  at  the  chiasma.  A 
partial  crossing  of  the  fibres  takes  place  in  the  chiasma,  the 
extent  of  decussation  varying  in  difterent  animals  and  being 
fairly  extensive  in  the  horse.  From  the  chiasma  the  two  optic 
tracts  pass  upwards  round  the  crura  cerebri  to  end  in  two 
divisions — 

1.  A  posterior  division  passing  to  the  anterior  colliculus 
of  the  tectum  on  the  same  side  (fig.  81,  A.G.Q.). 

2.  An  anterior  division  running  to  the  external  geniculate 
body  on  the  posterior  aspect  of  the  thalamus  opticus  (fig. 
81,  02).Th.). 

In  most  of  the  lower  animals  a  complete  decussation  of 
the  optic  fibres  takes  place,  so  that  the  nerve  fibres  from 
the  left  eye  go  to  the  right  side  of  the  brain,  and  vice 
versa.  But  in  man  and  apes  a  partial  crossing  of  the  fibres 
takes  place  in  the  chiasma — fibres  from  the  middle  and 
internal  part  of  the  retina  decussating,  those  from  the 
outer  part  remaining  on  the  same  side.  For  this  reason, 
section  of  the  right  optic  tract  leads  to  partial  blind- 
ness of  both  retinae — on  the  outer  part  of  the  right  eye 
and  on  the  inner  and  middle  part  of  the  left  eye,  so  that 
objects  on  the  left  side  of  the  field  of  vision  are  not  seen. 


NERVE 


163 


The  fibres  of  the  posterior  division  of  each  optic  tract  ends 
in  synapses  with  neurons  in  the  anterior  colHculus  of  the 
tectum,  and  the  fibres  of  these  neurons  pass  downwards  and 
control  the  oculo-motor  mechanism  already  described  (fig.  81). 

The  fibres  of  the  anterior  division  make  synapses  with 
other  neurons  in  the  external  geniculate  body  and  pulvinar 


Fig. 


Dec.  L. 

W. — The  Connections  of  the  Retinae  with  the  Central  Nervous  System. 
R,  retinae;  Gh.,  chiasnia  leading  to  optic  tract;  Op.Th.,  optic 
thalamus  ;  A.C.Q.,  anterior  coUiculus  of  the  tectum  ;  Oc.M.,  oculo- 
motor mechanism  (fig.  80)  ;  Occ.L.,  occipital  lobe  of  the  cerebrum. 


of  the  thalamus,  and  in  the  thalamus  visual  impressions 
become  associated  and  integrated  with  those  of  other 
receptors — touch,  hearing,  etc.  (fig.  81,  Op.Th.-,  see  also 
fig.  50,  p.  113). 

From  the  thalamus  a  great  band  of  fibres,  which  early 
get  their  white  sheath,  sweeps  outwards  and  backwards  to 
cortex  on  the  internal  aspect  of  the  occipital  lobe. 


164 


VETERINARY  PHYSIOLOGY 


2.  THE  VISUAL  CENTRE  IN  THE  CORTEX  CEREBRI. 

An  extensive  lesion  of  one — say  the  right — occipital  lobe, 
(fig.  82),  is  accompanied  by  no  loss  of  muscular  power 
but  by  blindness  for  all  objects  in  the  opposite  side  of  the 
field  of  vision — i.e.  the  right  side  of  each  retina  is  blind. 
The  central  spot  of  neither  eye  is  completely  blinded,  because 


ECTOSYLVrAN 
ECTOSYCVIAN.  B... 


PARIETAL 

VISUAL 


S  Orl-LtiUs.. 

S  CruciattLS. 
■S.Corona,lis. 
•"H  omologue 
oj  Rolauio 


■5Su,|)riSi)lviui 
S  Lateralis. 


SfctoUteralis. 

Caltirine 
Post-calcaTtne 


SPosUatevaliJ. 


i\.     TuefRCULJM    OLFACl 


OLFACTORY.  A.. 


OLFAOTORV  B 


oOf..rXTRA-RHINie. 


Fig.  82. — Superior  and  inferior  aspects  of  the  brain  of  the  dog  to  show  the  various 
sulci  and  the  distribution  of  the  chief  receiving  and  reacting  mechanisms. 
Sensory  is  body  sensibility. 


the  fibres  from  the  macula  lutea  only  partially  decussate  at 
the  chiasma. 

Stimulation  in  this  region  in  the  monkey  causes  move- 
ment of  the  eyes  to  the  opposite  side,  as  if  some  object  were 
perceived  there.  But  destruction  of  the  area  does  not 
paralyse  the  movements  of  the  eyes. 

In  man,  and  the  higher  apes,  the  cortex  in  this  region 
shows  a  definite  arrangement  of  cells,  and  a  well-defined 
band  of  white  fibres  in  the  middle  of  the  layer  of 
granules  (p.  183),  which  is  thus  duplicated  (fig.  92). 
This  is  the  band  of  Gennari.  In  long-standing  cases  of 
blindness  the  thickness  of  the  layer  of  Gennari  may  be 
reduced  by  50  per  cent.,  and  of  the  outer  layer  of  granules 
by  10  per  cent. 

The  special  characters  of  this  area  become  more  and  more 


NERVE  165 

defined  as  we  pass  from  the  simpler  mammals,  with  separate 
vision  for  the  two  eyes,  to  the  primates  with  combined  vision 
with  the  two  eyes. 

The  evidence  that  it  is  this  part  of  the  brain  which  must 
be  stimulated  in  order  that  visual  sensations  may  be  experi- 
enced is  convincing.  But  it  must  be  remembered  that  there 
are  lower  arcs  connecting  with  the  oculo-motor  mechanism 
from  the  optic  nerves — (1)  through  the  thalamus  and  red 
nucleus,  and  (2)  through  the  anterior  part  of  the  tectum,  and 
that  stimulation  of  the  eye  may  lead  to  movements  of  the 
body  and  of  the  eye  througli  these  without  consciousness 
being  involved.  This  is  seen  in  the  reaction  of  the  pupil  to 
light  and  in  the  reflex  movements  of  the  eyes  in  response  to 
unilateral  auditory  stimulation. 

The  centre  is  united  by  a  strong  band  through  the  middle 
peduncle  with  the  cerebellum  (see  p.  130),  and  in  the  higher 
apes  by  another  band  of  fibres  with  an  area  in  the  frontal 
region,  concerned  with  the  movements  of  the  eyes,  and  this 
is  again  connected  with  the  cerebellum  (fig.  58). 

Usually  the  visual  area  is  stimulated  by  changes  passing 
up  the  series  of  neurons  from  the  retina,  but  it  may  also  be 
stimulated  directly,  as  is  sometimes  seen  in  the  early  part  of 
an  epileptic  fit,  or  by  the  previous  action  of  other  chains  of 
neurons,  as  in  dreaming. 

The  strength  of  the  sensation  varies  with  the  strength  of 
the  stimulus,  and  the  smallest  difference  of  sensation  which 
can  be  appreciated  is  a  constant  factor  of  the  degree  of 
stimulation. 

The  sensation  lasts  longer  than  the  stimulus,  and  thus, 
if  a  series  of  stimuli  follow  one  another  at  sufficiently  rapid 
intervals,  a  fusion  of  sensations  is  produced.  If  a  wheel 
rotating  slowly  is  looked  at,  the  individual  spokes  are  seen, 
but  when  it  is  going  more  rapidly,  the  appearance  of  a 
continuous  surface  is  presented.  When  the  light  is  dim, 
this  fusion  takes  place  more  readily  than  when  the  light  is 
bright. 

The  visual  centre  of  each  side  must  be  regarded  as  a 
chart  of  the  opposite  field  of  vision,  each  part  corresponding 


166  VETERINARY   PHYSIOLOGY 

to  a  particular  part  of  the  field.  The  two  centres  acting 
together  give  the  whole  field  of  vision.  Since  the  blind  spot 
is  not  represented  in  the  centre,  it  is  not  perceived  in  the 
field  of  vision. 

The  centre  is  said  to  rectify  the  inverted  image  formed 
on  the  retina,  but  this  simply  means  that,  as  a  result  of 
experience,  we  have  learned  that  changes  in,  say,  the  lower 
part  of  the  retinae,  and  in  the  corresponding  parts  of 
the  visual  centres,  are  produced  by  light  from  above 
the  head. 

Since  the  retinal  changes  vary  only  with  the  degree  of 
illumination,  i.e.  the  amplitude  of  vibration  of  the  ethereal 
waves,  and  with  the  rate  of  these  waves,  and  since  the  part 
of  the  retina  acted  on  is  determined  by  the  direction  of  the 
rays,  we  have  the  means  of  getting  a  flat  picture  only  of  what 
we  look  at,  but  no  special  arrangements  for  having  different 
sensations  according  to  the  distance  of  an  object  or  according 
to  whether  it  is  flat  or  in  relief.  The  means  of  determining 
the  size,  distance,  and  form  of  objects  by  the  visual  mechan- 
ism is  very  limited  (p.  138). 


III.  FOR  VIBRATION  OF  AIR. 

SENSE  OF  HEARING. 

1.  General  Considerations. 

The  vibratory  changes  of  pressure  in  the  air,  known  to 
physicists  as  sound  waves,  stimulate  special  receptor  struc- 
tures placed  in  the  ear  of  higher  animals.  Even  simple 
organisms,  devoid  of  any  special  organ  of  hearing,  may  be 
affected  by  vibratory  changes,  and  in  fish  it  is  difficult  to  be 
certain  how  far  such  vibrations  produce  their  effect  through 
the  ear  or  through  the  body  generally.  But  in  higher 
vertebrates  it  is  chiefly  through  the  ear  that  they  act.  In  it 
there  is  a  special  arrangement  by  which  the  vibrations  of  the 
air  are  converted  into  vibrations  of  a  fluid  in  a  sac  situated 
in  the  side  of  the  head.  In  this  a  series  of  ciliated  cells  is 
situated,  and  round  these  are  the  dendritic  terminations  of 
the  auditory  nerve  fibres. 


NERVE  167 

The  importance  of  such  a  mechanism  in  the  anterior  part 
of  the  animal  in  warning  it  of  danger  or  making  it  aware  of 
the  presence  of  its  prey  is  manifest. 

In  mammals  the  organ  of  hearing  consists  of  the  external, 
the  middle,  and  part  of  the  internal  ear.  The  purpose  of 
the  first  is  to  conduct  the  vibrations  of  the  air  to  the  second. 
in  which  these  vibrations  produce  to-and-fro  movements  of  a 
bony  lever,  by  which  the  fluid  in  the  third  is  alternately 
compressed  and  relaxed. 

2.   External  Ear. 

The  structure  of  this  presents  no  point  of  special  physio- 
logical interest.  In  lower  animals  the  pinna  is  under  the 
control  of  muscles,  and  is  of  use  in  determining  the  direction 
from  which  sound  comes. 

3.   Middle  Ear. 

The  object  of  the  middle  ear  is  to  overcome  the  mechanical 
difiiculty  of  changing  vibrations  of  air  into  vibrations  of  a  fluid. 

It  consists  of  a  chamber,  the  tympanic  cavity,  in  the 
petrous  part  of  the  temporal  bone  (fig.  83).  Its  outer  wall 
is  formed  by  a  membrane,  the  membrana  tympani  {Ty.),  which 
is  attached  to  a  ring  of  bone.  Its  inner  wall  presents  two 
openings  into  the  internal  ear — the  fenestra  ovalis  (/.o.),  an 
oval  opening,  situated  anteriorly  and  above,  and  the  fenestra 
rotunda  (/.r.),  a  round  opening  placed  below  and  behind. 
Throughout  life  these  are  closed,  the  former  by  the  foot  of 
the  stapes,  which  is  attached  to  the  margin  of  the  hole  by  a 
membrane,  the  latter  by  a  membrane  only.  The  posterior 
wall  shows  openings  into  the  mastoid  cells,  and  presents  a 
small  bony  projection  which  transmits  the  stapedius  muscle. 
The  anterior  wall  has  (a)  above,  a  bony  canal  carrying  the 
tensor  tympani  muscle,  and  (6)  below  this,  the  canal  of  the 
Eustachian  tube  which  communicates  with  the  posterior  nares 
(fig.  83,  En.T.). 

In  the  tympanic  cavity  are  three  ossicles — the  malleus  {im.), 
incus  (i.),  and  stapes  (s.),  forming  a  chain  between  the  mem- 


168  VETERINARY  PHYSIOLOGY 

brana  tympani  and  the  fenestra  ovalis.  The  handle  of  the 
malleus  is  attached  to  the  membrana  tympani,  and  each  time 
a  wave  of  condensation  falls  upon  the  membrane  it  drives 
this  inwards,  and  with  it  the  handle  of  the  malleus.  This 
locks  the  malleo-incal  joint  (fig.  83,  7n.i.),  and  pushes  inwards 
the  long  process  of  the  incus,  which  thrusts  the  stapes  into 
the  fenestra  ovalis,  and  thus  increases  the  pressure  in  the 
enclosed  fluid  of  the  internal  ear.  By  the  "  give "  of  the 
membrane  in  the  fenestra  rotunda,  the  increased  pressure  is 
relieved.  The  bones  rotate  round  an  antero-posterior  axis 
passing  through  the  heads  of  the  malleus  and  incus.     They 


ZkM 


LnT. 

Fig.  83. — Diagram  of  the  Ear.  Ex.M.,  external  meatus;  Ty.,  tympanic 
membrane;  m.,  malleus;  i.,  incus;  s.,  stapes  ;  /.o.,  fenestra  ovalis; 
f.r.,  fenestra  rotunda;  En.T.,  Eustachian  tube;  v.,  vestibule  with 
the  utricle  and  saccule  ;  s.c,  semicircular  canal ;  Coch.,  cochlea. 

thus  form  a  lever  with  the  arm  to  which  the  power  is  applied 
— the  handle  of  the  malleus — longer  than  the  other  arm. 
The  advantage  of  this  is  that,  while  the  range  of  movement 
of  the  stapes  in  the  fenestra  ovalis  is  reduced,  its  force  is 
proportionately  increased,  A  further  increase  of  force  is 
gained  by  the  small  size  of  the  fenestra  ovalis  as  compared 
with  the  tympanic  membrane. 

On  account  of  the  fact  that  the  annular  ligament  is 
narrow  on  one  side  and  broader  at  the  other,  the  movement 
of  the  foot  of  the  stapes  is  not  a  straight  thrust  but  a  move- 
ment round  a  hinge. 


NERVE  169 

If  the  membrana  tympani  is  forced  violently  outwards  by 
closing  the  nose  and  mouth  and  forcing  air  up  the  Eustachian 
tube,  as  in  one  clinical  method  of  overcoming  obstruction  of 
the  Eustachian  tube,  the  incus  and  stapes  do  not  accompany 
the  malleus  and  the  membrane,  because  the  malleo-incal 
articulation  becomes  unlocked,  and  the  head  of  the  malleus 
slides  on  the  incus. 

The  membrana  tympani  is  so  loosely  slung  that  it  has  no 
proper  note  of  its  own,  and  responds  to  a  very  large  range 
of  vibrations.  By  the  attachment  to  it  of  the  handle  of  the 
malleus  it  is  well  damped  and  stops  vibrating  as  soon  as 
waves  of  condensation  and  rarefaction  have  ceased  to  fall 
upon  it. 

Two  slender  muscles  are  attached  to  the  ossicles — 

(1)  The  stapedius  is  carried  from  the  posterior  wall  of  the 
cavity  and  is  inserted  into  the  orbicular  process  of  the  stapes. 
It  tends  to  twist  the  stapes  in  the  oval  window  and  so  to 
limit  its  range  of  movement  inwards  and  to  favour  its  out- 
ward movement.  It  is  supplied  by  the  seventh  cranial  nerve, 
and  in  paralysis  of  this  loud  sounds  may  be  heard  with 
painful  intensity. 

(2)  The  tensor  tympani  passes  from  the  inner  walls  of  the 
cavity  outwards  to  the  base  of  the  handles  of  the  malleus.  It 
tends  to  pull  the  tympanic  membrane  inwards  and  to  thrust 
the  head  of  the  stapes  further  into  the  oval  window.  It  is 
supplied  by  the  fifth  cranial  nerve,  and  when  this  is  paralysed 
hearing  is  impaired. 

These  two  muscles  are  thus  antagonistic  to  one  another 
and,  as  in  the  case  of  other  antagonists,  their  action  is  prob- 
ably co-ordinated.  They  must  exercise  a  balancing  action 
on  the  movement  of  the  stapes  inwards  during  the  positive 
phase  of  a  sound  wave  and  outwards  during  the  succeeding 
negative  phase. 

The  Eustachian  tube  has  a  double  function.  It  allows  the 
escape  of  mucus  from  the  middle  ear,  and  it  allows  the 
entrance  of  air,  so  that  the  pressure  is  kept  equal  on  both 
sides  of  the  membrana  tympani.  Its  lower  part  is  generally 
closed,  but  opens  in  the  act  of  swallowing.  This  part  is 
surrounded  by  an  arch  of  cartilage  to  one  side  of  which  fibres 


170 


VETERINARY   PHYSIOLOGY 


of  the  tensor  palati  are  attached,  so  that,  when  this  muscle 
acts  in  swallowing,  the  arch  of  cartilage  is  drawn  down  and 
flattened,  and  the  tube  opened  up 
(fig.  84).  The  Eustachian  tube  may 
get  occluded,  as  a  result  of  catarrh  of 
the  pharynx,  and  the  oxygen  in  the 
middle  ear  is  then  absorbed  by  the  tissues, 
and  the  pressure  falls.  As  a  result,  the 
membrane  is  driven  inwards  by  the 
atmospheric  pressure,  and  does  not 
readily  vibrate,  and  hearing  is  impaired. 
When  this  blocking  occurs,  and  it  is 
necessary  to  force  air  into  the  middle 
ear,  a  tube  connected  with  a  rubber 
ball  is  inserted  into  a  nostril,  the 
mouth  and  other  nostril  are  closed, 
and,  as  the  patient  swallows,  air 
is  forced  into  the  naso  -  pharynx  and 
so  through  the  Eustachian  tube. 


Fig.  84.— Trans- 
verse Section 
through  the  car- 
tilaginous lower 
part  of  Eustachian 
Tube,  to  show  the 
cartilaginous  arch 
cut  across,  and  the 
way  in  which  it  is 
pulled  down  and 
the  tube  opened 
in  swallowing 
(shaded). 


4.   Internal  Ear. 

The  internal  ear  is  a  somewhat  complex  cavity  in  the 
petrous  part  of  the  temporal  bone,  the  osseous  labyrinth.  It 
is  filled  with  fluid,  the  perilymph.  It  consists  of  a  central 
space,  the  vestibule  (F.),  into  which  the  fenestra  ovalis  opens. 
From  the  anterior  part  of  this,  a  canal  makes  two  and  a  half 
turns  round  a  central  pillar.  This  is  the  osseous  cochlea 
(fig.  83,  Cock.).  From  the  central  pillar  a  bony  shelf 
(fig.  85,  L.)  projects  into  the  canal.  From  the  edge  of  the 
bony  shelf  the  basilar  membrane  extends  to  the  outer  wall  of  the 
cochlea  (fig,  85,  B.M.).  It  is  composed  of  fibres  radiating 
outwards.  The  inner  is  more  elastic  than  the  outer  part. 
At  the  base  of  the  cochlea  the  bony  lamella  is  broad,  but  at 
the  apex  its  place  is  chiefly  taken  by  the  membrane,  which 
there  measures  about  three  times  its  width  at  the  base. 

A  canal  which  communicates  with  the  vestibule,  into  which 
the  oval  window  opens,  called  the  scala  vestihuli  (fig.  85,  S.V.), 
thus  runs  above  the  bony  shelf  and  basilar  membrane,  while 
another  canal,  the  scala  tymimni,  runs  below  them,  and  ends 


NERVE 


171 


at  the  round  window.  They  communicate  with  one  another 
through  a  small  opening  at  the  apex,  the  helicotrema. 

The  semicircular  canals  (fig.  54)  also  open  from  the 
vestibule.      Their  functions  have  been  discussed  (p.  121). 

A  complex  membranous  bag,  the  membranous  labyrinth,  lies 
in  the  perilymph  of  the  bony  labyrinth.  This  has  an  outer 
fibrous  coat,  and  inside  this  a  homogeneous  layer. 

In  the  vestibule  the  membranous  labyrinth  consists  of  the 
utricle  and  saccule,   the  functions  of  which  are  related  to 


/  B.M. 


Fig.  85. — Transverse  Section  through  one  turn  of  the  Cochlea  to  show  the 
Organ  of  Corti  on  the  Basilar  Membrane.  S.M.,  scala  media;  S.V., 
scala  vestibuli ;  S.T.,  scala  tympani.  The  tectorial  membrane 
normally  lies  upon  the  hair  cells  of  Cortis  organ. 

those  of  the  canals.  From  the  saccule  a  narrow  canalis 
reuniens  runs  to  the  membranous  cochlea,  which  lies  upon 
the  basilar  membrane,  as  the  scala  media,  between  the  scala 
vestibuli  and  scala  tympani  and  ends  blindly  at  the  apex  of 
the  cochlea  (fig.  85,  8.M.). 

In  the  membranous  cochlea,  the  organ  of  Corti  (fig.  85)  is 
formed  by  a  special  development  of  the  epithelium  lining  the 
tube.  It  is  set  upon  the  inner  more  elastic  part  of  the 
basilar  membrane,  and  consists  from  within,  outwards,  of — 
1st.  A  set  of  elongated  supporting  cells ;  2nd.  A  row  of 
columnar  cells,  with  short,  stiff,  hair-like  processes  projecting 


172  VETERINARY   PHYSIOLOGY 

from  their  free  border  ;  Srd.  The  inner  rods  of  Corti,  each  of 
which  may  be  compared  to  an  ulnar  bone  attached  by  its 
terminal  end,  and  fitting  on  to  the  heads  of  the  outer  rods ; 
4!th.  The  outer  rods  of  Corti,  each  resembling  a  swan's  head 
and  neck — the  neck  attached  to  the  basilar  membrane,  and 
the  back  of  the  head  fitting  into  the  hollow  surface  of  the  inner 
rods.  The  two  sets  of  rods  thus  form  an  arch ;  5th.  Several 
rows  of  outer  hair  cells,  with  some  spindle-shaped  cells  among 
them  ;  6th.  The  outer  supporting  cells  ;  7th.  Lying  over  the 
inner  and  outer  hair  cells  is  the  membrana  reticularis, 
which  resembles  a  net,  and  through  the  meshes  of  which 
the  hairs  project  ;  8th.  Lying  upon  this  organ,  with  the 
cilia  of  the  hair  cells  embedded  in  it,  is  a  homogeneous 
membrane — the  membrana  tectoria,  the  inner  margin  of 
which  is  firmly  attached  to  the  denticulate  lamina  as  shown 
in  fig.  85  and  fig.  87. 

The  terminal  neurons  both  to  the  vestibule  and  to  the 
cochlea  end  in  dendrites  among  the  hair  cells. 

5.  Connections  with  the  Central  Nervous  System. 

The  dorsal  or  auditory  part  of  the  VIII.  nerve  is  the  true 
nerve  of  hearing,  (a)  Its  fibres  (fig.  86)  (Coch.R.)  begin  in 
dendrites  between  the  hair  cells  of  the  organ  of  Corti,  and  have 
a  neuron -cell  upon  their  course.  When  they  enter  the 
medulla,  they  branch  into  two  divisions,  which  end  either 
in  the  tuberculum  acusticum  or  in  the  nucleus  accessorius 
(N.Acc.),  where  they  form  synapses. 

(b)  From  these  cells  axons  pass  across  the  middle  line 
in  the  striae  meduUares  and  corpus  trapezoideum,  and, 
turning  sharply  upwards,  run  as  the  lateral  fillet  to  the 
posterior  colliculi  of  the  tectum  of  the  opposite  side.  Here 
many  of  the  fibres  form  synapses,  and  fresh  fibres  pass  to  the 
oculo-motor  mechanism.  When  the  cochlea  is  destroyed  in 
young  animals,  the  opposite  posterior  colliculus  atrophies. 

(c)  The  rest  of  the  fibres  run  onwards  to  the  medial 
geniculate  body  of  the  thalamus  where  synapses  are  formed. 

(d)  From  these  fresh  fibres,  which  get  their  white  sheath 
early,  course  outwards  to  the  cortex  cerebri  in  the  superior 
temporo-sphenoidal  lobe. 


NERVE 


173 


6.  Auditory  Centre  in  the  Cortex  Cerebri. 

Ferrier,  by  removing  the  superior  temporo-sphenoidal  lobe 
in  the  monkey,  produced  no  motor  disturbance,  but  found 
evidence  of  loss  of  hearing  in  the  opposite  ear.  When  the 
reo'ion  was  stimulated,  he  found  that  the  monkey  pricked  up 


Fig.  86. — Connections  of  Cochlea  with  the  Central  Nervous  System.  Coch.R., 
cochlear  root  of  eighth  nerve  ;  N.Acc.,  tuberculum  acusticum  and 
nucleus  accessorius  sending  fibres  to  the  cerebrum  (C.B.)  and  to  the 
oculo-motor  mechanism  (^'.  vi.)  through  the  posterior  colliculi ; 
C.B.L.,  cerebellum  ;  G.M.,  the  corpus  genieulatum  mediale. 

its  ears  and  looked  to  the  opposite  side,  and  he  considered 
that  these  observations  prove  the  existence  of  a  special 
localised  mechanism  for  the  reception  of  stimuli  from  the  ear. 

The   receptor  arrangements   in  the    ear  enables    loudness 
— amplitude  of  vibration  ;    pitch — rate     of    vibration  ;     and 


174  VETERINARY   PHYSIOLOGY 

quality — the  character  of  the  sound  given  by  the  over-tones 
to  be  distinguished.  The  perception  of  this  last  is  essentially 
a  perception  of  pitch. 

Loudness. — It  is  easy  to  understand  how  the  peripheral 
neurons  in  the  internal  ear  are  more  powerfully  stimulated 
by  the  greater  variations  in  the  degree  of  pressure  which  are 
produced  by  more  powerful  aerial  waves,  and  how  the 
greater  stimulation  of  the  receptive  centre  in  the  brain  will 
be  accompanied  by  a  sensation  of  greater  loudness. 

Pitch. — The  auditory  mechanism  has  an  extraordinary  power 


z 


T. 

C. 


i  y\^ ;W.:>  R.M. 


I.R.C.  O.R.C.  H.C. 


Fig,  87.  — To  show  the  movements  of  the  basilar  membrane  with  the  passage 
of  a  sound  wave,  and  the  manner  in  which,  by  the  displacement  of 
Cortis'  arch  I.R.C,  O.R.C,  the  reticular  membrane  pulls  upon  the 
cilia  C,  which  are  embedded  in  the  tectorium  T.,  and  thus  may 
stimulate  the  nerve  endings  N. 

of  enabling  the  appreciation  of  the  pitch  and  quality  of  sound, 
and  of  enabling  complex  sounds  to  be  analysed.  How  it 
does  so  is  still  somewhat  problematical.  Helmholtz  main- 
tained that  it  is  by  resonance,  the  fibres  of  the  basilar 
membrane  acting  like  the  strings  of  a  piano,  each  one  of 
which,  or  each  set  of  which,  is  made  to  vibrate  by  a  particular 
note  and  its  overtones,  and  thus  to  stimulate  the  nerve  endings 
among  the  hair  cells  situated  upon  these  parts  of  the  mem- 
brane. Some  experiments  on  dogs,  in  which  the  apprecia- 
tion of  notes  of  lower  pitch  was  apparently  lost  after  the 
upper  turns  of  the  cochlea  were  destroyed,  and  some  few 
cases   of  partial  destruction  of   the  cochlea    in    the  human 


NERVE  175 

subject  with  alterations  in  pitch  perception,  give  support  to 
this  theory. 

But  it  has  recently  been  pointed  out  by  Wrightson  that 
there  is  no  real  proof  of  this  theory,  and  that  it  assumes  that 
the  air  waves  are  propagated  up  the  scala  vestibuli,  through 
the  helicotrema,  and  down  the  scala  tympani. 

It  has  further  been  urged  that  the  small  size  of  the 
opening  at  the  helicotrema  and  the  large  extent  of  scala 
media  exposed  to  the  perilymph  must  tend  to  favour  a  trans- 
mission of  pressure  across  this  scala  from  the  scala  vestibuli 
to  the  scala  tympani,  and  so  to  the  round  window. 

Now  this  would  cause  a  displacement  of  the  basilar 
membrane — 

1st.   Downwards. 

2nd.   Back  to  the  horizontal. 

3rd.   Upwards. 

4th.  Back  to  the  horizontal,  as  in  fig.  87. 
This  would  result  in  a  lateral  displacement  of  Cortis'  arch 
round  the  base  of  the  internal  rods  as  a  hinge,  and  this 
would  lead  to  a  pull  first  in  one  direction  then  in  the  other 
of  the  reticular  membrane,  which  would  lead  to  a  bending  of 
the  cilia  embedded  in  the  tectorial  membrane,  which  is  firmly 
attached  to  the  denticulate  lamina,  and  this  might  stimulate 
the  cells,  thus  stimulating  the  nerve  endings  and  so  leading 
to  stimulation  of  the  auditory  centre. 

This  view  of  the  action  of  the  ear  brings  hearing  into 
accord  with  the  sense  of  touch,  and  it  is  of  interest  that  the 
otic  vesicle  is  developed  as  an  invagination  of  the  skin.  The 
mechanism  is  a  most  delicate  one  for  "weighing"  the  small 
variations  of  pressure  which  constitute  sound  waves. 

Such  a  theory  serves  to  explain  the  possibility  of  notes 
of  different  pitch  producing  different  effects  in  the  short 
stumpy  cochlea  of  the  bird,  which  is  difficult  to  understand 
on  Helmholtz's  theory. 


176  VETERINARY    PHYSIOLOGY 


IV.  THE  METHODS  BY  WHICH  THE   RECEIVING 
AREAS  HAVE  BEEN  LOCALISED. 

The  determination  of  the  exact  parts  of  the  cortex 
cerebri  concerned  with  the  reception  of  the  incoming  impres- 
sions from  the  different  peripheral  receptors  has  proved  to  be 
by  no  means  easy,  but  the  combination  of  various  methods 
has  overcome  these  difficulties  and  has  enabled  the  localisation 
to  be  made  with  exactness. 

1st.  ExpeTimental  Methods. — (a)  Sensations  are  the 
usual  accompaniment  of  the  activity  of  the  receiving 
mechanism.  But,  in  the  lower  animals,  it  is  not  possible  to 
have  a  direct  expression  of  whether  or  not  sensations  are 
experienced,  and  therefore,  in  determining  whether  removal 
of  any  part  of  the  brain  has  taken  away  the  power  of  receiv- 
ing impressions,  we  have  to  depend  upon  the  absence  of  the 
usual  mode  of  response  to  the  given  stimulus.  But  the 
absence  of  this  may  mean,  not  that  the  receiving  mechanism 
is  destroyed,  but  either  that  the  reacting  mechanism  is  out 
of  action,  or  that  the  channels  of  conduction  have  been 
interfered  with  (see  fig.  88). 

Thus,  if  light  be  flashed  in  the  eye  of  a  monkey,  it 
responds  by  glancing  towards  the  source  of  illumination  ; 
and  if  this  movement  is  absent  it  may  be  due  to  (1)  loss  of 
the  receiving  mechanism  ;  (2)  loss  of  the  mechanism  causing 
the  movements ;  or  (3)  interruption  of  the  channels  be- 
tween them. 

Again,  it  is  quite  possible  that,  after  removing  the 
receiving  mechanism  in  the  cerebrum,  external  stimuli  may 
lead  to  the  usual  response  hy  acting  through  lower  reflex 
arcs  (fig.  88).  If,  for  instance,  we  suppose  the  receiving 
part  of  the  cerebrum  connected  with  the  reception  of 
tactile  impressions  to  be  entirely  destroyed,  scratching  the 
sole  of  the  foot  may  still  cause  the  leg  to  be  drawn  up, 
just  as  if  a  sensation  had  been  experienced.  Here,  although 
the  upper  arc  is  out  of  action,  the  lower  arc  still  acts. 

(6)  In  the  lower  animals,  stimulation  of  a  part  of 
the  brain,  if  it  be  connected  with  the  reception  of  impres- 


NERVE 


177 


sions,  may  cause  the  series  of  movements  which  naturally 
follow  such  an  impression.  But  these  movements  may  also 
be  caused  by  directly  stimulating  the  reacting  mechanism. 

Wlien,  however,  removal  of  a  part  of  the  brain  causes  no 
loss  of  power  of  moveinent,  and  yet  prevents  a  stimulus 
frovi  causing  its  natural  response,  it  is  justifiahle  to 
conclude  that  that  p)art  of  the  brain  is  connected  with 
reception. 

2nd.    Clinical  and  Patholo- 
gical    Methods. — In     man,     the  ..-- •-._ 

chief  difficulty  of  obtaining  infor- 
mation is  in  finding  cases  where 
only  a  limited  part  of  the  brain 
is  affected.  But  such  cases  have 
been  observed.  Tumours  of  the 
inner  aspect  of  an  occipital  lobe, 
for  instance,  have  been  found  to 
be  associated  with  loss  of  visual 
sensations  without  loss  of  mus- 
cular power,  and  thus  the  con- 
clusion has  been  drawn  that  this 
part  of  the  occipital  lobe  is  the 
receiving  mechanism  for  stimuli 
from  the  eyes. 

3rd  Pathological.  —  As  a 
result  of  the  destruction  of 
certain  parts  of  the  nervous 
system,  in  the  region  of  the 
thalamus,  either  by  experiment 
in  the  lower  animals  or  by  disease 
in  man,  a  degeneration  of  nerve 
fibres  may  occur  to  some  definite 
region  of  the  cortex,  and  this  generally  shows  that  the  area 
is  a  receiving  one. 

4ith.  Anatomical  Methods. — When  it  has  been  found 
possible  to  assign  a  definite  function  to  any  area  of  the 
cortex,  its  extent  and  limits  may  be  determined  by  the 
extent  and  distribution  of  the  particular  character  of  the 
arrangement  and  structure  of  the  nerve  cells. 
12 


Fig.  88. — Diagram  to  Illustrate 
Different  possible  Channels  of 
Cerebral  Response  to  Stimula- 
tion, and  to  show  how,  through 
reflex  action  of  the  lower  arcs, 
the  action  of  the  higher  arcs 
may  be  simulated. 


178 


VETERINARY  PHYSIOLOGY 


oth.  Developmental  Methods. — Flechsig  has  found  that 
bands  of  fibres  going  to  certain  parts  of  the  cortex  get  their 
medullary  sheaths  earlier  than  others,  and  that  the  fibres  to 
each  part  of  the  cortex  become  medullated  at  a  definite  date. 
The  areas,  the  fibres  of  which  get  their  sheaths  first,  he  calls 
the  primary  projection  areas,  and  they  correspond  very 
closely  with  the  receiving  areas  determined  by  other  methods 
(figs.  51  and  52). 

Qth.  Comparative  Anatomy. — The  complexity  of  different 


BODY  SENSE: 

AUDI  TORY - 

OLFACTORY 

HiGHFR  Associative  ""I 


BODY  SENSES 
AUDITORY 
■OLFACTORY    ^ 


Fig.  89. — A  purely  Schematic  Diagram  to  show  the  relations  of  receiving, 
associating,  and  discharging  parts  of  the  cortex  cerebri,  and  to  illus- 
trate that  each  of  these  is  in  itself  receiving,  associating,  and  dis- 
charging. The  mechanism  of  reaction  to  visual  impressions  is  given, 
and  the  modifying  influence  of  other  incoming  stimuli  is  indicated. 


parts  of  the  cortex  is  very  different  in  different  animals,  and 
this  affords  some  indication  of  the  probable  function  of  the 
various  parts.  Thus,  the  sub-granular  layers  are  first 
developed  and  are  best  marked  in  the  lowest  mammals,  and 
it  may  be  concluded  that  they  have  specially  to  do  with 
simple  instinctive  activities.  The  granular  layer  is  best 
developed  in  what  other  evidence  indicates  to  be  receiving 
areas.  The  supra-granular  layers  are  the  last  to  develop  and 
attain  their  greatest  thickness  in  the  higher  mammals, 
especially  in  the  frontal  part  of  the  brain,  the  development 


NERVE  179 

of  which  is  speciall}^  associated  with  the  higher  mental 
activities  (fig.  90). 

Further,  in  the  group  of  mammals  which  depend  largely 
on  olfactory  impressions,  the  rhinencephalon  is  markedly 
developed,  while  in  those  in  which  smell  plays  a  sub- 
ordinate part  this  portion  of  the  brain  is  only  slightly 
developed  (fig.  61). 

It  must  at  once  be  recognised  that  if  such  special  parts 
exist,  each  must  he  in  nature  receiving,  reacting,  and,  to 
some  extent,  associative.  Thus,  if  one  part  of  the  cortex  is 
specially  connected  with  the  reception  of  impulses  from  the 
eye,  it  must  be  able  to  bring  about  appropriate  reactions 
either  by  sending  impulses  directly  outwards  or  by  acting 
upon  some  part  of  the  brain  which  has  the  function  of 
bringing  about  a  reaction.  And,  in  order  that  the  reaction 
may  be  appropriate,  some  associative  mechanism,  either  in 
these  parts  of  the  cortex  or  elsewhere,  must  be  brought  into 
play  (fig.  89). 


V.  THE  INTEGRATION  OF  SENSATIONS  IN 
THE  CORTEX. 

The  Cortex  Cerebri  and  Mental  Life. 

We  have  seen  that  stimulation  of  definite  parts  of  the 
-cortex  cerebri  may  lead  not  only  to  modification  of  move- 
ment through  the  spinal  arcs,  but  also  to  changes  of 
consciousness,  which  have  been  described  as  sensations. 

But  the  great  object  of  the  development  of  the  cerebral 
cortex  from  the  basal  ganglia  is  to  furnish  a  means 
by  which  these  sensations  are  associated  and  integrated,  so 
that  more  complex  changes  of  consciousness  may  result. 
A  full  study  of  this  is  beyond  the  domain  of  physiology  and 
encroaches  on  the  territory  of  the  psychologist. 

A  pine  tree  may,  through  the  visual  mechanism,  produce 
a  sensation  of  green  of  a  certain  extent  and  from  a  certain 
direction ;  the  odour  may  act  upon  our  olfactory 
mechanism  ;  the  wind  in  the  branches  may  stimulate  our 
hearing,    and    when  we    approach   and   touch    the   tree    our 


180 


VETERINARY   PHYSIOLOGY 


cutaneous  sensations  are  evoked  and  our  muscle-joint  sense 
is  aroused. 

But,  unless  these  sensations  are  brought  together  and 
integrated,  we  can  have  no  recognition  that  some  one  object, 
a  pine  tree,  is  calling  them  forth.  It  is  only  by  an 
association  of  the  sensations  that  wo  gain  the  knowledge  that 
all  are  due  to  the  object  which  we  agree  to  call  a  pine  tree. 

If,  at  a  later  date,  some  of  these  different  sensations 
called  forth  by  this  tree  are  again  elicited,  they  are  associated 


Total  . 
depth' 


lot*'     .^ 
depth  *» 


4MontKs  Foetxis 


New  bom  Child 


Normal  Hu/rvan  A.dull 


Fig.  90. — To  show  the  development  of  the  different  layers  of  the  cortex  in 
man,  and  their  condition  in  one  of  the  lower  mammals — the  mole.  The 
first  column  shows  the  two  layers,  from  the  lower  of  which  the  infra- 
granular,  granular,  and  supra-granular  layers  of  the  adult  cortex  are 
formed.  The  second  column  shows  the  cortex  at  the  time  of  birth. 
The  third  column  shows  the  growth  of  the  supra-granular  layers  as 
adult  life  is  reached.  In  the  fourth  column  the  cortex  of  the  adult 
mole  is  shown  for  comparison.     (After  Bolton.) 


with  the  past  impressions  and  again  call  forth  the  idea  of 
the  pine  tree  and  may  lead  us  to  conclude  that  we  are 
near  one. 

This  means  that  each  sensation  and  each  combination  of 
sensations  or  perceptions  of  the  object  producing  them  must 
leave  an  impress  on  the  brain  which  is  the  physical  basis  of 
memory,    and    that    the    repetition    of   some    of    them,    by 


NERVE 


181 


association  with  these  previous  impressions,  leads    to    their 
renewal,  leads  to  our  recollecting  the  past  experiences. 

1.  Structural  Development. 

In  the  lower  vertebrata  the  ditferentiation  of  the  cortex 
from  the  basal  ganglia  is  incomplete,  and  it  is  only  in  the 
higher  mammals — monkeys  and  man — that  the  cortex 
reaches  full  physiological  importance. 


Fig.  91. — The  passage  of  fibres  from  the  nuclei  of  the  thalamus  to  the 
cortex  cerebri  of  a  primitive  mammal.  Th.Op.,  thalamus  ;  L.P.,  lobus 
pyriformis  ; /.r.,  fissura  rhinica  ;  F///'",  auditory  area;  //'",  visual 
area;  ^.,  hippocampus.     (Elliot  Smith.) 

The  cerebrum  originally  developed  as  a  ganglion  in 
connection  with  the  organ  of  smell,  and  in  the  osmatic 
mammals — those  in  which  smell  plays  a  great  part  in 
guiding  their  actions — a  large  part  of  the  cerebrum  remains 
specially  connected  with  the  nose.  This  may  be  called  the 
rhinencephalon  (fig.  61,  p.  135). 

The  cortex  cerebri,  or  neopallium,  is  a  secondary  develop- 
ment from  the  thalamus,  with  which  it  remains  closely 
associated  by  outgoing  and  ingoing  neurons  (fig.  91). 

In  the  human  foetus  at  four  months,  two  layers  are  visible  in 
the  cortex — (1)  an  outer  molecular  layer  of  fibres,  and  under 
this  (2)  layers  of  unditferentiated  cells  (fig.  90). 

By  the  sixth  month  this  second  layer  has  become  divided 
into    two     by    a    well-developed    layer    of   small    cells,    the 


182  VETERINARY   PHYSIOLOGY 

granular  layer.  The  deeper  layer  becomes  differentiated 
into  a  layer  of  ^polymorphic  cells,  and  outside  of  this  a  layer  of 
fibres,  the  layer  of  Baillarger.  Outside  the  granular  layer 
a  layer  of  'pyramidal  cells  appears  below  the  outer  layer  of 
fibres. 

Histology. — At  birth,  the  cortex  in  the  neopallium, 
from  without  inwards,  consists  of  (fig.  92) — 

1 .  Outer  layer  of  fibres. 

2.  Layer  of  pyramidal  cells, 

3.  Layer  of  granules. 

4.  Inner  layer  of  fibres  (Baillarger's  layer). 

5.  Layers  of  polymorphic  cells. 

It  is  the  supra-granular  layers  which  increase  as  develop- 
ment advances.  The  adult  mole  has  a  cortex  like  that  of 
the  six-month  human  foetus  (fig.  90). 

The  cells  of  the  cortex  send  dendritic  processes  up 
towards  the  surface,  Avhere  they  form  a  complicated  series  of 
synapses,  and  they  also  send  axon-processes  downwards  into 
the  white  substance  of  the  brain. 

From  these  fibres,  collaterals  come  off  which  connect 
different  parts  of  the  cortex  of  the  same  side,  and  which  also 
connect  the  cortex  of  one  side  with  that  of  the  other,  and 
with  the  basal  ganglia  (fig.  93). 

In  the  rhinencephalon  three  layers  develop  in  the  cortex 
— (1)  the  outer  layer  of  fibres;  (2)  the  layer  of  granules, 
often  curiously  broken  up  into  nests  ;  and  (3)  the  layer  of 
polymorphic  cells. 


2.  Functional  Development. 

An  animal  at  birth  is  little  more  than  a  reflex  machine, 
and  the  functions  of  its  cortex  cerebri  are  still  in  abeyance. 
But  the  ingoing  tracts  from  the  visual,  auditory,  tactile  and 
other  receptors  are  fully  developed,  and  hence  from  the  first 
a  stream  of  stimuli  pours  in  upon  the  cortex. 

Although  these  may  not  at  first  be  properly  integrated, 
they  influence  and  colour  one  another,  giving  rise  to  the 
pleasurable  state  of  consciousness  in  the  creature  signified  by 


NERVE 


183 


^^ 


-^,:^ 


a  condition  of  rest,  and  to  the  unpleasant 
state  associated  with  restlessness  and  crying. 

Only  when  sensations  become  more 
completely  integrated  in  the  developing 
cortex  is  any  real  mental  life  possible. 
The  power  of  integrating  largely  de- 
pends upon  : — 

(1)  The  previous  history  of  the  brain, 
both  phylogenetic  and  ontogenetic.  For, 
;  just  as  in  the  spinal  cord  channels  of 
action  are  formed,  so  in  the  cerebrum, 
if  a  given  reaction  once  follows  a  given 
stimulus,    it    will  tend  to  follow  it  again. 


3  and  4 


[r?; 


^t.^ 


■  a'- 


Fig.  92. — A,  Section  of  cerebra  cortex  in  the  pre-central  lobe  (a  motor 
area).  (For  description  of  zones,  see  text.)  B,  Section  through 
cerebral  cortex  in  the  region  of  the  calcarine  fissure  (visual  area), 
stained  to  show  the  arrangement  of  the  fibres.  (For  description  of 
zones,  see  text.)     (Campbell.) 


184  VETERINARY  PHYSIOLOGY 

(a)  This  training  or  preparation  of  the  brain  is  in  part 
hereditary.  Each  member  of  a  species  is  born  with  well- 
estabhshed  lines  of  action  in  the  process  of  development,  and 
throughout  life  these  inherited  channels  play  an  important 
part  in  determining  the  results  of  stimulation.  In  young 
fowls,  as  soon  as  they  are  hatched,  the  acts  of  running  and 
of  pecking  are  at  once  performed,  and  in  many  families 
particular  gestures  or  expressions  follow  certain  modes  of 
stimulation    in     many    different    individuals    without     the 


Fig.  93. — Diagram  of  collateral  connections  of  different  parts  of  the  cere- 
bral cortex,  a,  b,  c,  pyramidal  cells  of  the  cortex,  all  connected  by 
collateral  branches  with  other  parts  of  the  cortex  in  the  same  and  in 
the  opposite  hemisphere,  a  give  off  the  pyramidal  fibres  to  the  cord. 
{After  Ramon  y  Cajal.) 

consciousness  of  the  person  being  involved.  They  are 
inherited  cerebral  reflexes.  (6)  Paths  may  also  have  been 
developed  in  the  individual  as  the  result  of  previous  activities 
of  the  nervous  mechanism.  For,  if  a  given  action  has  once 
followed  a  given  stimulus,  it  always  tends  to  follow  it  again. 
This,  in  fact,  is  the  basis  of  all  training  of  animals — to  open 
up  paths  in  the  nervous  system  by  which  the  most  suitable 
response  may  be  made  to  any  given  stimulus,  and  to  prevent 
the  formation  of  paths  by  which  inappropriate  reaction  may 
be  produced. 

(2)  The  nutrition  of  the  brain. — Not  only  will  the 
previous  training  of  the  brain  thus  act  as  the  directive  force 
in  the  response  to  stimuli,  but  the  nutrition  of  the  brain 
also  plays  an  important  part.  The  action  of  a  brain  when 
well  nourished  and  freely  supplied  with  pure  blood  is  often 
very  different  from  that  of  the  same  brain  when  badly 
nourished  or  imperfectly  supplied  with  blood. 


NERVE  185 

3.  Storing  and  Associating  Part  of  the  Cortex. 

The  existence  of  a  special  part  or  parts  of  the  brain 
connected  with  the  storing  of  impressions,  so  that  they 
may  be  associated  with  present  sensations,  is  indicated 
by  the  following  considerations  : — It  is  this  association  of 
present  stimuli  with  past  sensations  which  is  the  basis  of 
intellectual  life,  and  in  man  the  frontal  and  parietal  lobes  of 
the  brain  are  much  more  developed  than  in  the  lower  animals. 
So  far,  stimulation  of  these  has  failed  to  give  any  indication 
of  resulting  sensations,  or  to  produce  muscular  movements. 
They  may  be  extensively  injured  without  loss  of  sensation 
and  without  paralysis,  and  hence  it  has  been  concluded  that 
the  storing  and  associating  functions  must  be  chiefly  located 
in  them. 

In  these  regions  the  nerve  fibres  acquire  their  medullary 
sheath  at  a  very  late  date. 

4.  The  Relationship  of  Consciousness  to  Cerebral  Action. 

Cerebral  action  frequently  goes  on  without  consciousness 
being  implicated  ;  but,  so  far  as  we  know,  consciousness 
without  accompanying  cerebral  action  is  unknown,  and  there 
is  evidence  that  it  is  only  when  the  actions  of  the  various 
parts  of  the  cerebrum  are  co-ordinated  that  consciousness  is 
possible.  In  cases  of  Jacksonian  epilepsy,  as  a  result  of  a 
small  centre  of  irritation  on  the  surface  of  the  brain,  a 
violently  excessive  action  of  the  cerebral  neurons  starts  at 
the  part  irritated  and  passes  to  involve  more  and  more  of  the 
brain.  In  such  fits,  it  is  found  that  at  first  the  patient's 
consciousness  is  not  lost,  but  that,  when  a  sufficient  area  of 
brain  is  involved  in  this  excessive  and  inco-ordinated  action, 
consciousness  disappears. 

Unconsciousness  may  be  produced  by  many  conditions 
which  modify  the  nutrition  of  the  brain.  (1)  Many  drugs, 
of  which  chloroform  and  ether  may  be  taken  as  types,  poison 
the  brain  and  cause  loss  of  consciousness.  (2)  Similar 
poisons  may  develop  in  the  body  as  the  result  of  faulty 
metabolism,  as  is  seen  in  diabetic  coma.  (3)  A  sudden 
failure  of  the  supply  of  blood  to  the  higher  centres  may 
cause    the   loss  of  consciousness   which   occurs   in  fainting. 


186  VETERINARY  PHYSIOLOGY 

(4)  Hsemorrhage  into  the  brain,  or  a  tumour  inside  the  skull 
may  interfere  with  the  blood  supply  by  pressure  and  also 
cause  loss  of  consciousness. 

The  study  of  the  action  of  drugs  which  abolish  conscious- 
ness— e.g.  chloroform  and  morphine — on  the  dendrites  of 
brain  cells  suggests  a  physical  explanation  of  the  condition. 
It  is  found  that  these  drugs  cause  a  general  extension  of 
the  gemmules  of  all  the  dendrites  ;  and,  if  we  imagine 
that  the  co-ordinated  action  of  any  part  of  the  brain  is 
secured  by  definite  dendrites  of  one  set  of  neurons 
coming  into  relationship  with  definite  dendrites  of  another 
set  of  neurons  by  their  gemmules  so  as  to  establish 
definite  paths,  the  want  of  co-ordinate  relationship  estab- 
lished by  the  general  expansion  would  explain  the  dis- 
appearance of  the  definite  sensations  which  constitute 
consciousness. 

5.  Time  of  Cerebral  Action. 

The  cerebral  mechanism  takes  a  very  appreciable  time  to 
act,  and  the  time  varies  (1)  with  the  complexity  of  the  action 
and  (2)  with  the  condition  of  the  nervous  apparatus. 

Of  the  time  between  the  presentation  of  a  flash  of  light 
to  the  eye  or  a  touch  to  the  skin  and  a  signal  made  by  the 
person  acted  upon  when  it  is  perceived,  part  is  occupied  in 
the  passage  of  the  nerve  impulses  up  and  down  the  nerves 
and  in  the  latent  period  of  muscular  contraction,  but  a 
varying  period  of  something  over  one- tenth  of  a  second 
remains,  representing  the  time  occupied  in  the  cerebral 
action  {Practical  Physiology). 

Continued  action  of  the  nerve  centres  may  lead  first  to  a 
shortening  of  the  reaction  time  as  a  result  of  the  facilitation 
of  the  passage  of  the  impulses  over  the  synapses  {practice), 
but  this  is  soon  followed  by  a  prolongation  {fatigue). 
The  latter  condition  is  produced  by  the  action  of  alcohol, 
chloroform,  and  other  poisons. 

6.  Fatigue  of  the  Cerebral  Mechanism. 

Fatigue  of  the  cerebral  mechanism  is  manifested  {a)  by 
decrease  in  the  power  of  attention,  comparable  to  the  loss 


NERVE  187 

of  command  of  the  common  path  seen  in  the  spinal  reflex 
action  (p.  86)  ;  (b)  by  prolongation  of  the  reaction  time  ; 
and  (c)  by  a  more  rapid  decrease  of  the  force  of  muscular 
contraction. 

The  seat  of  the  change  is  in  the  cerebral  synapses,  and, 
after  these  have  failed  to  act,  the  spinal  arcs  may  still 
be  unaffected  and  spinal  reflexes  may  be  produced. 

The  cause  of  the  condition  is  probably  primarily  the 
accumulation  of  the  products  of  activity  and  the  lack  of  a 
free  supply  of  oxygen  to  the  brain,  and  the  condition  may 
often  be  removed  by  (a)  a  short  rest,  or  by  (6)  the  substitu- 
tion of  a  change  of  occupation,  or  by  (c)  muscular  exercise, 
which  increases  the  flow  of  blood  through  the  brain,  or  (cZ) 
by  sleep.  The  act  of  yawning  (p.  522)  is  simply  a 
reflex  which  increases  the  flow  of  blood  to  the  heart  and  thus 
on  to  the  brain.  It  is  Nature's  effort  to  overcome  cerebral 
fatigue. 

As  to  how  the  synapses  are  affected,  a  consideration  of 
the  possible  way  in  which  such  poisons  as  chloroform  act 
upon  the  dendrites  and  gemmules  to  abolish  definite  lines  of 
action  suggests  that  in  fatigue  the  same  thing  may  occur  to 
a  lesser  degree. 

Continued  action  also  leads  to  well-marked  changes  in 
the  cell  protoplasm  of  the  neurons.  The  Nissl's  granules 
diminish  and  the  nucleus  shrivels  and  becomes  poorer  in 
chromatin. 

7.  Sleep. 

Fatigue  of  the  cerebral  mechanism  is  closely  connected 
with  sleep.  As  the  result  of  fatigue,  external  stimuli  produce 
less  and  less  definite  effects,  and  thus  the  changes,  which  are 
the  physical  basis  of  consciousness,  become  less  and  less 
marked.  At  the  same  time,  by  artificial  means,  stimuli  are 
usually  so  far  as  possible  excluded.  Absence  of  light,  of 
noise,  and  of  tactile  and  thermal  stimuli  all  conduce  to  sleep. 
The  purpose  of  hypnotic  drugs  is  to  render  the  brain  less 
susceptible  to  external  or  internal  stimuli. 

As  sleep  advances  (1)  consciousness  fades  away,  and,  as 
the  cerebral  activity  diminishes,  (2)  the  arterioles  throughout 


188  VETERINARY   PHYSIOLOGY 

the  body  dilate,  the  arterial  blood  pressure  falls,  and  thus 
less  blood  is  sent  to  the  brain,  and  the  organ  becomes  more 
bloodless.  This  may  be  seen  in  cases  of  trephining,  where  a 
sinking  of  the  trephine  scar  occurs  during  sleep.  (3)  The 
eyeUds  close,  (4)  the  eyeballs  turn  upwards,  (5)  the  pupils 
contract,  and  (6)  the  voluntary  muscles  may  relax.  In  the 
horse,  ox,  etc.,  the  tonic  contraction  of  the  muscles,  with  the 
assistance  of  the  ligaments  (p.  237)  sustaining  the  body 
in  the  standing  position,  may  persist,  and  the  animal  may 
sleep  in  that  position. 

The  depth  of  sleep  may  be  measured  by  the  strength  of 
the  stimuli  required  to  overcome  it. 

The  prejudicial  effect  of  want  of  sleep  has  been  demon- 
strated by  observations  upon  young  dogs.  It  was  found  that 
they  died  in  five  days  if  prevented  from  sleeping,  while, 
if  allowed  to  sleep,  they  withstood  even  the  withdrawal  of 
food  for  no  less  than  twenty  days. 

More  sleep  is  required  by  young  than  by  old  animals. 

8.  Hypnosis. 

This  is  a  condition  in  some  respects  allied  to  sleep.  In 
many  of  the  lower  animals  it  is  readily  produced,  simply  by 
holding  the  animal  firmly  and  placing  it  in  any  unusual 
position,  as  on  the  back,  especially  if  stimuli  from  without 
are  cut  off— e.g.  by  bandaging  the  eyes.  It  may  then  lie  with- 
out moving  for  a  prolonged  period,  and  may  react  much  in  the 
same  way  as  if  decerebrated.  The  condition  may  be  induced 
in  many  people  by  powerfully  arresting  the  attention,  and  it  is 
probably  due  to  a  removal  of  the  influence  of  the  higher 
centres  over  the  lower.  The  respirations  and  pulse  become 
quickened,  the  pupils  dilate,  and  the  sensitiveness  of  the 
neuro  -  muscular  mechanism  is  so  increased  that  merely 
stroking  a  group  of  muscles  may  throw  them  into  firm 
contraction.  This  suggests  an  abrogation  of  the  cerebral 
function  and  a  dominance  of  the  tonic  vestibulo-cerebellor- 
spinal  arc.  The  individual  becomes  a  reflex  machine  even  as 
regards  the  cerebral  arcs,  and  each  stimulus  is  followed  by  an 
immediate  reaction. 


NERVE  189 


VI.  THE  DISCHARGING  SIDE  OF  THE  CEREBRAL  ARC. 

In  the  previous  section  the  way  in  which  changes  in  the 
external  world  act  upon  the  body  and  the  consciousness  has 
been  considered. 

The  reactions  of  the  body  through  the  spinal  arcs  in 
reflex  action  have  also  been  dealt  with,  and  the  way  in 
which  these  reactions  are  controlled  and  adjusted  by  the 
concomitant  action  of  the  proprioceptive  mechanisms  of 
the  muscles  and  joints  on  the  one  hand,  and  of  the  vestibule 
and  semicircular  canals  on  the  other,  has  been  explained 
(p.  121). 

I.  The  Basal  Ganglia. 

The  way  in  which  all  incoming  impulses  are  interrupted 
and  associated  in  the  thalamus  has  been  considered  (p.  118). 

The  thalamus  is  connected  not  only  with  the  cortex,  but 
also  with  the  different  nuclei  of  the  corpus  striatum  and 
with  the  red  nucleus  from  which  fibres  pass  down  the  cord 
in  front  of  the  crossed  pyramidal  tract  (fig.  97). 

In  man  and  apes  the  functions  of  these  connections  have 
been  largely  handed  over  to  the  control  of  the  cortex,  but  in 
lower  mammals  they  are  of  primary  importance. 

Corpus  Striatum. — There  is  some  evidence  that  the 
corpus  striatum  plays  a  part  in  controlling  temperature 
(p.  269).  It  was  found  that  stimulation  leads  to  increased 
heat  production,  which  must  be  due  to  increased  chemical 
change  in  the  muscles.  More  recently  it  has  been  found 
that  stimulation  by  njeans  of  cold  leads  to  a  rise  of  tem- 
perature, while  stimulation  by  heat  leads  to  a  fall,  the 
first  causing  a  constriction  of  the  blood-vessels  of  the  sl^in, 
the  second  causing  a  dilatation. 

II.  The  Discharging  Area  of  the  Cortex  Cerebri. 

The  way  in  which  movements  are  originated  and 
dominated  by  the  cortex  cerebri,  as  the  result  of  the 
integration  of  the  stimuli  falliug  on  the  body,  and  their 
association  with  one   another  and   with  the  impressions  of 


190 


VETERINARY   PHYSIOLOGY 


previous   stimulation,  so   that   the  resulting   action  may  be 
appropriate  to  the  surroundings,  must  now  be  studied. 

Ample  evidence  is  forthcoming  that  in  men  and  apes  the 
part  of  the  cortex  round  about  and  chiefly  in  front  of  the 
central  fissure  performs  this  function.  In  the  dog,  cat 
and  pig  it  is  situated  round  the  cruciate  fissure  (figs.  52, 
p.  115,  and  94). 


Fig.  94. — (a)  Surface  of  the  left  Cerebral  Hemisphere  of  a  Monkey  to  show 
the  situations  of  some  of  the  Discharging  Mechanisms  (front  to  left) ; 
(b)  Mesial  Surface  of  the  same  Hemisphere  (front  to  right). 


This  area  is  played  upon  by  all  the  sensory  and  associa- 
tive arrangements  in  the  cortex.  Like  every  other  part  of 
the  brain  it  receives  and  discharges  impulses  (fig.  89).  But 
unlike  the  receiving  areas,  which  have  been  already  considered, 
it  discharges  outwards  from  the  brain  upon  the  spinal  arcs. 


NERVE  191 

The  evidence  as  regards  its  position  is  both  clinical  and 
experimental. 

1.  Clinical  and  Pathological  Evidence. — Destructive  lesions 
of  this  area  on  one  side  cause  a  loss  of  the  so-called 
voluntary  action  of  groups  of  muscles  on  the  opposite  side  of 
the  body,  while  the  muscles  themselves  and  the  spinal  reflexes 
connected  with  them  are  not  interfered  with.  The  spinal 
reflexes  may  in  fact  become  more  active. 

Certain  lesions  may  directly  stimulate  these  centres, 
causing  them  to  act  without  the  previous  action  of  the 
other  cerebral  mechanisms  and  may  cause  convulsions. 
This  is  seen  in  Jacksonian  epilepsy,  where,  as  the  result 
of  a  spicule  of  bone  or  a  thickened  bit  of  membrane,  one 
part  of  the  cortex  is  from  time  to  time  excited.  This  pro- 
duces movements  of  certain  groups  of  muscles,  which  spread 
outwards  to  other  groups  as  the  stimulation  of  the  cortex 
extends  outwards  from  its  seat  of  origin,  till  finally  all  the 
muscles  of  the  body  are  involved  in  a  general  convulsion. 

2.  Experimental  Evidence.  —  Experimental  observations 
have  fully  confirmed  and  extended  the  conclusions  arrived  at 
from  such  pathological  evidence. 

(a)  Removal. — If  parts  of  these  convolutions  be  excised 
in  the  monkey,  the  animal  loses  the  power  of  voluntary 
movement  of  certain  groups  of  muscles  on  the  opposite  side 
of  the  body.  Movements  requiring  the  co-operative  action 
of  muscles  of  both  sides,  e.g.  movements  of  the  eyes  and 
trunk,  are  not  abolished  by  unilateral  destruction. 

In  these  motor  areas  the  lesion  must  be  extensive  to 
cause  coTnplete  paralysis  of  any  group  of  muscles.  A  limited 
lesion  may  simply  cause  a  loss  of  the  finer  movements. 
Thus,  a  monkey  with  part  of  the  middle  portion  of  the 
Eolandic  areas  removed  may  be  able  to  move  its  arm  and 
hand,  but  may  be  quite  unable  to  pick  up  objects  from  the 
floor  of  its  cage. 

Even  after  removal  of  a  fairly  extensive  part  of  these 
centres,  with  resulting  muscular  paralysis,  it  has  been  found 
that  after  a  time  more  or  less  complete  recovery  takes  place. 
Evidently  some  other  part  than  that  removed  can  take  upon 


192 


VETERINARY  PHYSIOLOGY 


itself  the  function,  is  capable  of  education,  just  as  the  lower 
spinal  centres  seem  capable  of  adaptation. 

(b)  Stimulation  by  electricity  causes  movements  of  group 
of  muscles  of  the  opposite  side  of  the  body  and  of  muscles 
on  both  sides  when  bilateral  co-operation  is  required. 

These  movements  are  elicited  by  much  weaker  currents 
than  are  required  to  produce  them  on  stimulating  the  typical 
receiving  area  already  studied. 

If  the  cortex  is  stripped  off  and  the  white  fibres  below 


Afuaiy^g^- 


£Ar-   / 

EueLid    /  citisure 
Noie 


Fig.  95. — Left  Hemisphere  of  Brain  of  Chimpanzee  to  show  the  results  of 
stimulating  different  parts.  The  Sulcus  Centralis  is  the  fissure  of 
Rolando.     (From  Grunbaum  and  Sherrington.  ) 

it  are  directly  stimulated,  the  latent  period  is  shortened  and 
the  strength  of  current  required  to  produce  movements  is 
greater. 

The  work  of  Griinbaum  and  Sherrington  on  the  brain 
of  anthropoid  apes  has  shown  that  the  discharging  mechanism 
is  chiefly  in  the  pre-central  convolution  (fig.  95),  extend- 
ing forwards  into  the  posterior  parts  of  the  superior,  middle 
and  inferior  frontal  convolutions,  with  a  patch  far  out  in  the 
frontal  area  by  stimulation  of  which  movements  of  the  eyes 
are  produced.  This  is  present  only  in  men  and  apes  which 
use    binocular    vision.       It    is    associated    by    a    band    of 


NERVE  193 

fibres  with  the  visuo-sensory  area,  and  fibres  pass  from 
it  down  the  cerebro-pontine  tracts  to  reach  the  cerebellum 
(fig.  58). 

The  discharging  part  of  the  cortex  may  be  con- 
sidered as  a  map  of  the  various  muscular  combinations 
throughout  the  body,  the  map  being  mounted  so  that  the 
lower  part  represents  the  face,  the  middle  part  the  arm,  and 
the  upper  part  the  leg,  probably  corresponding  closely  to  the 
map  of  cutaneous  and  muscle-joint  sensibility,  although  this 
may  lie  rather  more  posteriorly.  Each  large  division  is  filled 
in  so  that  all  the  various  combinations  of  muscular  movement 
are  represented  (fig.  95).  It  must  be  remembered  that 
these  centres  do  not  send  nerves  to  single  muscles,  but  that 
they  play  upon  the  spinal  centres  to  produce  combined  move- 
ments of  sets  of  muscles. 

These  movements  involve  inhibition  as  well  as  excita- 
tion, just  as  the  spinal  reflexes  do. 

This  is  very  clearly  shown  as  regards  the  eye  movements. 
In  the  monkey,  the  resting  position  of  the  eyes  is  straight 
forward  with  the  optic  axes  parallel.  If  all  the  nerves  to  the 
ocular  muscles  be  cut,  this  position  is  assumed,  and,  if  the 
position  of  the  eye  be  passively  altered,  upon  removing  the 
displacing  force,  it  springs  back  to  this  position.  If  the  III. 
and  IV.  nerves  of  the  left  side  be  cut  (p.  161),  so  that  the 
external  rectus  alone  is  unparalysed,  then,  exciting  a  part  of  the 
cortex  which  causes  movements  of  the  two  eyes  to  the  right, 
produces  not  only  a  movement  of  the  right  eye  in  that 
direction,  but  a  movement  of  the  left  eye  to  the  right  as  far 
as  the  middle  line — the  position  of  rest — showing  that  the 
VI.  nerve  has  been  inhibited. 

Stimulation  of  the  cortex  causes  flexion  more  readily 
than  extension,  apparently  because  the  inhibitory  mechanism 
for  the  extensors  is  better  developed  than  that  for  the  flexors. 
Sherrington  finds  that  this  condition  is  reversed  under  the 
influence  of  strychnine  or  of  tetanus  toxin,  and  that  stimuli, 
which  in  normal  conditions  will  cause  flexion,  now  cause 
powerful  extension,  and  hence  co-ordinated  movement  is 
impossible. 

13 


194 


VETERINARY  PHYSIOLOGY 


up 


III.  Fibres  passing  outwards  from  the  Discharging 
Parts  of  the  Cerebrum. 

The  fibres  coming  from  this  area  of  the  cortex  are  mixed 
with  the  ingoing  fibres  from  the  thalamus  ah-eady 
described.  But  they  are 
distinguished  from  them  by 
the  fact  that  they  get  their 
medullary  sheaths  at  a  late 
date,  so  that  they  may  be 
traced  right  down  into  the  , 
cord  as  the  pyramidal  tract 
in  the  following  situations  : — 
1st.  In  the  corona  radiata 
(figs.  96  and  97). 

2nd.  In  the  anterior  two- 
thirds  of  the  posterior  limb 
of  the  internal  capsule,  the 
face  fibres  lying  to  the  front, 
the  arm  fibres  behind  these, 
and  the  leg  fibres  furthest 
back  (figs.  96  and  97). 

Srd.  In  the  central  part 
of  the  crusta  of  the  crus, 
arranged  from  within  out- 
wards— face, arm,  leg  (fig.  97). 
Uh.  In  the  pons  Varolii, 
between  the  deep  and  super- 
ficial transverse  fibres.  The 
face  fibres  cross  here  (fig.  97). 
Hence  a  unilateral  tumour 
in  this  situation  may  cause 
paralysis  of  the  face  on  one 
side  and  of  the  limbs  on  the 
other — a  crossed  paralysis. 

5th.      In      the      anterior 

pyramids    of     the     medulla 

decussate    at   the  junction    of 


Fig.  96. — Diagrammatic  horizontal 
section  through  base  of  cerebral 
hemisphere,  showing  (1)  the  out- 
going fibres  for  the  leg,  arm,  and 
face  springing  from  the  cortex  of 
the  central  areas,  passing  through 
the  internal  capsule  between 
the  thalamus  and  the  lenticular 
nucleus.  The  face  fibres  cross  in  the 
pons,  the  leg  and  arm  fibres  in  the 
medulla.  (2)  The  incoming  fibres 
(fillet,  eye,  etc.)  have  cell  stations 
in  the  thalamus,  anfl  then  pass  on 
to  the  cortex. 

oblongata.       Most    of    them 

the  medulla  and  spinal  cord  (fig.  97). 


NERVE 


195 


6th.  In  the  spinal  cord  they  constitute  the  crossed 
pyramidal  tract  of  the  opposite  side.  Those  which  do  not 
cross  run  down  for  some  distance  in  the  antero-median 
tract  as  the  direct  pyramidal  fibres  to  cross  lower  down 
(fig.  98). 

7  th.  From  the  pyramidal  fibres  collaterals  come  off 
and  act,  through  intercalated  neurons,  on  the  outgoing 
neurons  from  the  anterior  horn  of 
grey  matter  to  the  skeletal  muscles 
(fig.  33).  These  pyramidal  fibres  de- 
generate when  the  motor  cortex  is 
destroyed  or  when  they  are  severed  by 
a  haemorrhage  into  the  internal 
capsule. 

Fibres  from  the  Basal  Ganglia. — De- 
generation of  the  rubro-spinal  and  of  the 
tecto-spinal  fibres  does  not  occur  unless 
this  interruption  is  below  the  level 
of  the  tectum  (p.  1 1 3). 

The  course  of  the  various  outgoing 
fibres  in  the  spinal  cord  in  man  is 
shown  in  fig.  98. 

In  addition  to  the  tracts  from  the 
cerebrum,  the  downgoing  fibres  forming  Fig.  97.— To  show  the 
the  outgoing  part  of  the  vestitalo-  ^CIT <:Ttlr<Si;^ 
cerebello  -  spinal     arc     are    also     shown       of  the  Precentral  Con- 

(fig.   98).  volution. 


IV.  Outgoing  Nerves  from  the  Spinal  Cord. 

These  outgoing  nerves  from  the  spinal  cord  emerge  in 
the  anterior  roots  (p.  107).  They  may  be  divided  into  two 
sets  : — 

1.  Somatic,  to  the  skeletal  muscles. 

2.  Visceral,  to  the  viscera,  blood-vessels,  glands,  etc, 

1.  Somatic. — The  course  and  distribution  of  these  are 
fully  studied  in  anatomy. 

2.  Visceral. — These  outgoing  fibres  are  characterised  by 
their  small  size.  They  take  origin,  as  described  on  p.  54, 
chiefly  in  a  lateral  column  of  cells,  which  is  well  developed 


196 


VETERINARY  PHYSIOLOGY 


in  the  dorsal  region  of  the  cord  (see  p.  54),  and  pass 
out  as  medullated  tibres  by  the  anterior  root.  From  this 
they  pass  by  the  white  root  to  a  sympathetic  ganghon, 
whence  they  may  proceed  in  one  of  two  ways  (fig.  46). 

1,  They  may  form  synapses  with  cells  in  the  ganglion^ 
and  fibres  from  these  cells  may  pass — 

(«)  Outwards  with  the  splanchnic  nerves. 

(6)  Back  into  the  spinal  nerve  by  the  grey  root,  and  so 
down  the   somatic   nerve    to    blood-vessels,  muscles  of   the 


Fig.  98. — Cross  section  of  the  Spinal  Cord  in  the  cervical  region  to  show 
the  position  of  the  various  groups  of  outgoing  fibres  :— 1,  The  crossed 
pyramidal  tract  ;  2,  the  direct  pyramidal  tract  ;  3,  the  rubro-spinal 
tract  ;  4,  the  tecto-spinal  fibres  ;  5.   the  vestibulo-spinal  fibres. 


hairs,  sweat  glands,  etc.  The  ganglia  from  which  fibres 
pass  back  into  spinal  nerves  are  known  as  lateral  ganglia.. 

(c)  Upwards  or  downwards  into  other  sympathetic 
ganglia, 

2.  They  may  pass  through  the  ganglion  to  one  more 
peripherally  situated  in  which  they  form  synapses  witb 
other  neurons  which  are  continued  onwards.  The  ganglia, 
from  which  fibres  do  not  pass  back,  are  called  collateral 
ganglia.       Before    their    first   interruption   these   fibres   are 


NERVE  197 

termed  pre-ganglionic  fibres ;  after  their  interruption  post- 
iglionic. 
3.  The  various   fibres,   after  their  interruption,   proceed, 


Fig.  99.— Scheme  of  Distribution  of  the  Thoracico-Abdominal  (the  true 
sympathetic)  and  the  Cranial  and  Sacral  (the  para-synipathetic) 
Splanchnic  Nerves. 

generally  as  non-medullated  or  grey  fibres,  to  their  termina- 
tion. Here  they  form  synapses  with  the  network  of  fibres 
and  cells  which  constitute  the  terminal  plexuses.  Upon 
these,  in  the  gut  at  least,  the  control  of  the  tissues  is  largely 
devolved. 


198  VETERINARY   PHYSIOLOGY 

The  interruption  of  fibres  in  ganglia,  or  their  passage 
through  these  structures,  has  been  determined  by  taking 
advantage  of  the  fact  that  nicotine,  in  one  per  cent,  solution 
when  painted  on  a  ganglion,  poisons  the  synapses  but  does 
not  influence  the  fibres.  Hence,  if  when  a  ganglion  is  painted 
with  nicotine,  stimulation  of  the  fibres  on  its  proximal 
side  produces  an  effect,  it  is  proved  that  the  break  is  not  in 
that  ganglion. 

The  Visceral  Nerves  may  be  divided  into  two  sets — 

I.  The  Thoracico-Abdominal  or  True  Sympathetic  Fibres,  which 
come  out  in  the  middle  region  of  the  spinal  cord  and  pass 
through  the  lateral  ganglia  of  the  sympathetic  chain  (fig.  99). 
They  are  distributed  to  the — 

(1)  Head  and  Neck. — These  leave  the  spinal  cord  by 
the  upper  five  dorsal  nerves  and  pass  upwards  in  the 
sympathetic  cord  of  the  neck  to  the  superior  cervical 
ganglion  where  they  have  their  cell  stations.  From  these, 
fibres  are  distributed  to  the  parts  supplied.  Stimulation  of 
these  fibres  causes — l.s^,  Vaso-constriction  of  the  vessels  of 
the  face  and  head  ;  2nd,  Dilatation  of  the  pupil  (see  p.  148) ; 
Srd,  Prominence  of  the  eyeball,  due  probably  to  the  stimula- 
tion of  visceral  muscular  fibres  in  the  eyelids  by  which  these 
are  drawn  apart  and  the  eye  exposed  and  allowed  to  bulge 
forward ;  4iA,  Secretion  of  the  salivary,  lachrymal,  and 
sweat  glands. 

(2;  Thorax. — The  fibres  to  the  thoracic  organs  also 
come  off  in  the  upper  dorsal  nerves.  They  have  their  cell 
stations  in  the  stellate  ganglion  in  the  dog,  and  pass  to  the 
heart  and  lungs. 

(3)  Abdomen. — These  fibres  come  off  in  the  lower 
dorsal  and  upper  lumbar  nerves.  They  course  through 
the  lateral  ganglia  and  form  synapses  in  the  collateral 
ganglia  of  the  abdomen — the  solar  plexus  and  the  superior 
and  inferior  mesenteric  ganglia.  From  these,  they  are 
distributed  to  the  abdominal  organs,  being  vaso-constrictor 
to  the  vessels,  and  inhibitory  to  the  muscles  of  the  stomach 
and  intestine. 

(4)  Pelvis. — The  fibres  for  the  pelvis  leave  the  cord  by 
the    lower    dorsal    and    upper     lumbar    nerves,    and    have 


NERVE  199 

their  cell  stations  in  the  inferior  mesenteric  ganglia,  from 
which  they  run  in  the  hypogastric  nerves  to  the  pelvic 
plexus.  Ttiey  are  vaso-constrictor,  inhibitory  to  the  colon, 
and  generally  motor  to  the  bladder,  uterus,  and  vagina. 

(5)  Arm. — These  fibres,  coming  out  by  the  middle  dorsal 
nerves,  have  their  synapses  in  the  ganglia  of  the  sympathetic 
chain,  and  passing  back  into  the  spinal  nerves  by  the  grey 
rami,  course  to  the  blood-vessels,  hairs,  and  sweat  glands 
of  the  limb. 

(6)  Leg. — The  fibres  take  origin  from  the  lower  dorsal 
and  upper  lumbar  nerves,  have  their  cell  stations  in  the 
lateral  ganglia,  and  pass  to  the  leg  in  the  same  way  as  do 
the  fibres  to  the  arm. 

II.   The   Cranial    and  Sacral,   or   Para  -  sympathetic    Fibres  — 

These  pass  out  from  the  upper  and  lower  ends  of  the  cord. 
They  do  not  pass  through  the  lateral  ganglia  but  have  their 
cell  stations  in  some  of  the  collateral  ganglia. 

(a)  Cranial — 

(1)  The  third  cranial  nerve  carries  fibres  which  have 
their  synapses  in  the  ciliary  ganglion,  and  pass  on  to  the 
sphincter  pupillse  and  ciliary  muscle. 

(2)  The  seventh  nerve  carries  fibres  through  the  chorda 
tympani  to  cell  stations  in  the  submaxillary  and  sublingual 
ganglia.  These  are  secretory  to  the  submaxillary  and  sub- 
lingual glands. 

(.3)  The  niiith  nerve  sends  fibres  to  the  parotid  gland, 
which  have  their  cell  station  in  the  otic  ganglion. 

(4)  The  vagus  sends  inhibitory  fibres  to  the  heart,  which 
form  synapses  in  the  cardiac  plexus.  It  also  sends  augmentor 
fibres  to  the  oesophagus,  stomach  and  intestine. 

(h)  Sacral — 

The  nervi  erigentes  or  iMlvic  nerves  come  off  from  the 
second  and  third  sacral  nerves,  and  pass  to  the  hypogastric 
plexus  near  the  bladder  where  the  fibres  have  their  cell 
stations.  They  are  the  vaso-dilator  nerves  to  the  pelvic 
organs  ;  they  inhibit  the  sphincter  of  the  bladder,  and  are 
motor  to  the  bladder,  colon,  and  rectum. 


200  VETERINARY   PHYSIOLOGY 

CRANIAL  NERVES. 
Although  the  cranial  nerves  contain  both  ingoing  and 
outgoing  fibreS;  they  may  be  dealt  with  at  this  point.  Their 
functions  should  be  studied  while  they  are  being  dissected. 
They  do  not  come  off  in  the  same  regular  fashion  as  do  the 
spinal  nerves,  although  they,  like  the  spinal  nerves,  must  be 
considered  as  forming  part  of  the  spinal  arcs.  The  outgoing 
fibres  of  each  spring  from  a  more  or  less  definite  mass  of 
ceUs.  The  ingoing  fibres  form  synapses  with  cells  generally 
arranged  in  definite  groups.  In  this  way  the  so-called 
nuclei  of  the  cranicU  nerves  are  formed.  The  position  of 
these  is  indicated  in  fig.  100.      In  many  of  the  cranial  nerves 


Fig.  100.— The  Nuclei  and  Roots  of  the  Cranial  Nerves.      (After  Edixger.) 

no  sharp  differentiation  into  anterior  and  posterior  roots  can 
be  made  out.  Nevertheless,  they  contain  the  same  com- 
ponent elements  as  the  spinal  nerves,  the  fibres  running 
either  together  or  separately. 

Ingoing  Fibres. — Somatic  and  splanchnic  fibres  (p.  54) 
enter  the  medulla  and  have  their  cell  stations  in  ganglia 
upon  the  nerves. 

Outgoing  Fibres. — Somatic  and  splanchnic  fibres  pass 
out,  the  latter  being  characterised  by  their  small  size,  and 
by  forming  synapses  before  their  final  distribution. 

The  XII.  (Hypoglossus)  is  purely  an  anterior  root  nerve, 
and  is  motor  to  the  muscles  of  the  tongue. 

The  X.  (Vagus)  and  the  XI.  (Spinal  Accessory)  are 
practically    one   nerve,    consisting    partly    of   posterior    and 


NERVE  201 

partly  of  anterior  root  fibres.  The  vagus  is  the  great 
ingoing  nerve  from  the  abdomen,  thorax,  larynx,  and  gullet, 
while,  by  outgoing  fibres,  passing  through  it  or  through  the 
accessorius,  it  is  augmentor  for  the  muscles  of  the  bronchi 
and  alimentary  canal,  inhibitory  to  the  heart,  dilator  to 
blood-vessels  of  the  thorax  and  abdomen,  and  motor  to  the 
muscles  of  the  larynx  and  to  the  levator  palati.  The 
accessorius  is  also  motor  to  the  sterno-cleido-mastoid  and 
trapezius. 

The  IX.  (Glossopharyngeus)  is  essentially  a  posterior 
root,  and  is  the  ingoing  nerve  for  the  back  of  the  mouth, 
the  Eustachian  tube,  and  tympanic  cavity.  It  also  contains 
outgoing  somatic  fibres  to  the  stylo-pharyngeus  and  middle 
constrictor  of  the  pharynx,  and  outgoing  visceral  fibres 
which  are  secretory  and  vaso-dilator  to  the  parotid  gland. 

The  VII.  (Facial)  is  almost  purely  an  anterior  root,  trans- 
mitting somatic  motor  fibres  to  the  muscles  of  expression 
and  to  the  stapedius  muscle,  visceral  secretory  fibres  to  the 
submaxillary  and  sublingual  glands  and  the  glands  of  the 
mouth,  and  vaso-dilator  fibres.  It  also  carries  ingoing 
fibres  from  the  anterior  two-thirds  of  the  tongue.  It  is  the 
cranial  nerve  most  frequently  paralysed,  and  this  condition 
is  often  caused  by  inflammatory  changes  in  the  canalis 
facialis  in  which  it  passes  through  the  temporal  bone.  The 
condition  in  man  is  known  as  BeWs  paralysis.  It  occurs  in 
the  horse,  and  it  is  characterised  by  a  drooping  of  the  ear 
on  the  affected  side,  by  the  upper  eyelid  being  pulled  to  the 
middle  line,  while  the  lower  eyelid  droops,  by  an  elongation 
of  the  nostril  and  difiiculty  of  breathing  through  it,  and  by 
a  pendulous  condition  of  the  lips  so  that  saliva  and  food 
dribble  from  the  mouth. 

The  V.  (Trigeminal)  is  chiefly  a  posterior  root,  but  it  has 

a  distinct  anterior  or  motor  root  which  joins  it,  and  it  carries 

the  motor  fibres  to  the  muscles  of  mastication  and  to  the 

tensor  tympani.     It  is  the  great  ingoing  nerve  for  all  the  face. 

The  VI.  (Abducens)  supplies  the  external  rectus  of  the  eye. 

The  IV.  (Trochlears)  supplies  the  superior  oblique. 

The  III.  (Oculo-motorius)  supplies  all  the  muscles  of  the 

eye  except  those  supplied  by  VI.  and  IV. 


C.   THE  EFFECTORS. 

MUSCLE. 

So  far  as  the  chemical  changes  in  the  body  are  concerned, 
muscle  is  more  important  than  nerve,  for  three  reasons — 1st. 
It  is  far  more  bulky,  making  up  something  like  45  per  cent, 
of  the  total  weight  of  the  body  in  meo,  and  38  per  cent,  in 
women.  2nd.  It  is  constantly  active,  for  even  in  sleep  the 
muscles  of  respiration,  circulation,  and  digestion  do  not  rest ; 
and  3rd.  The  changes  going  on  in  it  are  very  extensive, 
since  its  great  function  is  to  set  free  energy  from  the  food. 
So  far  as  the  metabolism  of  the  body  is  concerned,  muscle  is 
the  master  tissue.  For  muscle  we  take  food  and  breath, 
and  to  get  rid  of  the  waste  of  muscle  the  organs  of  excretion 
act.  Hence  it  is  in  connection  with  muscle  that  all  the 
problems  of  nutrition — digestion,  respiration,  circulation, 
and  excretion — have  to  be  studied. 

Muscle  is  the  great  liberator  of  energy  in  the  body,  and 
the  energy  is  used — 

1.  To  perform  mechanical  work. 

2.  To  heat  the  body. 


I.  Development  and  Structure. 

Most  free  protoplasmic  units  have  the  power  of  contract- 
ting  and  expanding.  A  muscle  cell  or  fibre  is  specially 
developed  to  contract  and  relax  in  one  direction  and  to  use 
a  large  proportion  of  the  energy  liberated  in  work. 

The  first  trace  of  the  evolution  of  muscle  is  found  among 
the  infusoria,  where,  in  certain  species,  in  parts  of  the 
protoplasm,  long  parallel  fibrils  run  in  the  direction  in  which 
the  cell  contracts  and  expands.  Such  a  development  has 
been  termed  a  myoid. 


MUSCLE 


203 


Even  a  cursory  examination  of  mam- 
malian muscles  shows  that  those  of  the 
trunk  and  limbs,  skeletal  muscles,  are 
different  from  those  of  such  internal  organs 
as  the  bladder,  uterus,  and  alimentary  canal, 
visceral  muscles. 


5?  //  ■•'■ 


Fig.  101. —(a)  Some 
of  the  sarcous 
substance  of  a 
fibre  of  skeletal 
muscle  teased 
to  show  the 
constituent 
fibrils  (sarco- 
stj-les)  with 
transverse 
markings  ;  (6) 
dim  band  ;  (c) 
clear  band  with 
Dobie's  line. 


1.  The  visceral  muscles  appear  to  be 
formed  from  cells  similar  to  ordinary  con- 
nective tissue  cells.  These  elongate  and  be- 
come deiinitely  longitudinally  fibrillated. 
They  thus  become  spindle-shaped  cells, 
varying  in  length  from  about  50  to  200 
micro  -  millimetres.  A  delicate  covering 
membrane,  the  sarcolemma,  is  said  to  be 
present,  but  bridges  of  protoplasm  may 
extend  from  one  fibre  to  another.  In 
mammals  the  nucleus  is  usually  long, 
almost  rod-shaped,  and  is  independent  of 
the  fibrilLe  of  the  protoplasm.  The  nerves 
which  pass  to  these  muscles  are  post- 
ganglionic, and  they  form  a  plexus  be- 
tween the  fibres.  In  many  situations, 
e.g.  the  wall  of  the  intestine,  well- 
jlgil  developed  peripheral  nerve  ganglia  are 
present. 

2.  The  skeletal  muscles  develop 
from  a  special  set  of  cells,  early  differ- 
entiated as  the  muscle-plates  in  the 
mesoblast  down  each  side  of  the 
vertebral  column  of  the  embryo. 
Each  cell  elongates.  The  nucleus 
divides  across,  but  the  cell,  instead  of 
also  dividing,  lengthens,  and  continues 
to  elongate  as  the  two  daughter  nuclei 
ivide.  A  longitudinal  fibrilla- 
tion develops  in  the  protoplasm,  and 
a  series  of  transverse  markings  appears 


Fig.  102.— To  illustrate 
the  possible  structure 
of  a  fibril  as  a  series  of  again 
potential  spheres  in 
the  relaxed  and  in  the 
contracting  condition. 


204 


VETERINARY  PHYSIOLOGY 


across  the  cell..  Lastly,  a  covering,  the  sarcolemvia,  is 
formed.  The  fully-formed  fibre  thus  consists  of  three 
parts. 

(1)  The  Sarcoiemma  is  a  delicate,  tough,  elastic  mem- 
brane closely  investing  the 
fibre,  and  attached  to  it  at 
Dobie's  lines. 

(2)  The  Muscle  corpuscles 
consist  of  little  masses  of 
protoplasm  each  with  a 
nucleus,  which  lie  just  under 
the  sarcoiemma. 

(3)  The  Sarcous  substance 
is  made  up  of  a  series  of  longi- 
tudinal fibrils,  each  made  up 
of  alternate  dim  and  clear 
bands  embedded  in  the  pro- 
toplasm or  sarcoplasm,  as  it 
is  generally  called.  The  dim 
bands  stain  deeply  with  eosin 
(fig.  101).  In  the  middle  of 
the  clear  band  is  a  narrow 
dark  line,  Dobie's  line.  The 
fibres  and  fibrils  tend  to 
break  across  in  the  region 
of  the  clear  band,  showing 
that  they  are  weakest  at 
that  part.  The  clear  band 
differs  from  the  dim  band, 
not  only  in  not  taking  up 
eosin,  but  also  in  the  fact 
that  it  entirely  prevents  the 

passage  of  polarised  light  except  in  one  position  of  the 
analysing  prism,  while  the  dim  band  allows  polarised 
lio-ht  to  pass,  whatever  be  the  position  of  the  prism. 
The  fibrils  of  a  muscle  fibre  must  be  considered  as 
a  distinct  development  of  the  sarcoplasm  in  which 
they  lie  embedded.  They  have  been  called  sarcostyles. 
Various  explanations  of  the  cross  striping  have  been  given. 


Fig.  10.3. — To  show  the  nerves  con- 
nected with  skeletal  muscle.  .3, 
The  ingoing  proprioceptive  fibres  ; 
e,  the  nerve  cells  of  the  anterior 
horn,  from  which  spring  the  true 
somatic  fibres  which  have  been  cut 
in  the  anterior  root  ;  the  degen- 
erated part  is  shown  in  the  dotted 
line  ;  1,  the  fibres  of  the  sympath- 
etic system  with  their  cell  stations 
in  the  sympathetic  ganglion.  The 
post-ganglionic  part  is  not  degen- 
erated, and  its  endings  persist  in 
the  muscle,  as  shown  in  fig.  105. 
(Barenxe.) 


MUSCLE  205 

It  is  clear  that  the  substance  of  the  dim  band  and  of  Dobie's 
line  is  different  from  that  of  the  clear  band,  and  the  view 
has  been  advanced  that  each  sarcostyle  is  a  tube  divided 
into  compartments  by  a  membrane  at  Dobie's  line,  and  with 
a  spongy  material  in  the  dim  band,  into  and  from  which  the 
more  fluid  constituents  of  the  tube,  situated  in  the  clear 
band,  may  flow  in  contraction  and  in  relaxation. 

It  would  perhaps  be  better  to  regard  each  fibril  as  made 
up  of  a  series  of  potential  spheres  drawn  out  and  elongated 
in  the  long  axis  of  the  fibril  in  the  resting  condition,  but 
capable  of  approaching  the  spherical  form  when  their  surface 
tension  is  increased,  as  it  probably  is  in  contraction  (fig.  102). 

Two  types  of  fibres,  the  white  and  the  red,  are  present, 
sometimes  forming  separate  muscles,  as  in  the  fowl,  some- 
times mixed  in  one  muscle.  In  the  white  the  sarcostyles 
are  most  developed,  while  in  the  red  the  sarcoplasm  is  more 
abundant. 

These  skeletal  muscle  fibres  are  generally  about  30  to 
40  mm.  in  length,  but  they  may  be  as  much  as  30  cm 
In  diameter  they  also  vary  greatly — from  about  0  01  mm. 
to  0*1  mm. 

In  muscles  the  fibres  are  joined  end  to  end  through  their 
sarcolemma.  They  are  held  side  by  side  by  fibrous  tissues 
which  carries  the  blood-vessels,  lymph-vessels,  and  nerves. 

The  somatic  nerves  to  these  fibres  are  medullated,  and 
the  neurolemma  joins  the  sarcolemma,  while  the  axon 
spreads  into  dendrites  on  the  sarcous  substance  forming  the 
neuromyal  junction  (fig.  104).  It  is  difficult  to  say  atwhat 
point  nerve  ends  and  muscle  begins. 

There  is  considerable  evidence  that  post-ganglionic 
sympathetic  fibres  also  end  in  the  muscle  fibres  (figs.  108 
and  104). 

The  origin  in  muscle  of  the  ingoing  nerves  has  been 
already  considered  (p.  105). 

3.  The  muscle  of  the  heart  consists  of  a  syncytial 
network  of  fibres  which  have  a  longitudinally  fibrillated  and 
transversely  striped  sarcous  substance,  with  nuclei  placed 
deeply  in  this  substance  and  surrounded  by  undifferentiated 
protoplasm.      The  sarcolemma  is  apparently  absent. 


206 


VETERINARY   PHYSIOLOGY 


^' 


Fig.  104. — The  endings  of  true  somatic  outgoing  fibres  in  skeletal 
muscle  fibres. 


Fig.  105. — The  endings  of  sympathetic  nerve  fibres  in  skeletal  muscle  after 
section  of  the  anterior  roots  of  spinal  nerves.      (Boeke  and  Barenne.) 


MUSCLE 


207 


II.  Chemistry  of  Muscle. 

Like  all  other  living  tissues,  muscle  is  largely  composed 
of  water.  It  contains  about  75  per  cent.  The  25  per  cent, 
of  solid  constituents  is  made  up  of  a  small  quantity,  about 
2  per  cent,  of  ash,  and  22  per  cent,  of  organic  sub- 
stances. 

The  following  table  gives  an  idea  of  the  average  com- 
position of  mammalian  muscle  freed  from  visible  fat  : — 


Water 

.      74  per  cent. 

Solids     .... 

26        „ 

Proteins    .... 

18 

Collagen   ... 

2 

Fat 

2 

Glycogen  .... 

under  1 

Creatin      .... 

.        0-3     „ 

Other  Organic  Substances    . 

Ash           .... 

2 

1.  Proteins. — Of  the  organic  constituents,  by  far  the 
greater  part  is  made  up  of  Proteins. 

They  may  be  extracted  either  by — 

A.  Cooling  the  muscle  at  once  to  near  the  freezing-point 
and  expressing  the  juice  in  a  press. 

When  the  temperature  is  raised  the  fluid  tends  to  clot, 
becoming  a  gel  on  account  of  some  change  in  the  colloidal 
complex.  The  clot  is  myosin,  and  it  is  of  the  nature  of  a 
globulin. 

B.  Rubbing  the  muscle  in  a  mortar  with  NaCl,  and 
then  diluting  so  as  to  make  a  5  per  cent,  solution  of 
the  salt.  By  this  method  the  proteins  may  be  divided 
into — 

(1)  Those  soluble  in  neutral  salt  solutions. 

(2)  Those  insoluble  in  them. 

(1)  These  belong  to  the  class  of  globulins.  They 
are  : — 

(a)  Myosinogen,  which  constitutes   about  75   per  cent,   of 


208  VETERINARY  PHYSIOLOGY 

the  proteins,  and  which  coagulates  at  between  55°  and 
65°  C. 

(6)  Paramyosinogen,  which  constitutes  about  18  per  cent., 
and  coagulates  at  46°  to  51°  C. 

Both  of  these,  after  the  death  of  the  muscle,  change  into 
an  insoluble  form,  Myosin.  In  the  case  of  myosinogen  this 
takes  place  in  two  stages — first,  the  formation  of  a  soluble 
myosin,  coagulating  at  a  very  low  temperature — 35°  to  40°  C.  ; 
and  second,  the  insoluble  myosin.  The  soluble  myosin  exists 
preformed  in  the  muscles  of  cold-blooded  animals. 

(c)  Myoglobulin,  another  globulin  which  does  not  clot,  is 
present  in  the  muscles  of  fish  and  amphibia. 

Whether  such  separate  proteins  actually  exist  in  living 
muscle,  or  whether  they  are  the  result  of  some  change  in 
the  colloidal  complex  at  death,  it  is  impossible  to  sa}^  but 
the  fact  that  the  living  muscle  when  gradually  heated 
undergoes  a  sharp  shortening  at  the  temperatures  at  which 
these  proteins  coagulate  favours  the  view  that  they  do 
exist. 

(2)  The  insoluble  protein  of  muscle,  Myostromin,  seems 
to  be  of  the  nature  of  a  nuclein,  and  probably  forms  the 
framework  of  the  fibrils. 

(3)  In  the  residue  after  extraction  of  the  globulins,  it  is 
always  mixed  with  the  Collagen  of  the  fibrous  tissue  of 
muscle,  from  which  it  may  be  separated  by  dissolving  it  in 
carbonate  of  soda  solution,  and  reprecipitating  by  weak  acetic 
acid  (Ghemical  Physiology). 

2.  In  addition  to  the  proteins,  small  quantities  of  other 
organic  substances  are  found  in  muscle — 

(1)  Carboliydrates. — Glucose  (CgH^^Oe)  is  present  in  muscle, 
as  in  all  other  tissues. 

Glycogen  a;(C6H],Q05) — a  substance  closely  allied  to 
ordinary  starch,  but  giving  a  brown  reaction  with  iodine — is 
always  present  in  muscle  at  rest.  The  amount  is  variable 
from  0"3  to  I'O  per  cent.,  but  the  mass  of  muscle  is  so 
great  that  about  one-half  of  all  the  glycogen  of  the  body  is 
in  the  muscles,  the  rest  being  chiefly  in  the  liver.  If  the 
muscle  has  been  active,  the  glycogen  diminishes,  being 
probably  converted  to  glucose,  and  used  for  the  nourishment 


MUSCLE  209 

of  the  tissue.      (For  the  chemistry  of  the  carbohydrates,  see 
p.  285.) 

(2)  Fat  is  present  in  small  quantities  in  the  fibres,  and 
often  in  very  considerable  quantities  in  the  fibrous  tissue 
between  them,  especially  in  some  animals,  e.g.  the  pig. 

(3)  Inosite,  formerly  called  muscle  sugar,  is  present  in 
small  amounts.  It  is  an  isomere  of  glucose  CeHjoOg,  but  it 
is  a  cyclic  compound  and  not  a  carbohydrate. 

(4)  Sarcolactic  Acid. — Hydroxy-propionic  acid — 

H  OH  O 

I       I      II 
H— C— C— C— 0— H 

I     ! 

H    H 

This  dextro-rotatory  isomere  of  ordinary  lactic  acid  is 
hardly  to  be  detected  in  resting  muscle  when  the  latter  is 
rapidly  removed  and  at  once  placed  in  ice-cold  alcohol.  But 
it  is  rapidly  formed  in  excised  muscle,  and  it  is  markedly 
increased  both  in  muscle  and  in  the  urine  by  hard  exercise 
and  by  mal-oxygenation.  Along  with  COg  and  NaHgPO^  it 
plays  a  part  in  causing  the  phenomena  of  fatigue. 

(5)  From  muscle,  after  removal  of  the  proteins,  a 
series  of  bodies  containing  nitrogen  may  be  extracted. 
The  chief  of  these  is  Creatin,  methyl-guanidin-acetic  acid. 
Guanidin  C.NH(NH2).,  is  a  near  ally  of  urea  CO(NH2)2. 

Methyl-guanidin  is  produced  by  replacing  an  H  in 
guanidin  by  CHg,  and  in  creatin  this  is  linked  to  acetic 
acid — 

H    O 


H— N  CH3 

II       I 
H,N— C— N- 

Methyl-guanidin 


_C— C— OH 

I 

H  acetic  acid 


There  is  some  evidence  that  methyl-guanidin  may  exist 
as  such  in  muscle. 

The  amount  of  creatin  in  the  muscles  of  various  species 
of  animals  is  very  constant.  In  man  about  0  4  per  cent, 
may  be  extracted.  Some  maintain  that  it  does  not  exist 
14 


210  VETERINARY   PHYSIOLOGY 

free,  but  as  part  of  the  muscle  complex.  It  is  more 
abundant  in  white  than  in  red  muscle,  and  when  muscle 
degenerates  it  decreases  in  amount.  How  it  is  formed  is 
not  known,  but  it  may  be  produced  from  methyl-guanidin 
by  union  with  acetic  acid,  methyl-guanidin  being  toxic  while 
creatin  is  inert.  The  fate  of  creatin  will  be  considered 
later. 

Purin  Bodies  (see  Appendix),  in  the  form  of  the  amino- 
purins,  adenin  and  guanin,  are  present  in  small  quantities. 

7.  The  Colour  of  Muscle  varies  considerably,  according  to 
the  preponderance  of  one  or  other  type  of  fibre,  some  muscles 
being  very  pale,  almost  white  in  colour — e.g.  the  breast 
muscles  of  the  fowl  ;  others  again  being  distinctly  red,  even 
after  all  the  blood  has  been  removed.  This  red  colour  is,  in 
some  cases,  due  to  the  presence  of  the  pigment  of  blood, 
hcemoglohin,  but  in  certain  muscles  it  is  due  to  a  peculiar 
set  of  pigments,  onyohctmatins,  giving  different  reactions  from 
the  blood  pigment. 

8.  Inorganic  Constituents. — The  ash  consists  chiefly  of 
potassium  and  phosphoric  acid,  with  small  amounts  of 
sulphuric  and  hydrochloric  acids  and  of  sodium,  magnesium, 
calcium,  and  iron.  The  sulphuric  acid  is  derived  from  the 
sulphur  of  the  proteins,  and  a  part  of  the  phosphoric  acid  is 
derived  from  the  phosphorus  of  the  nucleins  of  muscle,  and 
probably  from  other  organic  combinations. 


III.  Physical  Characters  and  Physiology. 

(1)  Muscle  is  translucent  during  life,  but,  as  death-stiffen- 
ing sets  in,  it  becomes  more  opaque. 

(2)  Muscle  is  markedly  extensile  and  elastic.  A  small 
force  is  sufficient  to  change  its  shape,  but,  when  the  distort- 
ing force  is  removed,  it  returns  completely  to  its  original 
shape,  provided  always  that  the  distortion  has  not  over- 
stepped the  limits  of  elasticity. 

When  a  distorting  force  is  suddenly  applied  to  muscle — 
e.g.  if  a  weight  is  suddenly  attached — the  distortion  takes 
place  at  first  rapidly,  and  then  more  slowly,  till  the  full 
effect  is  produced.      If  now  the  distorting  force  is  removed, 


MUSCLE  211 

the  elasticity  of  the  muscle  brings  it  back  to  its  orio-inal 
form,  at  first  rapidly,  and  then  more  slowly  {Practical 
Physiology).  This  is  seen  in  muscle  relaxing  after  con- 
traction. 

The  advantages  of  these_  properties  of  muscle  are,  that 
every  muscle,  in  almost  all  positions  of  the  parts  of  the  body, 
is  stretched  between  its  point  of  origin  and  insertion.  When 
it  contracts  it  can  therefore  act  at  once  to  bring  about  the 
desired  movement,  and  no  time  is  lost  in  preliminary 
tightening.  Again,  the  force  of  contraction,  acting  through 
such  an  elastic  medium,  causes  the  movement  to  take  place 
more  smoothly  and  Avithout  jerks,  and  a  force  acting  through 
such  an  elastic  medium  produces  more  work  than  when  it 
acts  through  a  rigid  medium  because  less  energy  is  lost  as 
heat. 

The  extensibility  of  muscle  is  of  value  in  allowing 
muscles  to  act  without  being  strongly  opposed  by  their 
antagonistic  muscles,  which,  however,  are  actively  relaxed 
through  the  action  of  nerves  (p.  86).  The  elasticity  of 
muscles  tends  to  bring  the  parts  back  to  their  normal 
position  when  the  muscles  have  ceased  to  contract. 

The  extensibility  of  muscle  is  increased  when  it  is  stimu- 
lated, so  that  the  application  of  a  weight  causes  a  greater 
lengthening  than  when  the  muscle  is  unstimulated. 

(3)  Tonus  of  Muscle. — The  tense  condition  of  resting 
muscle  between  its  points  of  origin  and  insertion  is  not  due 
to  passive  elasticity,  but  is  caused  by  a  continuous  con- 
traction or  tone  kept  up  by  the  action  of  the  nervous  system. 
If  the  nerve  to  a  group  of  muscles  be  cut,  or  the  spinal  cord 
destroyed,  the  muscles  become  soft  and  flabby  and  lose  their 
tense  feeling. 

Alterations  in  the  tonicity  of  the  muscles  is  an  essential 
feature  in  certain  diseases.  It  is  lost  in  infantile  paralysis 
where  the  cells  presiding  over  the  nerves  to  the  muscles  are 
destroyed,  and  it  is  increased  in  the  condition  of  myotonia, 
a,  disease  the  cause  of  which  is  not  known,  in  the  idiopathic 
tetany  of  infants,  in  animals  after  removal  of  the  para- 
thyreoids  (p.  603),  and  after  decerebration  (p.  113).  In 
<lecerebration  rigidity  Sherrington  finds  that  the  extension 


212  YETERINARY   PHYSIOLOGY 

tone  may  be  overcome  by  force,  but  that  the  limb  then 
remains  fixed  in  the  position  in  which  it  is  placed.  This  has 
been  termed  plastic  tone. 

It  has  been  found  that  the  rate  of  chemical  change  in 
muscle  in  decerebration  tonus  is  low  when  compared  with 
the  rate  in  ordinary  contraction  (p.  266).  There  seems  to 
be  some  difference  between  the  processes.  It  has  been 
suggested  that  the  sarcostjdes  are  the  essential  elements  in 
contraction,  and  that  the  sarcoplasm  may  be  responsible  for 
tonus,  but  evidence  is  wanting. 

Some  have  maintained  that  the  tone  of  the  muscles  is 
due  to  the  presence  of  visceral  fibres  in  the  motor  nerves 
(fig.  105).  Such  fibres  have  been  demonstrated  in  many 
muscles,  e.g.  in  the  extrinsic  muscles  of  the  eye  after  the 
third  nerve  has  been  cut.  Certainly  tone  depends  upon  the 
integrity  of  the  reflex  arc  (p.  82). 

(4))  Heat  Production. — Muscle,  like  all  other  living  proto- 
plasm, is  in  a  state  of  continued  chemical  change,  constantly 
undergoing  oxidation  and  reconstruction.  As  a  result  of 
this  chemical  change,  heat  is  evolved.  But  the  heat 
evolved  by  muscle  at  rest  is  trivial  when  compared  with 
that  evolved  during  contraction,  and  heat  production  will 
later  have  to  be  considered  fully  (p.  266). 

(5)  Electrical  Conditions. — (1)  Uninjured  resting  muscle, 
when  at  rest,  is  iso-electric,  but  if  one  part  is  injured,  it  acts 
to  the  rest  like  the  zinc  plate  in  a  galvanic  battery — becomes 
electro-positive.  Hence,  if  a  wire  passes  from  the  injured  to 
the  uninjured  part  round  a  galvanometer,  the  needle  is 
deflected,  indicating  a  current  passing  along  the  wire  from 
the  uninjured  to  the  injured  part  ;  just  as,  when  the  zinc 
and  copper  plates  in  a  galvanic  cell  are  connected,  a  current 
is  said  to  flow  through  the  wire  from  copper  to  zinc 
(fig,  106).  This,  \?,  \X\Q  Current  of  Injury.  In  investigating 
the  electrical  condition  of  muscle,  non-polarisable  electrodes 
must  be  used. 

(2)  When  a  muscle  contracts  certain  electrical  changes 
occur.  These  may  best  be  studied  in  the  ventricle  of  the 
heart  of  a  frog,  which  is  a  muscle  which  can  be  exposed 
without  injury. 


MUSCLE 


213 


By  applying  one  non-polarisable  electrode  to  the  base 
and  the  other  to  the  apex,  and  leading  off  round  a  galvano- 
meter, it  is  found — 1st,  that,  when  the  heart  is  at  rest,  it  is 
of  equal  electrical  potential  throughout,  and  that  no  current 
passes  round  the  galvanometer  ;  2nd,  that,  with  each 
contraction  of  the  ventricle,  an  electric  current  is  set  up,  the 
base,  where  the  contraction  begins,  first  becoming  "  zincy  "  to 
the  apex,  and  later  the  apex,  which  contracts  later,  becoming 
"  zincy  "  to  the  base.     There  is  a  diphasic  variation. 

This  means  that,  when  the  contraction  occurs,  the  part 
which  first  contracts  becomes  of  a  higher  electric  potential 
(more  "  zincy  ")  than  the  rest  of  the  muscle.      The  contract- 


FiG.  106. — To  show  electric  current  of  action  in  a  muscle  (a)  compared  with 
that  in  a  galvanic  cell  [b).  The  contracting  part  of  the  muscle  is 
shaded,     {g)  Galvanometer. 


ing  part  is  thus  similar  to  the  positive  element  of  a  battery — 
the  zinc — the  uncontracting  part  to  the  negative  element. 
The  wire  coming  from  the  contracting  part  will  therefore 
correspond  to  the  negative  pole — that  from  the  uncontracting 
part  to  the  positive  pole.  This  current  of  action  precedes 
the  period  of  contraction. 

When  a  muscle,  in  which  the  current  of  injury  exists,  is 
stimulated,  the  contracting  part  becomes  electrically  more 
like  the  injured  part  ;  thus  the  difference  of  electric  potential 
is  decreased,  and  a  negative  variation  of  the  current  of 
injury  occurs. 

The  electric  variations  may  be  demonstrated  by  laying 
the  nerve  of  one  muscle-nerve  preparation  over  the  muscle  of 
another  muscle-nerve  preparation,  or  over  the  beating  heart, 


214  VETERINARY   PHYSIOLOGY 

when  it  will  be  found  that  the  first  muscle  contracts  with 
each  contraction  of  the  second,  being  stimulated  by  the 
current  of  action  {Practical  Physiology). 

These  electrical  changes  in  muscle  are  of  importance — 
because  by  means  of  them  the  tetanic  nature  of  voluntary 
contraction  of  muscle  has  been  demonstrated  (p.  230),  and 
because  they  are  now  used  in  the  diagnosis  of  heart  disease 
(p.  428). 

In  medicine  the  string  galvanometer  is  generally  used. 
This  consists  of  a  silvered  quartz  fibre  stretched  between  two 
powerful  magnets.  When  a  current  passes  through  the 
fibre  it  is  deflected,  and  the  reflected  light  is  thrown  upon  a 
moving  photographic  plate. 

IV.    Death  of  Muscle. 

The  death  of  the  muscle  is  not  simultaneous  with  the 
death  of  the  individual.  For  some  time  after  somatic  death 
the  muscles  remain  alive  and  are  capable  of  contraction 
under  stimulation.  Gradually,  however,  their  irritability 
diminishes  and  finally  disappears.  They  are  then  dead,  and 
necrobiotic  changes  begin.  The  first  of  these — Rigor  Mortis — 
is  a  disintegrative  chemical  change  whereby  carbon  dioxide 
and  sarcolactic  acid  are  set  free,  and,  at  the  same  time,  the 
soluble  myosinogen  changes  to  the  insoluble  myosin  and  the 
muscle  becomes  contracted,  less  extensile,  less  elastic,  and 
more  opaque.  The  contraction  is  a  feeble  one,  and  since  it 
affects  flexors  and  extensors  equally,  it  does  not  generally 
alter  the  position  of  the  limbs,  although  it  may  sometimes 
do  so.  As  these  changes  occur,  heat  is  evolved  and  the 
muscles  become  warmer. 

The  time  of  onset  of  rigor  varies  with  the  condition  of 
the  muscles.  If  they  have  been  very  active  just  before 
death  stiffening  tends  to  appear  rapidly.  It  may  appear  in 
in  from  10  minutes  to  about  7  hours.  It  generally  begins 
in  the  head  and  passes  downwards,  and  it  disappears  in  the 
same  order. 

It  lasts  for  a  period  which  varies  with  the  species  of 
animal  and  with  the  condition  of  the   muscles,   and   as   it 


MUSCLE  215 

disappears  the  muscles  again  become  soft,  and  the  body 
becomes  limp.  In  all  probability  this  latter  change  is  due 
to  a  solution  of  the  myosin  by  a  proteolytic  enzyme  which 
seems  to  exist  in  all  the  tissues,  and  to  lead  to  autolysis  or 
self-digestion.  It  acts  in  the  presence  of  an  acid,  and  the 
appearance  of  sarcolactic  acid,  therefore,  allows  it  to  come 
into  play. 

It  is  doubtful  if  a  condition  of  rigor  mortis  occurs 
in  visceral  muscle.  The  shortening  is  due  to  the  fall  of 
temperature. 


V.  Muscle  in  Action. 
A.  VISCERAL  MUSCLE. 

1 .  Independence  of  the  Nervous  System. — The  great  character 
of  visceral  muscle  is  its  independence  of  the  central  nervous 
system  which  seems  to  have  granted  it  an  autonomy.  As 
already  indicated  (p.  54),  neuroblasts  migrate  outwards  in 
great  numbers  and  form  plexuses  associated  with  the  layers 
of  visceral  muscle  which  exist  in  the  walls  of  the  various 
viscera.  The  part  played  by  these  plexuses  will  be  considered 
in  discussing  the  movements  of  the  intestines  (p.  332). 

The  nerves  passing  from  the  central  nervous  system  are 
of  two  kinds — augmentors  and  inhibitors. 

2.  Tonus. — When  all  these  nerves  are  cut,  the  muscles  still 
manifest  a  continuous  semi-contraction  or  tone  which  may  be 
modified  by  the  conditions  in  which  they  for  the  time  exist. 
Thus,  stretching  of  the  muscles,  e.g.  by  slowly  injecting 
fluid  into  the  bladder,  may  cause  a  decrease  of  tone  and 
allow  the  viscus  to  be  distended.  But  a  point  is  reached 
at  which  the  tone  is  increased  and  a  forcible  contraction  is 
produced,  and  this  occurs  sooner  if  the  injection  is  made 
rapidly. 

3.  Rhythmic  Variations  of  Tone. — In  most  situations  these 
muscles  manifest  a  rhythmical  increment  and  decrement  of 
tone,  an  apparent  rhythmic  contraction.  This  is  best  seen 
when  they  are  slightly  stretched.  If  a  strip  of  the  intestine 
of  a  rabbit  be  placed  in  Ringer's  solution  supplied  with 
oxygen  and  kept  at  the  temperature  of  the  body,  and  if  it  be 


216  VETERINARY   PHYSIOLOGY 

attached  to  a  lever  with  a  slight  load  upon  it,  the  strip  of 
tissue  will  manifest  a  beat  at  a  rate  of  about  12  per  minute 
(Practical  Physiology). 

4.  Stimulation. — Any  sudden  change  will  cause  these 
muscles  to  contract. 

(1)  Mechanical. — A  blow  or  a  pinch  to  the  gut  will  set  up  a 
contraction,  as  is  seen  when  the  gut  is  pinched  in  strangu- 
lated hernia.     Any  sudden  stretching  may  cause  a  contraction. 

(2)  Thermal. — A  lowering  of  temperature  causes  a  con- 
traction, and  the  more  sudden  the  change  the  more  powerful 
is  the  stimulus.  Exposure  to  a  continued  low  temperature 
sets  up  a  sustained  contraction,  as  may  be  seen  in  the  gut 
after  death.  By  warming  the  gut  this  contraction  may  be 
made  to  pass  olT. 

A  sudden  increase  of  temperature  also  stimulates,  and 
this  is  sometimes  taken  advantage  of  in  obstetrical  practice 
by  injecting  hot  solution  to  make  the  uterus  contract. 

(8)  Electrical. — The  make  and  break  of  a  galvanic 
current  will  cause  a  contraction  as  it  does  in  skeletal  muscle 
(p.  219),  and  it  is  more  effective  than  the  more  sudden 
variation  of  a  farad ic  current. 

(4)  Chemical. — Some  chemical  substances  cause  con- 
traction, e.g.  salts  of  barium,  while  some  cause  relaxation,  e.g. 
nitrites. 

If  any  stimulus  is  sufficient  to  make  visceral  muscle 
contract,  it  causes  a  full  contraction  because  all  the  fibres  are 
stimulated  at  once.  Further,  the  continued  repetition  of  a 
stimulus,  insufficient  to  cause  a  contraction,  may  produce  one, 
just  as  such  repetition  may  liberate  a  reflex  action  fp.  84). 

5.  Characters  of  Contraction. — The  contraction  which  is 
produced  is  a  slow  one.  A  distinct  interval  occurs  between 
the  application  of  the  stimulus  and  the  contraction — the  latent 
period.  This  may  occupy  nearly  a  second.  A  slow  contrac- 
tion and  a  slow  relaxation  follow.  These  may  be  recorded 
by  the  methods  used  for  skeletal  muscle  (p.  222). 

6.  Refractory  Period. — If  a  second  stimulus  is  given 
before  the  contraction  has  passed  off,  probably  no  second  con- 
traction will  be  produced.  But,  if  the  relaxation  phase  is  in 
progress,  a  second  contraction  may  be  superimposed  upon  the 


MUSCLE  217 

first.  Evidently  a  refractory  2:)e7'iod  follows  stimulation  so  that 
the  muscle  refuses  to  respond  again  until  it  has  nearly  finished 
relaxing.     It  is  almost  impossible  to  produce  a  tetanus  (p.  2  28). 

7.  Propagation  of  Contraction. — When  a  contraction  is  set 
up  in  any  part  of  a  sheet  of  visceral  muscle  it  passes  over  the 
rest  as  a  wave.     The  importance  of  this  will  be  considered  later. 

The  physiology  of  Heart  Muscle,  which  closely  resembles 
visceral  muscle,  will  be  dealt  with  when  describing  the  action 
of  the  heart  (p.  423). 

B.  SKELETAL  MUSCLE. 
1.  Direct  Stimulation  of  Muscle. 

Skeletal  muscle  is  directly  under  the  control  of  the 
central  nervous  system.      It  remains  at  rest  indefinitely  until 


Fig.  107. — Curare  Experiment,  to  show  sciatic  nerves  exposed  to  curare,  but 
nerve  endings  protected  on  the  left  side  ;  while  on  the  right  side  the 
curare  is  allowed  to  reach  the  nerve  endings  in  the  muscle. 

Stimulated  to  contract,  usually  by  changes  in  the  nerves, 
produced  by  changes  in  the  central  nervous  system. 

Ca7i  skeletal  muscle  he  made  to  contract  without  the 
intervention  of  nerves — can  it  be  directly  stimulated  ? 

(1)  To  answer  this,  some  means  of  throwing  the  nerves 
out  of  action  may  be  resorted  to.  If  curare,  a  South 
American  arrow  poison,  be  injected  into  a  frog,  the  brain  of 
which  has  been  destroyed,  it  soon  loses  the  power  of  moving. 
When  the  nerve  to  a  muscle  is  stimulated,  the  muscle  no 
longer  contracts.  But  if  the  muscle  be  directly  stimulated, 
in  any  of  the  various  ways  to  be  afterwards  mentioned,  it 
at  once  contracts. 

It  might  be  urged  that  the  curare  leaves  unpoisoned  the 
endings  of  the  nerve  in  the  muscle,  and  that  it  is  by  the 
stimulation  of  these  that  the  muscle  is  made  to  contract. 
But  that  these  are  poisoned  is  shown  by  the  fact  that,  if  the 
artery  to  the  leg  be  tied,  just  as  it  enters  the  muscle,  so  that 


218  VETERINARY   PHYSIOLOGY 

the  poison  acts  upon  the  whole  length  of  the  nerve  except 
the  nerve  endings  in  the  muscle,  stimulation  of  the  nerve 
still  causes  muscular  contraction.  Only  when  curare  is 
allowed  to  act  upon  the  nerve  endings  in  the  muscle  does 
stimulation  of  the  nerve  fail  to  produce  any  reaction  in  the 
muscle,  while  direct  stimulation  of  the  muscle  causes  it  to 
contract.  This  clearly  shows  that  it  is  the  nerve  endings 
which  are  poisoned  by  curare,  and  that  therefore  the  appli- 
cation of  stimuli  to  the  muscle  must  act  directly  upon  the 
muscular  fibres  (fig.  107),  (Practical  Physiology). 

(2)  The  action  of  various  chemical  substances  also  demon- 
strates that  muscle  maybe  directly  stimulated.  Thus,  ammonia 
vapour  fails  to  stimulate  nerve,  but  stimulates  even  those  parts 
of  muscle  which  are  devoid  of  nerve  fibres.  Glycerol,  on  the 
other  hand,  stimulates  nerve  but  does  not  act  on  muscle. 

Muscle,  although  it  can  be  directly  stimulated,  is 
more  readily  made  to  contract  through  its  nerves,  and  a 
knowledge  of  the  points  of  entrance  of  the  nerves  into 
muscles,  the  motor  points,  is  of  importance  in  medicine,  in 
indicating  the  best  points  at  which  to  apply  electrical  stimu- 
lation. Charts  of  the  various  parts  of  the  body  are  given  in 
clinical  text-books. 

2.  Methods  of  Stimulating. 

In  investigating  the  direct  stimulation  of  muscle  apart 
from  nerve,  it  is  necessary  to  throw  the  nerves  out  of  action  by 
curare,  since  it  is  most  easily  stimulated  through  nerve,  and 
since  nerve  responds  to  most  of  the  stimuli  which  act  on  muscle. 

\st.  Various  chemical  substances  when  applied  to  a  muscle 
make  it  contract  before  killing  it,  while  others  kill  it  at  once 
{Practical  Physiology). 

Alterations  in  the  proportions  of  the  salts  or  of  the 
cat-ions  normally  present  in  muscle — sodium,  potassium, 
and  calcium — may  modify  its  excitability  and  may  actually 
cause  contraction.  An  excess  of  sodium  ions  has  a  special 
action  in  this  direction  which  is  checked  by  the  addition  of 
calcium  ions.  Potassium,  on  the  other  hand,  although  the 
chief  base  of  muscle,  when  added  in  the  free  ionic  condition, 


MUSCLE  219 

checks  excitability,  produces  the  phenomena  of  fatigue,  and 
may  finally  kill.  A  balance  between  the  proportion  of  these 
ions,  such  as  exists  in  the  lymph  bathing  the  tissues,  is 
necessary  for  normal  action.  Ringer's  solution  is  an  attempt 
to  reproduce  the  balance  required  for  the  tissues  of  the  frog. 

2nd.  A  sudden  mechanical  change  such  as  may  be  pro- 
duced by  pinching,  tearing,  or  striking  the  muscle  will  cause 
it  to  contract.  This  may  be  seen  in  fractures,  and  in  opera- 
tive interference  with  muscles  {Practical  Physiology). 

8rd  Any  sudden  change  of  temperature,  either  heating 
or  cooling,  stimulates.  A  slow  change  of  temperature  has 
little  or  no  effect.  Muscle,  however,  passes  into  a  state  of 
contraction,  heat  rigor,  when  a  temperature  sufficiently  high 
to  coagulate  its  protein  constituents  is  reached — in  mammals 
about  46°  C.    This,  however,  is  not  a  true  living  contraction. 

4:th.  Muscle  like  nerve  is  stimulated  by  any  sudden 
change  in  the  strength  of  an  electric  current  passed  through 
it,  and  what  has  been  said  of  the  stimulation  of  nerve  (pp.  62 
to  67)  applies  also  to  the  stimulation  of  muscle. 

The  slower  make  and  break  of  the  galvanic  current  is  a  more 
effective  stimulus  to  muscle  than  the  more  rapid  combined  make 
and  break  of  the  faradic  current,  which  is  more  effective  upon 
nerve  (p.  67). 

3.  The  Changes  in  Muscle  when  Stimulated. 

1 .   Change  in  Shape. 

The  result  of  stimulation  of  a  muscle  is  the  sudden 
development  of  tension.  It  is  as  if  a  piece  of  string  passing 
between  two  points  were  suddenly  changed  into  a  stretched 
rubber  band.  If  the  two  ends  are  fixed,  tension  develops,  but 
the  muscle  cannot  shorten.  If  the  ends  are  not  fixed  the  most 
manifest  change  is  shortening  and  thickening  of  the  muscle. 
This  any  one  can  see  in  the  contracting  biceps  muscle. 

In  skeletal  muscle  the  development  of  tension  and  the 
shortening  and  thickening  of  the  muscle  as  a  whole  is  due 
to  the  development  of  tension  in  and  the  shortening  and  thicken- 
ing of  the  individual  fibres  and  their  fibrils. 

In    these  fibrils    the  shortening  and  thickening'  is  most 


220  VETERINARY   PHYSIOLOGY 

marked  in  the  dim  band.  The  clear  band  also  slightly  shortens, 
and  at  the  same  time  appears  to  become  darker.  These 
appearances  may  best  be  explained  on  the  assumption  that 
the  fibrils  are  the  part  of  the  fibre  which  shorten  and 
thicken,  that  these  fibrils  chiefly  change  in  the  dim 
band,  and  that,  by  the  contraction  of  the  fibrils  in  the  clear 
band,  adjacent  dim  bands  are  pulled  nearer  to  one  another, 
and  so  cast  a  shadow  over  the  clear  band. 

That  no  fundamental  chemical  change  takes  place  in 
either  band  is  indicated  by  the  fact  that  they  retain  their 
reaction  to  polarised  light  and  to  staining  reagents  (p.  204). 

Various  theories  have  been  advanced  as  to  how  these 
changes  are  produced,  and  it  will  be  shown  later  that  the 
change  is  of  the  nature  of  an  increase  in  surface  tension. 
Suppose  a  fibril  to  be  made  up  of  elongated  potential 
spheres  in  the  dim  bands,  an  increase  in  the  surface  tension 
of  each  would  cause  it  to  assume  a  more  spherical  form,  and 
this  change  occurring  all  along  the  fibril  would  lead  to  a 
shortening  (fig.  102). 

It  has  been  maintained  that  the  sarcoplasm  is  also 
capable  of  contraction,  and  that  the  tonic  contraction  of 
muscle  is  due  to  this  (p.  211). 

Usually,  in  a  maximum  contraction,  all  the  fibres  contract 
simultaneously.  In  a  lesser  contraction  fewer  fibres  are 
involved  but  they  contract  simultaneously.  This  is  because 
a  nerve  fibre  passes  to  every  muscle  fibre.  When,  as 
sometimes  occurs  in  disease,  the  nervous  mechanism  acts 
abnormally,  the  nmscular  fibres  may  not  all  act  at  once,  and 
a  ipeculisir  fibrillar  twitching,  a  jerky  contraction  of  one  set 
of  fibres  apart  from  the  other,  may  be  produced,  a  condition 
which  most  people  have  experienced  in  the  orbicularis  oculi 
or  in  some  other  muscle. 

If  the  muscle  be  directly  stimulated  at  any  point, 
the  contraction  starts  from  that  point,  and  passes  as  a 
wave  of  contraction  outwards  along  the  fibres.  This  may  be 
seen  by  sharply  percussing  the  fibres  of  the  pectoralis  major 
in  the  chest  of  an  emaciated  individual.  The  rate  at  which 
the  wave  of  contraction  travels  is  ascertained  by  finding  how 


221 


Fig.  108.— a,  Method  of  Recording  Muscular  Contraction.  B,  Key  to  parts 
of  Apparatus.  AI,  Muscle  attached  to  crank  lever  L.  p.c.  Primary 
circuit,  and,  s.c,  secondary  circuit  of  an  induction  coil  with  short  cir- 
cuiting key,  k^,  in  secondary  circuit.  B,  Galvanic  cell,  and,  k,  a  mercury 
key  for  closing  and  opening  the  primary  circuit.  T.M.,  a  lever  moved 
by  an  electro-magnet  placed  in  the  primary  circuit  and  marking  the 
moment  of  stimulation.  In  A,  a  tuning-fork  beating  100  times  per 
second  is  shown  recording  its  vibration  on  the  drum. 


222 


VETERINARY  PHYSIOLOGY 


long  it  takes  to  pass  between  any  two  points  at  a  known 
distance  from  one  another.  Its  velocity  is  found  to  vary 
much  according  to  the  kind  of  muscle  and  the  condition  of 
the  muscle.  In  the  frog  in  good  condition  it  travels  at 
something  over  three  metres  per  second.  When  the  muscle 
is  in  bad  condition  the  wave  passes  more  slowly,  and  in 
badly  nourished  muscle,  e.g.  in  advanced  phthisis,  it  may 
remain  at  the  point  of  stimulation  {Practical  Physiology). 

Contraction  of  a  Muscle  as  a  whole  may  best  be  studied 
under  the  following  heads  :  — 

\8t   The  course  of  contraction. 
2nd.   The  extent  of  contraction. 
3nZ.  The  force  of  contraction. 


\st.  Course  of  Contraction  (fig.  108). 
By  attaching  the  muscle  (ili)  to  the  short  limb  of  a  lever 
{L),  (fig.  108),  and  allowing  the  point  of  the  lever  to  mark 
upon    some    moving    surface,    a    magnified    record    of    the 
shortening  of  the  muscle  when  stimulated  may  be  obtained. 

A  revolving  cylinder  covered  with  a  smoke-blackened 
glazed  paper  is  frequently  used  for  this  purpose,  and,  to 
stimulate  and  mark  the  moment  of  stimulation,  an  induction 

coil  (p.c,  s.c),  with  an 
electro  -  magnetic  marker 
(T.M.),  is  introduced  in  the 
primary  circuit. 

To  find  the  duration  of 
the  contraction,  a  tuning- 
fork,  vibrating  100  times 
per  second,  may  be  made 
to  record  its  vibration  on 
the  surface  (fig.  108,  A) 
{Practical  Physiology). 

In    this     way     such     a 

tracing  as  is  shown  in  fig. 

109  is  produced.     The  appearance  of  the  trace  will,  of  course, 

vary  with  the  rate  at  which  the  recording  surface  is  moving. 


/^?^A^/v\AAA/v6^vv^ 


Pig.  109.  —  Trace  of  Simple  Muscle 
Twitch  (1)  showing  periods  of  lat- 
ency {a-b),  contraction  (b-c),  and  re- 
laxation {c-d) ;  record  of  moment  of 
stimulation  (2)  ;  and  a  time  record 
made  with  a  tuning  -  fork  vibrating 
100  times  per  second  (3). 


MUSCLE  223 

being  more  spread  out  the  greater  the  velocity  of  the 
movement  of  the  surface. 

(a)  From  such  a  trace  it  is  evident  that  the  muscle  does 
not  contract  the  very  moment  it  is  stimulated,  but  that  a 
short  latent  period  supervenes  between  the  stimulation  and 
the  contraction.  In  the  muscle  of  the  frog,  attached  to  a 
lever,  this  usually  occupies  about  0-01  second  ;  but,  if  the 
change  in  the  muscle  be  directly  photographed  without  any 
lever  being  attached  to  it,  this  period  is  found  to  be  very 
much  shorter,  only  about  0  0025  second. 

(6)  The  latent  period  is  followed  by  the  period  of  contrac- 
tion. At  first  it  is  sudden,  but  it  becomes  slower,  and 
finally  stops.  Its  average  duration  in  the  frog's  muscle  is 
about  0  04  second. 

(c)  The  period  of  relaxation  follows  that  of  contraction, 
and  it  depends  essentially  on  the  elasticity  of  the  muscle, 
whereby  it  tends  to  recover  its  shape  when  the  distorting 
force  is  removed  (p.  210),  The  recovery  is  therefore  at  first 
fast  and  then  slow,  and  it  lasts  in  the  frog's  muscle  about 
O'Oo  second. 

The  whole  contraction  thus  lasts  only  about  0"1  second 
in  the  frog's  muscle.  In  mammalian  muscle  it  is  much 
shorter,  in  the  rabbit  about  O'OT  second  ;  in  the  muscle 
of  insects  shorter  still,  about  0  003  second. 

2nd.  Extent  of  Contraction. 

From  the  height  of  the  trace  the  actual  shortening  of  the 
muscle  may  be  calculated  by  measuring  the  two  limbs  of  the 
lever  (Practical  Physiology). 

While,  as  will  be  afterwards  considered,  the  extent  of 
contraction  is  modified  by  the  strength  of  stimulus  and  the 
state  of  the  muscle,  the  total  extent  of  contraction  is  primarily 
determined  by  the  length  of  the  muscle.  If  a  muscle  of  two 
inches  contracts  to  one-half  its  length,  the  amount  of  con- 
traction is  one  inch,  but  if  a  muscle  of  four  inches  contracts 
to  the  same  amount,  it  shortens  by  two  inches  (fig.  1 24,  p.  243). 

Srd.  Force  of  Contraction. 

The  force  of  contraction,  that  is  the  tension  developed,  is 


224  VETERINARY  PHYSIOLOGY 

measured  by  finding  what  weight  the  muscle  can  lift 
(Practical  Physiology). 

The  absolute  force  of  a  muscle  may  be  expressed  by  the 
weight  which  is  just  too  great  to  be  Hfted  by  it.  The  lifting 
power  of  a  muscle  depends — (1)  upon  its  thickness  or 
sectional  area,  i.e.  on  the  number  of  fibres.  The  absolute 
force  of  a  muscle  may  therefore  be  expressed  per  unit  of 
sectional  area.  In  mammals  the  absolute  force  per  1  sq.  cm. 
is  probably  5000  to  10,000  grams. 

(2)  Upon  the  length  of  the  muscle  luheyi  stimulated. 
In  the  body  a  muscle  may  be  in  different  conditions  of 
length  between  its  origin  and  insertion.  The  force  of  con- 
traction or  tension  is  greatest  when  the  muscle  is  extended 
slightly    beyond    its    greatest    normal    length,    and    rapidly 


Fig.  110. — To  show  the  relationship  of  tension  developed  to  the  length  of 
the  muscle.  Tlie  abscissa  represents  the  length  of  the  muscle,  |  is  the 
normal  length  in  the  body.     The  curve  represents  the  tension  developed. 

decreases  if  it  is  shorter,  e.g.  when  the  brachialis  anticus  is 
stimulated  in  partial  flexion  of  the  elbow. 

The  relationship  of  length  to  the  tension  developed  is 
shown  in  fig.  110.  This  relationship  is  of  importance,  for 
it  indicates  that  the  development  of  tension  is  a  surface 
tension  phenomenon. 

The  force  of  contraction  during  different  parts  of  the  con- 
traction period  may  be  recorded  by  making  the  muscle  pull 
upon  a  strong  spring,  so  that  it  can  barely  shorten.  The 
slight  bending  of  the  spring  may  be  magnified  and  recorded 
by  a  long  lever,  and  in  this  way  it  is  found  that  the  ordinary 
curve  of  contraction  gives  a  somewhat  untrue  representation 
of  the  force,  inasmuch  as  the  lever,  from  its  inertia,  is  carried 
too  high  and  seems  to   represent  a  continuance  of  tension 


MUSCLE  225 

beyond  the  point  at  which  it  has  ceased.  This  method  of 
recording  the  force  of  contraction  is  sometimes  called  the 
isometric  method,  in  distinction  to  the  isotonic  method  of 
letting  the  muscle  act  upon  a  light  lever. 

Muscle  may  start  contracting  isometrically,  overcoming 
the  inertia  of  the  weight,  and  then  proceed  in  isotonic 
contraction  while  lifting  the  weight ;  or  the  converse  con- 
dition may  occur. 

In  clinical  medicine  the  dynamometer  is  used  for 
measuring  the  force  of  muscular  contraction.  This  has 
generally  the  form  of  an  oval  steel  spring  which  is  com- 
pressed by  the  hand.  The  degree  of  compression  is  indicated 
on  a  dial  {Practical  Physiology). 

The  contraction  of  the  unexposed  muscles  in  the  body  of  the 
mammal  may  be  studied  by  recording  their  thickening  by 
means  of  Marey's  muscle  forceps.  These  consist  of  a  tam- 
bour, placed  between  one  pair  of  the  limbs  of  forceps  hinged 
in  the  centre.  This  is  pressed  upon  by  the  contraction 
of  a  muscle  or  group  of  muscles  held  between  the  opposite 
limbs.  The  pressure  is  transmitted  by  a  tube  to  another 
tambour  which  carries  a  recording  lever  {Practical  Phy- 
siology). 

It  may  also  be  investigated  by  fixing  a  limb  and  attaching 
the  tendons  of  the  muscles  to  be  studied  to  levers.  This 
method  has  been  used  by  Sherrington  in  his  work  upon  reflex 
action. 

The  Factors  modifying  Contraction. — 1.  Kind  of  Fibre. — 

In  skeletal  muscles,  the  pale  fibres  contract  more  rapidly  and 
completely  than  the  red  fibres,  which  contain  more  sarco- 
plasm  and  nuclei.  The  peculiarities  of  the  contraction  of 
visceral  muscles  have  been  already  considered. 

2.  Species  of  Animal. — In  vertebrates,  the  contraction  of 
the  muscles  of  warm-blooded  animals  is  more  rapid  than  the 
coatraction  in  cold-blooded  animals,  in  the  rabbit  about 
0-07  sec.  The  most  rapidly  contracting  muscles  are  met 
with  in  insects. 

o.  State  of  the  Muscle. — (1)  Continued  Exercise.-— li  a 
15 


226  VETERINARY  PHYSIOLOGY 

muscle  is  made  to  contract  repeatedly,  the  contractions  take 
place  more  and  more  sluggishly.  At  first  each  contraction 
is  greater  in  extent,  but,  as  they  go  on,  the  extent  diminishes 
as  fatigue  becomes  manifest,  and 
stimulation  finally  fails  to  call 
forth  any  response  (fig.  Ill) 
{Practical  Physiology). 

This  condition  is  probably 
caused  by  the  accumulation  of 
the  products  of  activity  in  the 

Fig.  111. — Influence  of  continued  i  „i     „ •   ii        u,      „^ 

Exercise  on  Skeletal  Muscle       ^"^^^^^^^     and     especially     by    an 

■    —(1)  The  first  trace ;  (2)  a     increase    in    the    hydrogen    ion 
trace   after  moderate  exer-     concentration.    The  Same  pheno- 

cise;  (3)  a  trace  when  fatigue  i        •     t  -,     ^ 

has  been  induced.  mena  may  be  mduced    by  the 

application  of  dilute  acids  and 
certain  other  drugs,  and  ma}'^  be  removed,  for  a  time,  by 
washing  out  the  muscle  with  salt  solutions  or  very  dilute 
alkalies.  These  products  act  first  upon  the  nerve  endings 
in  the  muscle,  and  thus,  in  ordinary  conditions  of  stimulation 
through  the  nerve,  the  muscle  is  protected  against  the  onset 
of  fatigue. 

As  a  result  of  repeated  stimulation  a  condition  of 
incomplete  relaxation — contracture — may  appear  and  then 
gradually  wear  off.  Its  onset  may  explain  the  stiffness  of 
muscles  when  first  brought  into  play,  and  the  advantages  of 
' '  warming  up  "  exercises  before  an  athletic  contest. 

Later,  as  fatigue  becomes  manifest,  a  contracture  due  to 
the  very  slow  relaxation  may  be  observed. 

(2)  Temperature. — If  a  muscle  be  tvarmed  above  the 
normal  temperature  of  the  animal  from  which  it  is  taken, 
all  the  phases  of  contraction  become  more  rapid,  and  the 
contraction  may  be  at  first  increased  in  extent,  but  is  sub- 
sequently decreased. 

If,  on  the  other  hand,  a  muscle  be  cooled,  the  various 
periods  are  prolonged.  At  first  the  force  and  extent 
of  contraction  becomes  greater,  a  fact  which  indicates 
that  the  development  of  tension  must  be  of  the  nature 
of  a  change  in  surface  tension  (p.  205).  As  the  cooling 
process  goes  on,  the  contraction  becomes  less  and  less,  until 


MUSCLE 


227 


finally  the  most  powerful  stimuli  produce  no  effect.  Cooling 
has  thus  practically  the  same  effect  on  contraction  as  con- 
tinued exercise  (fig.  Ill)  {Practical  Physiology). 

(3)  Many  drugs  modify  muscular  contractions,  e.g.  veratrin 
enormously  prolongs  the  relaxation  period. 

4.  Strength  of  Stimulus. — A  stimulus  must  have  a  certain 
intensity  to  cause  a  contraction.  The  precise  strength  of  this 
minimum  effective  stimulus  depends  upon  the  condition  of  the 
muscle.  The  application  of  stronger  and  stronger  stimuh  to 
the  nerve  causes  the  muscular  contraction  to  become  more 
and  more  rapid,  more  and  more  complete,  and  more  and 
more  powerful,  by  involving  more  and  more  fibres.  Each 
fibre  when  stimulated  undergoes  the  "  all  or  nothinsf "  chansfe 


Con. 


5 


A    a 


Fig.  112, — Influence  of  increasing  the  Strength  of  the  Stimulus  upon  the  con- 
traction of  Skeletal  Muscle.  St.,  the  stimulus;  Con.,  the  resulting 
contraction.  -.-1,  a  subminimal  stimulus  ;  B,  the  minimum  adequate 
stimulus  ;  C,  the  optimum  stimulus  ;  ,  stimuli ;  -  -  -,  contractions. 


already  described  (p.  68).  But  increase  in  the  contraction  is 
not  proportionate  to  the  increase  in  the  stimulus.  If  the 
stimulus  is  steadily  increased,  the  increase  in  contraction 
becomes  less  and  less.  This  may  be  represented  diagram- 
matically  in  the  accompanying  figure,  where  the  continuous 
lines  represent  the  strength  of  the  stimuli  and  the  dotted 
lines  the  extent  of  the  contractions  (fig.  112). 

After  a  certain  strength  of  stimulus  has  been  reached, 
further  increase  of  the  stimulus  does  not  cause  any  increase 
in  the  muscular  contraction,  since  all  the  fibres  have  been 
made  to  contract.  This  smallest  stimulus  which  causes  the 
maximum  muscular  contraction  may  be  called  the  optimum 
stimulus. 

Increasinof    the   strength   of  the    stimulus    shortens   the 


228  VETERINARY   PHYSIOLOGY 

latent  period,  but  lengthens  the  periods  of  contraction  and 
relaxation,  and  thus  lengthens  the  whole  period  of  con- 
traction. 

5.  Resistance  to  Contraction — Weight  to  be  Lifted. — A  small 
weight  attached  to  the  muscle  may  actually  increase  the 
extent  of  contraction  by  lengthening  the  fibres  (p.  224  (2)), 
but  greater  weights  diminish  it,  and  when  a  sufficient  weight 
is  applied,  the  muscle  no  longer  contracts,  but  may  actually 
slightly  lengthen,  because  its  extensibility  is  increased  during 
stimulation  (fig.  113,  a). 

The  application  of  weights  to  a  muscle  causes  the  latent 
period  and  period  of  contraction  to  be  delayed,  while  it 
renders  the  period  of  relaxation  more  rapid,  and  an  over- 
extension may  be  produced  followed  by  a  recovery  resembling 


1 

Fig.  113. — Influence  of  Load  on  a  Muscular  Contraction,  (a)  The  effect  of 
increasing  the  load  on  the  extent  of  contraction  ;  (h)  the  effect  of  load 
on  the  course  of  contraction. 


a  small   after-contraction  and  due  to  the   elasticity  of  the 
muscle  (fig.  113,  h),  {Practical  Physiology). 

6.  Electrotonus. — As  already  explained  in  dealing  with 
nerve  (p.  63),  the  passage  of  a  galvanic  current  through  a 
muscle  decreases  its  excitability,  and  hence  its  contractility, 
at  the  anode  and  increases  them  at  the  cathode. 

7.  Successive  Stimuli. — So  far,  we  have  considered  the 
influence  of  a  single  stimulus  on  the  shape  of  muscle.  But, 
in  nearly  every  muscular  action,  the  contraction  lasts  much 
longer  than  O'OT  of  a  second,  w^hich  is  about  the  time  taken 
by  a  single  contraction  of  mammalian  muscle. 

How  is  this  continued  contraction  of  muscles  produced  ? 
To  understand  this  it  is  necessary  to  study  the  influence  of 
a  series  of  stimuli. 

(1)  If,  to  a  frog's  muscle  that  takes  O'l  of  a  second  to 
contract  and  relax,  stimuli  at  the  rate  of  say  7  per  second  are 


MUSCLE 


229 


applied,  it  is  found  that  a  series  of  simple  contractions,  each 
with  an  interval  of  O'Oo  of  a  second  between  them,  is  pro- 
duced (fig.  114  (1)).  If  the  stimuh  follow  one  another  at 
the  rate  of  10  per  second,  a  series  of  simple  contractions  is 
still  produced,  but  now  with  no  interval  between  them. 

(2)  If  stimuli  be  sent  more  rapidly  to  the  muscle,  say  at 
the  rate  of  12  per  second,  the  second  stimulus  will  cause 
contraction  before  the  contraction  due  to  the  first  stimulus 
has  entirely  passed  off  (fig.  114  (2)).  The  second  contrac- 
tion will  thus  be  superimposed  on  the  first,  and  it  is  found 
that  the  second  contraction  is  more 
complete  than  the  first,  and  the 
third  than  the  second.  But,  while 
the  second  contraction  is  markedly 
greater  than  the  first,  the  third  is 
not  so  markedly  greater  than  the 
second,  and  each  succeeding  stimu- 
lus causes  a  less  and  less  increase 
in  the  degree  of  contraction,  until, 
after  a  certain  number,  no  further 
increase  takes  place,  and  the  degree 
of  contraction  is  simply  maintained. 
When  the  contractions  follow  one 
another  at  such  a  rate  that  the 
relaxation  period  of  the  first  con- 
traction has  begun,  but  is  not  com- 
pleted, before  the  second  contraction  takes  place,  a  lever 
attached  to  the  muscle,  and  made  to  write  on  a  movino- 
surface,  produces  a  toothed  line.  The  contraction  is  not 
uniform,  but  is  made  up  of  alternate  shortenings  and 
lengthenings  of  the  muscle.  This  constitutes  "  incomjdete 
tetanus"  (fig.  114  (2)). 

(3)  If  the  second  stimulus  follows  the  first  so  rapidly 
that  it  reaches  the  muscle  before  the  contraction  period  has 
given  place  to  relaxation,  then  the  second  contraction  will  be 
superimposed  on  the  first,  the  third  on  the  second,  and  so 
on  continuously  and  smoothly  without  any  relaxations, 
and  thus  the  lever  will  describe  a  smooth  line,  rising  at 
first  rapidly,  then  more  slowly,  till  a  maximum  is  reached 


a  t>   c 

.A  A  A 

1 

1 

Fig.  114.— Effect  of  a  series  of 
Stimuli  onSkeletalMuscle. 

(See  text.) 


230  VETERINARY   PHYSIOLOCxY 

and  being  maintained  at  this  level,  till  the  series  of  stimuli 
causing  the  contraction  is  removed,  or  until  fatigue  causes 
relaxation  of  the  muscle.  This  is  the  condition  of  "  com- 
plete tetanus"  (fig.  114  (3)),  (Practical  Physiology). 

The  rate  at  which  stimuli  must  follow  one  another  in  order 
to  produce  a  tetanus  depends  upon  a  large  number  of  factors. 
An37thing  which  increases  the  duration  of  a  single  contrac- 
tion decreases  the  number  of  stimuli  per  second  sufficient  to 
produce  a  tetanus,  and  thus,  all  the  various^  factors  modifying 
a  single  muscular  contraction  modify  the  number  of  stimuli 
required  (p,  225). 

The  red  fibres  in  tetanus  give  a  more  powerful  con- 
traction than  do  the  white  fibres. 

The  reason  -udiy  a  very  rapid  series  of  stimuli  cause 
a  tetanus  in  skeletal  muscle  is  that  the  refractory  period  is 
so  very  short  (p.  216).  Hence,  if  the  stimuli  follow  one 
another  sufficiently  rapidly  they  fail  to  produce  a  tetanus. 

Every  voluntary  contraction  of  any  group  of  the  muscles 
is  probably  of  the  nature  of  a  tetanus ;  and  the  question 
thus  arises  : — At  what  rate  do  the  stimuli  which  cause  such 
a  tetanus  pass  from  the  spinal  cord  to  the  muscles  ? 

Taking  advantage  of  the  electrical  change  which  accom- 
panies each  muscular  contraction  (p.  212),  and  using  the 
string  galvanometer  (p.  214),  it  has  been  shown  that,  in 
sustained  voluntary  contraction,  electric  variations  occur  at 
a  more  or  less  definite  rate  in  different  muscles,  faster  in  the 
shorter,  and  slower  in  the  longer  muscles. 

Kate  per  Second. 

Masseter  muscle.  .  .  .  .  88  to  100 

Arm  muscles  (flexors) .         .         .         .  47   ,,     50 

Quadriceps  extensor  femoris  .  .  38  ,,     41 

Hence  it  may  safely  be  concluded  that  impulses  at  these 
various  rates  pass  from  the  spinal  cord  to  the  muscles  to 
produce  the  sustained  contraction  of  voluntarj'  action. 

Besides  change  in  shape  muscle  when  stimulated  undergoes 
electrical  changes  (p.  212),  changes  in  elasticity  (p.  211), 
changes  in  temperature,  heat  production  (p.  246),  and  chemical 
changes  (p.  254). 


MUSCLE  231 

4.  Mode  of  Action  of  Muscles. 

The  skeletal  muscles  act  to  produce  movements  of  the 
body  from  place  to  place,  or  movements  of  one  part  of 
the  body  on  another.  This  they  do  by  pulling  on  the  bony 
framework  to  cause  definite  movements  of  the  various 
joints. 

The  muscles  are  arranged  in  opposing  sets  in  relation  to 
each  joint — one  causing  movement  in  one  direction,  another 
in  the  opposite  direction — and  named  according  to  their 
mode  of  action,  flexors,  extensors,  adductors,  abductors,  etc. 
In  the  production  of  any  particular  movement  at  one  joint 
— say  flexion  of  the  phalanx  of  a  foot — the  opposing 
muscles,  the  extensors,  have  their  activity  suppressed  or 
inhibited  (p.  86). 

Other   muscles  which   give  the  support  needed   for  the 


If  '- 


Fig.  115. — The  three  types  of  lever  illustrated  by  the  movements 
at  tne  ankle-joint. 

movement  also  come  into  action.  This  may  be  called  their 
Co-operative  Antagonism.  These  muscles,  when  acting  alone, 
cause  a  movement  in  the  opposite  direction  to  that  being 
produced,  e.g.  extension  instead  of  flexion.  If  the  part  of 
the  brain  which  causes  flexion  of  the  hand  of  the  monkey 
be  stimulated  and  the  nerve  to  the  flexors  divided,  the 
co-operative  action  of  the  supporting  extensors  brings  about 
an  extension  of  the  hand. 

It  is  very  probable  that  the  red  muscles  act  specially  in 
this  fixation  of  joints  to  enable  the  pale  muscles  to  bring 
about  different  kinds  of  movements.  Thus,  with  the  ankle 
fixed,  the  gastrocnemius  may  flex  at  the  knee  ;  with  the  knee 
fixed,  it  may  extend  at  the  ankle. 

The  muscles  act  upon  the  bones,  arranged  as  a  series  of 
levers  of  the  three  classes  at  the  various  joints  (fig.  115). 
These  may  be  illustrated  by  the  foot  in  man. 


232 


VETERINARY  PHYSIOLOGY 


1st  Class. — Fulcrum  between  power  and  weight.  In  the 
ankle  this  is  seen  Avhen,  by  a  contraction  of  the  gastroc- 
nemius, we  push  upon  some  object  with  the  toes. 

2nd  Class. — Weight   between   fulcrum  and  power.       In 


M.  interosseus 


M.  flexor  digitorum  _ 
sublimis 


Sesamoid  bone 


M.  flexor  digitorum 
profundus 


Tliird  metaLarpal  bone 


M.  extensor  digitorum  communis 


"Ergot' 


Ligg.  sesamoidea  obliqua 
Lig.  sesamoideum  lectum 

M.  flexor  digitorum  profundus 


Lig.  plialangosesainoideum 


Cuneus  ("  frog' ) 

Cuneate  m»trix  Digital  torus  Solar  matrix 

Fig.  116. — Longitudinal  Section  of  Digit. 

1  and  1',  2  and  2',  3  and  3'  =  Joint  capsules.  4  =  Synovial  sheath  of 
flexor  tendons.  5  =  Synovial  sheath  of  deep  flexor  tendon.  {From 
"  The  Limbs  of  the  Horse."     Bradley.) 

rising  on  the  toes,  the  base  of  the  metatarsals  is  the  fulcrum,  the 
weight  comes  at  the  ankle  and  the  power  on  the  os  calcis. 


MUSCLE  233 

3rd  Class. — Power  between  fulcrum  and  weight.  In 
raising  a  weight  placed  on  the  dorsal  aspect  of  the  toes  by 
the  contraction  of  the  extensors  of  the  foot,  we  have  the 
weight  at  the  toes,  the  power  at  the  tarsus,  and  the  fulcrum 
at  the  ankle. 

In  the  joints  of  animals  actions  involving  tbe  principle  of 
each  of  these  levers  may  be  found. 

5.  Special  Mechanisms  in  the  Horse. 

(1)   The  Limbs  of  the  Horse. 

The  condition  of  the  feet  and  legs  is  the  chief  limiting 

factors    in    work    production    in    the    horse.      An   intimate 

knowledge  of  the  anatomy  of  the  various  structures  involved 

must  be  obtained  in  the  dissecting  room. 

The  Foot. — The  weight  of  the  horse  is  transmitted 
through  the  second  phalanx  to  the  foot  (fig.  116). 

The  articular  surface  of  the  second  phalanx  is  larger  than 
that  of  the  third,  so  that  its  posterior  part  rests  upon  the 
sesamoid  bone  which  is  supported  by  the  tendons  of  the  flexor 
muscles  and  further  held  in  position  by  the  sesamoid  ligaments. 
This  yielding  articulation  assists  in  reducing  and  distributing 
shock.  The  tendon  of  the  flexor  muscle  rests  upon  the 
digital  torus  (the  plantar  cushion).  This  is  a  pyramidal 
shaped  mass  of  yellow  elastic  and  white  fibrous  tissue  which 
stretches  between  the  cartilages  of  the  third  phalanx  to 
which  its  edges  are  attached  (fig.  117).  The  superficial 
(volar)  surface  of  this  structure  is  arched,  but  becomes 
flattened  out  under  pressure.  The  cartilages  to  which  it  is 
attached  are  elastic  and  yield  under  pressure,  being  pushed 
out  slightly.  The  digital  torus  rests  upon  the  elastic  cuneus 
(the  frog),  which  in  the  normal  unshod  hoof  is  on  a  level 
with  the  wearing  edge  of  the  wall.  The  sole  of  the  foot 
(fig.  118)  is  concave.  When  the  weight  of  the  animal  is 
put  on  it,  the  arch  tends  to  flatten  out,  and  at  the  same 
time  there  is  slight  expansion  of  the  heel.  This  is  provided 
for  by  the  walls  at  the  heel  being  deflected  inward  to  form 
the  bars  instead  of  being  continuous  to  complete  the  circle. 


234 


VETERINARY   PHYSIOLOGY 


The  bars,  which  are  also  elastic,  receive  part  of  the  weight 
of  the  animal.  The  posterior  part  of  the  foot  which  first 
comes  to  the  ground  and.  receives  the  impact  is  thus  well 
adapted  for  the  absorption  and  dissipation  of  shock. 

If  the   toe  of  the  foot  were   provided  with  these  elastic 
yielding  structures  the  propulsive  force  would  be  dissipated. 


Third  metacarpal  bone 


First  phalanx 


Second  phalanx 


Cartilage  of  third  phalanx 
Line  of  border  of  hoof 


Third  phalanx 


Fig.  117. — Lateral  Aspect  of  the  Phalanges  and  the  Cartilage  of  the 
Third  Phalanx.      (Br.\dlf.y.  ) 


Rigidity  is  required  to  transmit  the  force  from  the  toe  which 
acts  as  the  fulcrum.  In  the  toe,  therefore,  the  matrix  of 
the  hoof  is  directly  adherent  to  the  bone,  and  the  horny 
part  of  the  hoof  is  thicker  and  less  elastic  than  at  the  heel, 
so  that  the  force  of  the  thrust  with  the  toe  is  passed  direct 
from  the  hoof  to  the  bony  column  of  the  leg. 


MUSCLE 


28.= 


The  breadth  of  the  bearing  edge  of  the  hoof  is  greatest 
at  the  toe  where  it  is  about  10  mm.  It  becomes  less 
towards  the  heel,  where  it  is  about  half  that  of  the  toe. 
The  inner  surface  of  the  wall  of  the  hoof  is  marked  off  from 
the  border  of  the  sole  by  a  line  of  pale  soft  horn — "  the 
white  line."  This  line  is  taken  as  an  indication  of  the  limit 
of  safety  in  driving  shoe  nails  to  avoid  the  sensitive  part 
of  the  foot.  The  histology  of  the  hoof  Tnust  he  studied 
l^ractically. 

The  Leg. — The    conformation    of    the    lesfs    is    of    great 


Layer  of  pale  horn 
uniting  wall ; 


Inflected  part  of  - 
wall  ("  bar  ") 
Cruro-parietal  groove 


Intercrural  groove 


Base  of  cuneus  ("  bulb  ") 


Apex  of  cuneus 


Crus  of  cuneus 


Fig.  118.— Volar  Aspect  of  the  Hoof.      (Bradley.) 

importance,  as  the  resultant  forward  thrust  of  the  pressure 
of  the  toe  on  the  ground  is  diminished  unless  the  line  of 
action  of  the  force  be  in  the  true  direction. 

To  obtain  this  action,  in  the  fore  leg  viewed  from  the 
front  a  vertical  line  from  the  middle  of  the  scapulo-humeral 
articulation  should  divide  the  leg  into  two  equal  halves  and 
meet  the  ground  at  the  middle  of  the  toe.  In  the  hind 
leg,  viewed  from  behind,  a  vertical  line  dropped  from  the 
tuberosity  of  the  ischium  should  divide  the  leg  into  two 
equal  halves  and  reach  the  ground  in  the  middle  of  the  heel 
(fig.  119,  A).     When  the  legs  are  as  indicated,  the  line  of 


236 


VETERINARY  PHYSIOLOGY 


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MUSCLE  237 

progression  of  the  hoof  is  a  straight  line  and  the  full  force 
of  the  thrust  is  obtained  in  moving  the  mass  of  the  body 
forward. 

When  the  conformation  deviates  from  this  standard,  as  in 
fig.  119,  B,  C,  D,  the  line  of  progression  of  the  foot  is  in 
segments  of  circles.  Part  of  the  force  is  lost  and  the  hoof, 
or  in  the  shod  animal,  the  shoe,  wears  unevenly. 

The  shape  of  the  hoof  and  the  slope  of  the  phalanges 
are  of  great  importance.  The  slope  of  the  wall  of  the  hoof 
and  of  the  phalanges  should  be  the  same — about  45°  to  50° 
in  the  front  foot,  and  about  50°  to  55''  in  the  hind  foot. 
When  the  phalanges  are  too  perpendicular,  as  in  the  "  upright 
pastern"  or  "boxy  foot"  (fig.  120,  A),  the  concussion  absorbing 
mechanism  is  less  effective,  as  more  of  the  shock  is  trans- 
mitted direct  from  the  third  phalanx  to  the  second.  When 
the  phalanges  are  too  obliquely  placed  (fig.  120,  C)  undue 
strain  is  thrown  upon  the  tendons  and  ligaments  supporting 
the  sesamoid  bones.  These  are  consequently  more  liable  to 
injury.  A  high  heel  is  usually  associated  with  upright 
phalanges  and  a  low  heel  with  sloping  phalanges. 

The  more  upright  the  foot  the  shorter  and  lower  the 
stride.  Fig.  120  illustrates  how  the  course  of  the  flight  of 
the  foot  is  determined  by  its  shape. 

(2)   Action  of  the  Limbs, 

(1)  In  standing  the  weight  of  the  body  is  chiefly  slung  by 
the  serrati  magni  on  the  scapulas,  which  are  supported  by 
the  bony  columns  of  the  fore  limbs.  The  flexor  and 
extensor  muscles  maintain  the  condition  of  partial  flexion 
at  the  elbow  joint.  The  metacarpo-phalangeal  articulation 
(the  fetlock  joint)  is  supported  chiefly  by  the  tendons  of  the 
flexor  muscles  and  by  the  interosseus  muscle  (the  suspensory 
ligament).  This  is  a  muscle  which  has  become  tendinous  in 
form  and  function.      By  it  the  sesamoid  bone  is  suspended. 

The  tendons  of  the  flexor  muscles,  which  are  the  chief 
supports  for  the  fore  legs,  are  connected  to  the  bony  column 
by  fibrous  bands  (the  check  ligaments).  These  act  as 
mechanical  stays  to  the  limb,  relieving   the   muscles   from 


238 


VETERINARY   PHYSIOLOGY 


& 


Upright  Foot  usually  associated  with  short  Second 
Metacarpal  and  high  heel.  Stride  is  short,  and 
foot  reaches  highest  point  in  second  half  of  flight. 


Foot  of  medium  slope.     Flight  of  hoof  is  an  arc  of 
circle,  with  highest  point  at  centre. 


Acute  Angled  Foot,  usually  associated  with  long 
Second  Metacarpal  and  low  heel.  Stride  is  long, 
and  foot  reaches  highest  point  in  first  half  of 
flight. 


Fig.  120. — Diagram  showing  influence  of  slope  of  foot  (fore)  on  flight 
of  hoof  in  walking.      (After  Lungwitz.) 


MUSCLE  239 

strain  when  the  standing  position  is  long  continued.  A  like 
arrangement  of  mechanical  braces  exists  in  the  hind  limbs. 

The  centre  of  gravity  is  in  a  vertical  plane  passing  about 
6  inches  behind  the  elbow.  In  standing,  therefore,  the 
greater  weight,  some  10  to  20  per  cent,  more,  is  borne  by 
the  fore  legs  than  by  the  hind.  The  proportion  depends 
largely  on  the  position  of  the  head.  As  the  head  comes 
down  and  forward  as  in  sleep,  the  centre  of  gravity  moves 
forward,  and  the  proportion  of  weight  carried  by  the  fore 
legs  is  increased. 

In  standing  for  any  length  of  time  the  hind  legs  are  used 
alternately  to  support  the  weight  of  the  posterior  part  of  the 
body,  the  one  not  in  use  being  partly  flexed  and  resting  on 
the  toe.  From  the  fact  that  the  hind  legs  are  less  straight 
than  the  fore  legs,  more  work  is  required  by  the  leg  muscles 
in  using  them  as  supports. 

2.  Lying. — Owing  to  the  sharp  edge  of  the  sternum  the 
horse  cannot  lie  vertically.  It  either  lies  inclined  to  one 
side  with  the  four  feet  tucked  under  the  body,  or  flat  on  the 
side  with  head  and  legs  extended. 

3.  Rising. — In  rising  the  head  is  raised,  the  fore  feet  are 
placed  on  the  ground  in  front,  and  the  hind  legs  are  placed 
well  below  the  body  and  push  it  up.  The  raising  of  the 
head  is  the  first  part  of  the  act  of  rising,  and  if  it  be  kept 
down  the  animal  cannot  rise. 

4.  The  movements  of  the  horse  at  the  different  paces 
have  been  analysed  by  instantaneous  photography. 

Walk. — The  body  being  balanced  on  three  legs,  as  shown 
in  fig.  121,  one  fore  leg  is  advanced,  the  body  moves  forward 
on  the  corresponding  hind  leg,  and  the  opposite  hind  foot 
leaves  the  ground  before  the  fore  foot  reaches  it,  so  that  for 
a  moment  the  horse  is  balanced  on  diagonal  legs  (2).  The 
hind  foot  which  has  left  the  ground  is  now  advanced,  and 
before  it  is  planted  the  corresponding  fore  foot  is  lifted  (3), 
and  thus,  at  this  stage,  the  animal  is  balanced  on  the  fore 
and  hind  leg  of  the  same  side  (4).  As  the  hind  foot  comes 
to  the  ground,  the  condition  described  at  the  starting  is 
again  reached  and  the  process  is  repeated. 

Normally  in  walking  the  heel  comes  first  to  the  ground. 


240 


VETERINARY   PHYSIOLOGY 


but  in  drawing  a  heavy  load  the  step  is  shortened  and  the 
toe.  reaches  the  ground  first. 

The  speed  attained  in  walking  is  usually  about   4  miles 


Fio.  121.— The  Walk. 

per  hour.  Among  draught  horses  the  greatest  speed  is  got 
in  the  Clydesdale,  which  has  been  bred  largely  for  the  quality 
of  the  feet  and  legs. 

Trot. — -The  body  is  driven  forward  by  the  alternate  pro- 
pulsive action  of  the  diagonal  fore  and  hind  legs.     The  off 


Fig.  122.— The  Trot. 


fore  and  near  hind  feet  leave  the  ground  together,  propelling 
the  body  upwards  and  forwards  (fig.  122,  1),  and  are  then 
advanced  to  again  reach  the  ground,  when  the  near  fore  and 
off  hind  feet  repeat  the  same  movements  (3). 


MUSCLE 


241 


The  length  and  stride  of  the  trot  is  about  8  or  9  feet. 
The  pace  is  usually  about  7  or  8  miles  an  hour,  but  in  horses 
trained  for  speed  in  trotting  the  pace  may  approach  that  of 
the  gallop. 

Amble. — Here  the  two  legs  of  the  same  side  act  together 
as  do  the  diagonal  legs  in  trotting. 

Gallop.  — At  one  stage  of  the  pace  all  the  feet  are  off  the 
ground  and  well  tucked  under  the  body  (fig.  123,  1).      One 


Fig.  123.— The  Gallop. 

foot,   say    the  off,  first  reaches  the  ground  (2), 


and 


hind 

immediately  after  the  opposite  hind  foot  is  planted  in 
advance  of  it  (3).  The  off  fore  now  comes  to  the  ground, 
and  as  it  does  so  the  off  hind  is  lifted  and  the  horse  rests 
on  diagonal  fore  and  hind  legs  (4).  Then  the  off  hind  foot 
leaves  the  ground  and  the  animal  is  now  on  the  off  fore 
foot  (5).  The  near  fore  foot  is  now  planted  and  the  off  fore 
leaves  the  ground  (6),  and  finally  the  near  fore  is  also  raised 
and  the  horse  is  again  in  the  air. 
16 


242  VETERINARY   PHYSIOLOGY 

The  stride  in  the  gallop  is  about  ]5  to  20  feet.  The 
speed  is  variable.  Race  horses  attain  a  speed  of  30  to  85 
miles  an  hour  on  a  race  course  of  one  to  two  miles  length. 

Canter. — The  canter  is  a  less  energetic  gallop.  At  one 
moment  all  the  feet  are  off  the  ground,  and  they  are  planted 
in  the  same  order  as  in  the  gallop — near  hind,  off  hind, 
near  fore.  But  while  in  the  gallop  the  near  hind  has  left 
the  ground  before  the  near  fore  is  planted,  in  the  canter  all 
these  are  on  the  ground  at  once,  and  it  is  only  as  the  off 
fore  comes  to  the  ground  that  the  near  hind  followed  by  the 
near  fore  is  raised.  The  off  hind  and  then  the  off  fore  next 
follow,  and  all  the  feet  off  the  ground.  Both  the  length  of 
the  stride  and  the  speed  are  less  in  the  canter  than  in  the 
gallop. 

Jump. — The  fore  legs  propel  the  body  upwards,  and  the 
hind  legs  give  a  further  forward  and  upward  propulsion,  and 
are  then  fully  flexed  under  the  body  to  clear  the  obstacle. 
The  animal  alights  on  its  fore  feet,  one  reaching  the  ground 
before  the  other. 

6.  Work  of  Muscle. 

As  the  result  of  the  changes  in  shape,  muscle  performs 
its  great  function  of  doing  mechanical  work  ;  and  the  most 
important  question  which  has  to  be  considered  in  regard  to 
muscle,  as  in  regard  to  other  machines,  is  the  amount  of 
work  it  .can  do.  The  work  unit  generally  employed  is  the 
kilogram-metre — the  work  required  to  raise  one  kilogram  to 
the  height  of  one  metre  against  the  force  of  gravity. 

Since  the  work  done  depends  upon  the  weight  moved 
and  the  distance  through  which  it  is  moved,  the  work-doing 
power  of  muscle  is  governed  by  the  (a)  force  of  contraction, 
i.e.  the  tension  developed  which  determines  the  weight 
which  can  be  lifted,  and  by  (6)  the  amount  to  which  the 
m^iiscle  can  shorten,  for  this  governs  the  distance  through 
which  the  weight  may  be  moved. 

It  has  been  already  shown  that  the  force  of  contraction 
depends  upon  the  sectional  area  of  a  muscle.  A.  thick 
muscle  is  stronger  than  a  thinner  one.  On  the  other  hand, 
the  extent  of  contraction  depends  upon  the  length   of  the 


MUSCLE 


243 


muscle,  since  each  muscle  can  contract  to  a  fixed  proportion 
of  its  original  length.     A  glance  at  the  diagram  will  at  once 
make  this  plain  (fig.  124).     The  size  of  the  muscle  is  thus 
the  first  great  factor  which  governs  its  work-doing  power. 
But  the  tension  developed  also  depends  upon  the  length 


■^ 


m. 


FiG.  124. — Influence  of  the  length  of  a  Muscle  upon  the  work  done.  A  is 
a  muscle  of  two  inches,  and  in  contracting  to  half  its  length  it  lifts  a 
weight  to  one  inch.  B  is  a  muscle  of  one  inch.  It  lifts  the  weight  to 
half  an  inch. 

of  the  muscle  at  the  moment  of  stimulation  (p.  224);  this 
and  every  factor  which  influences  the  force  of  muscular 
contraction  also  influences  the  work  which  can  be  done  (see 
p.  245  et  seq.). 

One  factor,  the  eftect  of  the  load,  requires  special  con- 
sideration. It  has  already  been  shown  that  as  this  is  increased 
the  lift  or  extent  of  contraction  is  diminished. 

The  following  experiment, represented  in  fig.  125,  illustrates 
the  influence  of  increasing  the  load  on  the  work-doing  power 
■of  a  muscle — 


Fig.  125. — To  show  the  influence  of  Load  on  the  Work  done. 

Load. 

Lift. 

—    —    —    —  Work. 

It  will  be  seen  that  increasing  the  load  at  first  increases  the 
amount  of  work  done,  but  that,  after  a  certain  weight  is 
reached,   it   diminishes    it.       There   is,    therefore,    for   every 


244 


VETERINARY  PHYSIOLOGY 


muscle,  so  far  as  its  working  power  is  concerned,  an  optimum 
load. 

In  studying  the  amount  of  work  which  a  muscle,  or  set  of 
muscles  can  do,  the  element  of  time  must  always  be  considered. 
Obviously,  contracting  muscles  will  do  more  work  in  an  hour 
than  in  a  minute. 

Further,  the  rate  at  which  the  work  is  done  has  an 
important  influence,  and  the  amount  of  work  which  can 
be  done  per  unit  of  time  will  depend  not  merely  on  the 
condition  of  the  onuscle  and  the  load,  but  upon   the  rate  at 


Uf.t 


Work  output 

4-7  kgm. 

per  unit  of  time. 


Strong  stimuli. 


Lift 


I   I   I   I   I   I   I   I   I 


Work  output 
I       I  6  kgm. 

per  unit  of  time. 


Weaker  stimuli. 


Fig.  126. — To  show  the  influence  of  varying  the  strength  of  stimulation 
on  the  work  done  per  unit  of  time. 

which  the  lifting  is  performed.  This  will  depend  upon 
(1)  the  strength  of  the  stimuli  and  (2)  the  rate  of  stimula- 
tion. 

(1)  Increasing  the  rate  of  work  by  the  application  of  a 
very  strong  stimulus  at  shor-u  intervals  may  rapidly  lead  to 
fatigue,  and  thus  to  a  comparatively  low  output  of  work  in 
the  time  of  the  experiment,  while  a  smaller  stimulus  at  the 
same  rate  may  cause  a  much  longer  continuance  of  the 
response  of  the  muscle  and  an  actually  greater  output  in 
the  total  time  (fig.  126). 

(2)  On  the  other  hand,  a  stimulus  too  rapidly  applied  may 
soon  lead  to  fatigue,  while  if  more  slowly  applied  the  onset 
of  fatigue  may  be  postponed  and  the  work  done  in- 
creased (fig.  127). 

The  same  is  seen  when  the  work  of  muscles  in  bulk  is 
studied.      Experiments  by  Zuntz  showed  that  in  the  horse, 


MUSCLE 


245 


as  the  speed  in  walking  rose  above  78  metres  per  minute, 
the  rate  of  expenditure  of  energy  per  unit  of  distance  covered 
was  increased.  This  is  confirmed  by  experiments  done 
on  man.  It  has  been  found  that  in  marching,  the  optimum 
rate — the  rate  at  which  the  most  work  can  be  done  on  the 
least  expenditure  of  energy — is  something  over  3  miles  an 
hour.  To  force  the  pace  above  this  leads  to  a  dispro- 
portionate demand  for  energy  and  is  less  economical. 

Fig.   128,  showing  the  extent  of  combustion  in  muscle 
measured  by  the  CO2  produced,  illustrates  this. 


Lift 


Rapid  succession  of  stimuli. 


Lift 


Work  done 

4-7  kgm. 

per  unit  of  time. 


Work  done 

6  kgm. 

per  unit  of  time. 


Slow  succession  of  stimuli. 

Fig.  127. — To  show  the  influence  of  varying  the  rate  of  stimulation 
on  the  work  done  per  unit  of  time. 

The  efficiency  of  a  muscle  as  a  machine  thus  depends 
upon  the — 

1.  Load  lifted. 

2.  Rate  at  which  work  is  done. 

Hence  the  efficiency  of  a  machine  is  often  expressed  in 
terms  of  Horse  Power — the  unit  being  76  kgm.  per  sec. 

Measurement  of  work— 

1.   In  the  isolated  muscle — 

(a)  The  work  done  by  the  single  isolated  muscle  of  the  frog 
in  a  single  twitch  is  got  by  multiplying  the  weight  lifted  by 
the  extent  of  shortening  of  the  muscle  {Practical  Physiology). 

(b)  If  the  work  during  a  series  of  contractions  is  to  be 
measured,  some  means  of  adding  the  effect  of  each  contrac- 
tion to  its  predecessor  must  be  employed,  as  is  done  by 
the  work-collector  of  Fick  {Practical  Physiology). 


246 


VETERINARY  PHYSIOLOGY 


2.  In  groujys  of  viuscles  in  the  body. — When  the 
work  done  by  groups  of  muscles  within  the  body  has  to  be 
studied,  some  form  of  work-measurer  or  ergometer  must  be 
devised.  A  horse  may  be  made  to  walk  upon  a  platform 
which  moves  against  a  known  resistance,  or  the  pull  exerted 
may  be  measured  by  means  of  a  dynamometer,  a  simple  form 


s:^ 
i 

i^ 

/ 

5 

X 

J 

^^ 

o    / 

•1-0 

S 

/ 

'' 

/ 

/ 

^ 

^ 

.--^ 

S 

1 

0 

IlLES    P 

ZH  Hou 

\ 

»-(V- 

Fig.  128. — Graph  to  show  the  effect  of  increasing  the  pace  of  walking  upon 
the  expenditure  of  energy  measured  by  the  oxygen  consumed  and 
carbon  dioxide  given  off.     (Briggs.  ) 

of  which  would   be  a  spring  balance  inserted  between  the 

horse  and   the  object  pulled.  By  these  means  the  capacity 

of   the    horse    for    work     (p.  252)    has    been     determined 
experimentally. 

7.  Heat  Production  in  Muscle. 

In  muscle,  as  in  a  steam-engine  or  any  other  machine,  by 
no  means  the  whole  of  the  energy  is  used  for  the  production 
of  mechanical  work.  Much  of  the  energy  is  lost  as  heat. 
But  while  this  is  an  actual  loss  in  the  steam-engine,  the  pro- 


MUSCLE  247 

Juction  of  heat  is  necessary  in  warm-blooded  animals  to 
maintain  the  temperature  of  the  body  at  a  level  at  which 
the  chemical  changes  essential  for  life  are  possible. 

That  heat  is  given  off  by  muscles  in  contraction  is  shown 
by  the  fact  that,  after  muscular  exercise,  the  temperature  of 
the  body  rises  for  a  short  time.  Some  delicate  method  of 
measuring  the  temperature  must  be  employed  to  demonstrate 
heat  production  in  single  isolated  muscles.  The  mercurial 
thermometer  is  hardly  sufficiently  sensitive,  and,  therefore, 
the  thermo-electrical  method  is  most  generally  employed. 
Various  forms  of  thermopile  may  be  used  (Appendix). 

The  rise  of  temperature  in  a  muscle  after  a  single  con- 
traction is  extremely  small,  but  after  a  tetanic  contraction, 
lasting  for  two  or  three  minutes,  it  is  much  greater. 

By  the  use  of  extremely  delicate  thermopiles  it  has  been 


Fig.  129. — The  continuous  line  shows  the  contraction  of  the  heart  of  the 
terrapin  :  the  dash  line  the  production  of  heat,  and  the  dotted 
line  the  temperature. 


shown  that  in  contraction  most  of  the  heat  is  evolved  in  the 
relaxation  phase.  This  is  more  easily  demonstrated  in  the 
slow  contraction  of  the  heart  of  the  terrapin  than  in  skeletal 
muscle. 

The  above  diagram  shows  the  relation  of  mechanical 
shortening  to  heat  production  (fig.  129). 

The  amount  of  heat  produced  by  a  single  isolated  muscle 
may  be  calculated  if  (a)  the  weight  of  the  muscle,  (6)  its 
temperature  before  and  after  contraction,  and  (c)  the  specific 
heat  of  muscle,  are  known. 

The  specific  heat  of  muscle  is  slightly  greater  than  that 
of  water,  but  the  difference  is  so  slight  that  it  may  be 
disregarded.  If,  then,  a  muscle  of  ten  grams  had  a  tempera- 
ture of  15°  C.  before  it  was  made  to  contract,  and  a  tempera- 


248  VETERINARY  PHYSIOLOGY 

ture  of  15 '05^  C.  after  a  period  of  contraction,  then  Oo 
gram- degrees  of  heat  have  been  produced  ;  i.e.  heat  sufficient 
to  raise  the  temperature  of  0*5  gram  of  water  through  1°  C. 

The  amount  of  energy  liberated  as  heat  may  be  cal- 
culated by  the  various  methods  considered  in  studying 
metabolism  (p.  259). 

The  heat  units  employed  are  the  small  and  large  calories — 
the  small  calorie,  the  heat  required  to  raise  one  gram  of 
water  through  one  degree  Centigrade,  and  the  large  Calorie — 
generally  written  with  a  large  C — the  heat  required  to  raise 
a  kilogram  of  water  through  one  degree  Centigrade. 

8.  The  Relationship  of  Work  Production  to  Heat 

Production. 

The  Mechanical  Eflaciency  of  Muscle. 

The  proportion  of  work  to  heat  is  not  constant  in  muscle 
any  more  than  it  is  in  an  engine.  If  an  unloaded  muscle  is 
made  to  contract,  no  work  is  done  and  all  the  energy  is 
given  off  as  heat,  and  the  same  thing  happens  when,  in 
isometric  contraction,  a  muscle  is  so  loaded  that  it  cannot 
contract  when  stimulated. 

Since  it  is  possible  to  measure  the  tension  exercised  upon 
a  spring  in  isometric  contraction  and  to  measure  the  amount 
of  heat  produced,  and  since  in  such  a  contraction  all  the 
energy  is  finally  given  off  as  heat,  it  is  possible  to  calculate 
the  proportion  between  these.  The  efficiency  in  such  con- 
ditions is  found  to  be  nearly  100  per  cent.  ;  but  this  is  no 
measure  of  the  actual  efficiency  of  the  muscles. 

The  point  of  practical  importance  to  decide  is — How 
much  of  the  energy  liberated  by  muscle  in  normal 
conditions  is  available  under  favourable  circumstances  for 
mechanical  work,  and  how  much  is  lost  as  heat  ? 

To  determine  this,  the  way  in  which  muscle  develops 
tension  when  stimulated  must  be  studied. 

The   Development   of  Tension   by   Muscle. 
It  has  for  long  been  recognised  that  muscle  like  other 
protoplasm   gets    its   energy   from    food.      But  the  problem 


MUSCLE  249 

to  be  considered  is  whether  muscle  develops  the  tension  by 
which  it  can  shorten  and  do  work  by  a  process  of  direct 
oxidation,  or  in  some  indirect  way. 

Pfliiger  long  ago  deprived  a  frog  of  all  free  oxygen  in  an 
air-pump  and  then  kept  it  in  an  oxygen-free  atmosphere  and 
found  that  it  moved  and  gave  off  COo.  He  concluded  that 
the  process  of  oxidation  is  not  a  direct  one. 

Subsequent  experiments  have  confirmed  this  view. 

It  has  been  found  that — 

1.  The  muscle  of  a  frog  may  be  made  to  go  on  contract- 
ing for  some  time  in  nitrogen  in  the  absence  of  oxygen, 
but  that  fatigue  is  soon  manifested  and  is  not  removed   by 


Fig.  130.— (To  be  read  from  right  to  left).  Contractions  of  muscle.  A,  in 
oxygen  ;  B,  in  nitrogen  with  no  oxygen.  In  both,  note  onset  of 
fatigue  ;  in  A  recovery  after  brief  rest,  in  B  no  recovery.     (Fletcher.  ) 

rest.      Sarcolactic  acid  is  liberated  but  no  carbon  dioxide  is 
produced  (fig.  180,  B). 

2.  In  the  presence  of  oxygen,  sarcolactic  acid  does  not 
accumulate.  It  is  oxidised  to  COg  and  HgO.  The  muscle 
recovers  from  fatigue  after  a  short  rest  (fig.  ISO,  A), 

3.  Hence  contraction  is  not  due  to  oxidation,  but  to  the 
throwing  out  of  sarcolactic  acid,  i.e.  to  an  increase  of  H  ions 
which  produces  changes  in  surface  tension  with  the  develop- 
ment of  "  tension."  The  tension  varies  with  the  length  of 
the  fibre,  which  indicates  that  it  is  a  surface  phenomenon 
(Appendix),  When  shortening  occurs  the  tension  is  decreased, 
and  the  potential  energy  becomes  kinetic.  Muscle,  therefore, 
in  contracting  does  not  act  as  a  heat  engine  by  liberating 
energy  by  combustion,  but  as  a  compression  engine  which 
liberates  energj^  already  stored. 


250 


VETERINARY  PHYSIOLOGY 


•i.  The  work  of  restoring  the  potential  energy  is  due  to 
the  oxidation  of  sarcolactic  acid  and  of  carbohydrates, 
proteins,  and  fats.  These  also  yield  the  materials  in  which 
the  energy  is  latent  for  reconstruction.  From  these  latter 
"  foods  "  the  muscle  molecule  is  regenerated  and  the  energ}-, 
therefore,  ultimately  comes  from  them. 
This  is  a  process  requiring  oxidation, 
and  hence  CO2  and  H2O  are  liberated 
and  heat  is  produced  (p.  247)  in  pro- 
portion to  the  work  done  by  the 
muscles. 

5.  In  the  contraction  phase  the 
efficiency  may  be  nearly  100  per 
cent.,  but,  when  the  process  of  restitu- 
tion is  included,  only  at  most  50  per 
cent.  In  considering  the  efficiency  of 
muscle  in  liberating  the  energy  stored 
in  the  food  this  point  must  be  taken 
into  consideration. 

Muscle,    like    secreting   epithelium 
Fig.    131.— To  show  the    (p.  35)^   thus  does  its  work  by  storing 

changes    in    the    effici- 


r 

J  ■ 

/ 

1 

/ 

.V 

^ 

/ 

Jl 

/ 

7 

A' 

it- 

f 

1-4 

t 

^ 

i^ 

t 

_j. 

ency  of  the  muscles  as 
the  work  done  is  in- 
creased. The  decrease 
in  efficiency  is  indi- 
cated by  the  divergence 


the  material  from  which  energy  may  be 
liberated  during  its  resting  phase,  and 
this  process  dominates  the  metabolism 
of  muscle.  For  this,  muscle  requires 
between  the  lines.   The   jqq^  ^q  ^i'v^^  the  material  and  energy 

ordinates    on    the    left     „  *^      .  ,      ,  ,  .   , 

lor  restoration  and  the  oxygen  which 
is  necessary  to  carry  out  the  recupera- 
tion. 

The     efficiency    of    the    muscles     of 
the  body  may  be  measured   by  deter- 


are  Calories  per  minute, 
those  on  the  right  indi- 
cate work  units  con- 
verted to  Calories.  The 
abscissae  are  the  num- 
ber of  revolutions  of 
the  bicycle  at  constant 
work. 


mining    the  total  energy  liberated 


doing  a  measured  amount  of  work 
upon  some  form  of  ergometer  by  direct  or  indirect  calorimetry 
(p.  259).  It  has  been  found  that  some  30  per  cent,  of  the 
total  energy  liberated  is  about  the  riiaximum  mechanical 
efficiency. 

The  efficiency  of  muscle  thus  compares  favourably  with 
that     of     an     ordinary    steam-engine     which    3'ields     some 


MUSCLE  251 

5  to  20  per  cent,  of  the  energy  of  the  coal  in  mechanical 
work. 

In  the  steam-engine,  where  heat  energy  is  converted 
to  work,  the  etficiency  depends  upon  the  difference  of 
temperature  between  the  cylinder  and  condenser.  The 
high  efficiency  of  muscle,  with  an  absence  of  marked 
temperature  variations,  shows  at  once  that  it  is  not  a  heat 
engine  (p.  247).  Compared  with  other  energy  transformers, 
muscle  does  not  stand  so  high.  A  Diesel  engine  gives  a 
theoretical  efficiency  of  about  75  per  cent,  and  a  practical 
efficiency  of  about  50,  and  an  electric  battery  from  80  to  90 
per  cent. 

It  must,  however,  be  recognised  that  the  heat  produced 
by  muscle  is  necessary  to  keep  the  temperature  of  the  body 
at  such  a  level  that  the  chemical  changes,  which  are  the 
basis  of  life,  may  go  on. 

Probably  a  return  of  30  per  cent,  is  seldom  yielded 
under  ordinary  working  conditions  by  the  whole  muscular 
system. 

Within  certain  wide  limits  of  work  done,  the  efficiency  of 
muscle  remains  constant,  that  is,  the  total  energy  evolved  is 
directly  proportionate  to  the  work  done. 

But  if  excessive  work  is  put  upon  muscles,  or  if  the  work 
has  to  be  done  at  a  rate  greater  than  the  optimum,  the 
mechanical  efficiency  decreases  and  the  total  energy  expendi- 
ture rises  out  of  proportion  to  the  work  done,  just  as,  when 
a  steamer  is  driven  above  a  certain  speed,  the  coal  consump- 
tion is  increased  out  of  proportion  to  the  increased  speed. 
This  is  well  illustrated  by  the  increased  production  of  CO2, 
which  occurs  when  the  rate  of  marching  is  forced  from 
3  to  5  miles  an  hour  (fig.  128). 

Fig.  131  shows  the  relationship  of  the  work  done  (effective 
work  performed;  to  the  total  output  of  energy  (total  heat 
output).  Such  a  figure  indicates  very  clearly  that,  while 
for  moderate  increments  of  work  the  mechanical  efficiency  of 
the  muscle  is  fairly  constant,  with  greater  increments  the 
efficiency  becomes  less,  i.e.  the  total  expenditure  of  energy 
increases  out  of  proportion  to  the  work  done. 


252  VETERINARY   PHYSIOLOGY 

9.  The  Efl&ciency  of  the  Horse  as  a  Machine. 

The  total  amount  of  energy  liberated  by  the  horse  in 
performing  a  measured  amount  of  work  lias  been  estimated 
by  the  indirect  method  of  calorimetry  (p.  260).  By  this 
means  the  efficiency  of  the  horse  as  a  machine  has  been 
determined  by  Zuntz  and  others. 

The  total  energy  expended  in  work,  as  in  drawing  a  load, 
is  the  sum  of  three  separate  items  : — 

(1)  The  amount  for  maintenance  purposes.  This  is 
required  whether  the  animal  is  working  or  at  rest. 

(2)  The  amount  spent  in  moving  the  body  of  the 
animal. 

(3)  The  amount  spent  in  moving  the  load. 

The  proportion  which  the  amount  of  useful  work  done  bears 
to  the  total  energy  liberated  (1  +  2  +  3)  is  called  the  gross 
efficiency.  The  proportion  which  the  work  done  bears  to  the 
part  of  the  energy  expended  in  moving  the  load  is  called  the 
net  efficiency.  The  amount  spent  solely  in  moving  the  load  is, 
of  course,  the  total  output  (1  +  2  +  3)  minus  the  output 
when  the  animal  is  moving  without  a  load  (1  +  2).  The 
net  efficiency  under  the  most  favourable  conditions  of  load 
and  speed  is  found  to  be  30  to  35  per  cent.,  which  is  about 
equal  to  that  obtained  in  man. 

10.  Capacity  of  the  Horse  for  Work. 

Draught. — The  amount  of  work  performed  by  the 
draught  horse  has  been  measured  by  means  of  the  moving 
platform  (p.  246).  Over  two  million  kilogram-metres  (about 
7000  foot  tons)  has  been  registered  in  an  experimental  day's 
work,  and  the  amount  of  work  which  the  horse  is  capable 
of  performing  daily  has  been  estimated  as  high  as  6000 
foot  tons  (1,854,720  kilogram-metres).  F.  Smith,  however, 
considers  that  5000  foot  tons  (1,545,600  kilogram-metres) 
is  a  severe  day's  work,  and  that  3000  foot  tons  (927,360 
kilogram-metres)  is  a  fair  average.  The  capacity  of  the 
draught  horse  for  work  is  doubtless  being  increased  by 
selective  breeding   designed  to    increase  the  weight  of  the 


MUSCLE  253 

animal  and  improve  the  conformation  and  qualities  of  feet 
and  legs.  The  former  has  improved  in  Shires  and  the 
latter  in  Clydesdales. 

The  force  that  a  liorse  can  extend  on  a  steady  pull  has 
been  found  by  F.  Smith  to  be  about  75  per  cent,  of  its 
body  weight.  The  load  which  can  be  pulled  depends  largely 
on  the  condition  of  the  road  and  the  nature  of  the  vehicle. 
On  a  level  road  a  total  weight — vehicle  plus  load — of  2-5 
to  4-5  times  the  weight  of  the  animal  can  be  pulled  at  a 
walking  pace. 

On  a  rising  gradient,  in  addition  to  the  pull  to  overcome 
the  inertia  of  the  load  on  starting  and  the  friction  in  motion, 
the  load  and  the  animal  itself  must  be  raised  against  gravity. 
The  load  that  can  be  drawn  up-hill  therefore  decreases  very 
rapidly  as  the  gradient  rises. 

As  the  co-ordination  of  muscle  of  two  or  more  horses 
pulHng  together  is  not  so  perfect  as  that  of  a  single  animal, 
the  amount  that  can  be  pulled  by  more  than  one  horse  is 
always  less  than  the  sum  of  the  amounts  each  can  pull 
separately. 

Carrying. — The  horizontal  spine  of  the  horse  is  not 
adapted  for  carrying  weights.  About  one-fifth  of  the  body 
weight  is  the  maximum  load  a  horse  can  carry  comfortably 
for  any  length  of  time.  In  this  respect  it  is  less  efficient 
than  man  Avith  his  vertical  spine.  An  infantry  soldier  can 
march  carrying  more  than  a  third  of  his  weight.  The 
smaller  breeds  of  horse  can  carry  more  in  proportion  to 
their  weights  than  the  larger.  Consequently  a  nuile  or  pony 
is  a  more  efficient  pack  animal  than  a  large  horse. 

Over-work. — -A  liorse  that  is  worked  beyond  its  capacity 
rapidly  deteriorates  and  is  liable  to  injury  of  the  feet  and 
legs  caused  by  excessive  strain.  When  the  muscles  are 
tired  undue  strain  is  allowed  to  fall  upon  the  supporting 
tendons  Avhich  become  injured.  Co-ordination  of  tired 
muscle  is  less  perfect,  and  "  brushing  "  and  stumbling  occur. 
Severe  work,  necessitating  a  propulsive  force  as  in  heavy 
draught,  or  concussion  as  in  high  speed,  greater  than  the 
elasticity  of  the  structures  of  the  foot  can  accommodate,  is 
apt  to  injure  the  matrix  and   produce   laminitis.       Horses 


254  VETERINARY  PHYSIOLOGY 

are  oftener  off  work  for  lameness  than  for  any  other  ailment, 
and  the  chief  cause  of  lameness  is  work  that  is  excessive 
either  in  amount  or  in  rate. 

The    nature   of   the   chemical    changes   in   muscle — the 
metabolism  of  muscle — must  next  be  studied. 


11.  The  Chemical  Changes  in  Muscle. 
Metabolism  of  Muscle- 
It  has  been  already  pointed  out  that,  on  account  of  (1) 
its  bulk,  (2)  its  constant  action,  (3)  the  extent  of  its 
chemical  changes,  the  metabolism  of  muscle  dominates  the 
metabolism  of  the  body  as  a  whole.  As  already  stated,  it  is 
to  supply  energy  to  muscle  that  food  is  taken.  It  is  to 
oxidise  this  food  and  to  liberate  its  energy  that  air  is  drawn 
into  the  lungs,  and  it  is  to  get  rid  of  the  carbon  dioxide 
formed  in  muscle  that  air  is  breathed  out.  It  is  to  adjust 
the  reaction  of  the  fluid  bathing  the  muscle  and  to  get  rid 
of  the  products  of  muscular  metabolism  that  urine  is 
secreted.  It  is  to  prepare  food  for  muscle  that  the  digestive 
organs  work.  It  is  to  carry  food  and  oxygen  to  muscle 
that  the  flow  of  blood  is  maintained,  and  it  is  to  regulate 
the  supply  of  food  to  the  muscle  that  the  liver  performs  its 
functions. 

Hence  the  study  of  the  metabolism  of  muscle  is  really  the  study 
of  the  general  metabolism  of  the  body.  The  intake  and  output 
of  matter  and  of  energy  are  alike  dominated  by  the 
requirements  of  the  muscles. 

One  caution,  however,  must  be  given.  The  amount  of 
food  taken  is  not  always  regulated  by  the  requirements  of 
the  body,  and  hence  any  surplus  over  what  is  required  must 
either  be  stored  for  future  use  (p.  351),  or  be  got  rid  of 
from  the  body,  just  as  the  coals  thrown  into  a  furnace  in 
excess  of  the  requirements  of  the  engines  are  burned  away. 
In  studying  the  influence  of  various  factors  upon  muscular 
metabolism  the  influence  of  food  has  therefore  always  to  be 
considered,  and  should  either  be  eliminated  by  fasting  or 
be  kept  constant  throughout  the  observations. 

(1)   The  Oxidation  in  Muscle:  Coefacient  of  Oxidation. — The 


MUSCLE  255 

amount  of  oxygen  used  in  muscles  at  rest  and  in  action  is 
ascertained  by  determining  (1)  the  decrease  in  the  amount  of 
oxygen  in  the  blood  leaving  the  muscles  (p.  498)  ;  (2)  the 
amount  of  blood  flowing  through  the  muscles  per  unit  of 
time  (p.  473):  and  (3)  the  weight  of  the  muscle.  This 
allows  the  coefficient  to  be  stated  in  terms  of  c.cm.  of  oxygen 
per  grm.  of  muscle  per  minute.  In  the  skeletal  muscles  of 
mammals  the  following  variations  have  been  found  according 

to   the   condition  of  activity  : oxygen  per  minute  per 


Nerve  cut  :  Tone  absent 
Tone  existing  in  rest 
Gentle  Contraction  , 
Active  Contraction 


grm 


of  muscle  in  c.i 

0-003 
0-006 
0-020 
0-080 


With  active  contraction  the  consiimption  of  oxygen  may 
he  increased  more  than  twenty-fold. 

Similar  results  are  obtained  when  the  influence  of 
muscular  work  upon  the  oxygen  consumption  of  the  body 
as  a  whole  is  studied  (p.  26 6 j. 

Is  this  increased  oxidation  in  contraction  accompanied 
by  any  change  in  the  composition  of  the  muscle  ?  So  long 
as  the  blood  stream  is  intact  it  is  probable  that  any  change 
which  may  occur  is  at  once  made  good.  But  if  muscle 
deprived  of  its  blood  supply  is  investigated  it  is  unsafe  to 
conclude  that  a  change  in  composition  is  the  result  of 
contraction  and  is  not  due  to  the  decreased  supply  of  blood. 
It  has  been  shown  that  when  muscle  contracts  without  a 
supply  of  oxygen,  sarcolactic  acid  accumulates  ;  but  that 
when  it  contracts  in  oxygen,  this  is  oxidised  to  carbon  dioxide 
(p.  249).  In  contraction  the  supply  of  glycogen  in  muscle 
is  decreased,  and  after  fasting  and  strychnine  convulsions,  it 
may  practically  disappear. 

(2)  Material  Oxidised  by  Muscle. — The  study  of  the  co- 
efficient of  oxidation  of  muscle  leads  to  the  consideration  of 
what  is  oxidised  to  yield  the  energy. 

It  has  been  found  that  only  the  three  great  constituents 
of  the  body  and  of  the  food — the  proteins,  carbohydrates, 
and  fats — are  freely  oxidised  to  yield  energy  in  the  body, 
although  some  other  substances,  e.g.  alcohol,  are  capable  of 
a  limited  oxidation. 


256  VETERINARY   PHYSIOLOGY 

(3)  The  Mode  of  Oxidation. — Every  one  knows  that,  at  the 
temperature  of  the  body,  proteins,  fats,  and  carbohydrates 
do  not  undergo  combustion.  To  bring  this  about,  the 
action  of  something  comparable  to  an  enzyme  with  an 
activator  is  necessary. 

The  enzy]ne-like  substance  appears  to  be  formed,  possibly 
from  the  lipoids  of  the  protoplasm,  by  an  auto-oxidation  by 
which  a  peroxide  is  formed,  which  has  a  greater  power  of 
oxidising  than  free  oxygen  has. 

But  peroxides  alone  are  not  sufficient  to  bring  about  the 
combustion  of  sugars,  lactic  acid,  etc.,  an  activator  is 
necessary,  and  such  an  activator  has  been  prepared  from 
various  cells,  and,  since  it  activates  the  peroxide,  it  has  been 
termed  a  peroxidase. 

In  the  oxidation  of  food- stuffs  in  muscle  and  other 
protoplasm  the  following  steps  seem  necessary  : — First,  the 
formation  of  a  peroxide  in  which  oxygen  is  stored  at  a  high 
Dotential  ;  second,  the  activating  action  of  a  peroxidase. 
These  together  may  be  called  an  oxydase. 

(4)  Energy  Value  of  Proteins,  Fats,  and  Carbohydrates.— To  de-^ 
termine  the  amount  of  energy  yielded  by  the  oxidation 
of  each,  all  that  i.-'  necessary  is  to  find  the  amount  of  heat 
evolved  by  its  combustion.  This  is  done  by  burning  a  known 
weight  in  a  water  calorimeter,  an  apparatus  by  which  all  the 
heat  evolved  is  used  to  heat  a  known  volume  of  water.  The 
Bomb  Calorimeter  is  generally  used  for  this  purpose.  It  consists 
of  a  thick  metal  case  in  which  a  weighed  quantity  of  the  food 
to  be  investigated  can  be  placed  in  oxygen.  By  means  of 
wires  it  can  be  completely  burned  by  an  electric  spark. 
The  metal  case  is  placed  in  a  known  quantity  of  water  and 
the  heat  given  off  from  the  food  goes  to  heat  this  water.  By 
taking  the  temperature  before  and  after  the  combustion  the 
amount  of  energy  in  heat  units,  or  calories,  may  be  calculated. 
Suppose  that  1  gram  of  starch  was  put  in  the  bomb  calorimeter 
and  the  case  then  placed  in  1  litre,  i.e.  1  kilogram  of  water  ; 
suppose  the  temperature  of  the  water  was  raised  4-1''  C,  this 
would  mean  that  1  gram  of  starch  had  liberated  4-1  Calories 
of  energy. 

It   is    known    that,   in   the  normal   individual,    fats    and 


MUSCLE  257 

carbohydrates  are  burned  completely  to  COg  and  HgO,  and 
therefore  yield  the  same  amount  of  energy  as  in  the 
calorimeter. 

On  the  other  hand,  the  proteins  are  not  completely  burned 
in  the  body,  the  process  of  combustion  stopping  at  the  form- 
ation of  urea,  CONgH^.  Something  like  3  grm.  of  protein 
yield  1  grm.  of  urea. 

In  the  case  of  fats  and  carbohydrates  where  the  com- 
bustion in  the  body  is  the  same  as  in  the  calorimeter,  this 
method  is  at  once  applicable  to  determine  the  energy  which 
muscle  can  liberate.  The  'physical  and  'physiological  avail- 
ability is  the  same. 

1  grm.  fat— 93  Calories. 

1  grm.  carbohydrate— 41  Calories. 

But  in  the  case  of  proteins  it  is  necessary  to  determine — 
(1)  The  energy  evolved  in  complete  combustion  in  the 
calorimeter.  (2)  The  energy  evolved  by  the  combustion  of 
the  urea  formed,  and  to  subtract  the  latter  from  the  former. 

Complete  combustion  of  1  grm.  yields  about  5-6  Calories — 
the  physical  energy  equivalent.  The  combustion  of  the  urea 
formed  from  1  grm.  of  protein  yields  about  1-3  Calories. 
Hence  about  4-3  Calories  are  available  in  the  body — the 
j^hysiological  energy  eqidvalent.  Variations  occur  according 
to  the  nature  of  the  protein  used,  and  generally  it  is  accepted 
that — 1  grm.  protein  yields  in  tlie  body  41  Calories. 

5.  The  Determination  of  the  Amount  of  Proteins,  Fats,  and 
Carbohydrates  oxidised  in  the  Body. — To  determine  the  amounts 
of  each  of  these  proximate  principles  oxidised  in  the  body, 
it  is  necessary  to  investigate — 

A.  Proteins. — The  amount  of  nitrogen  excreted  gives 
a  measure  of  the  protein  used  since  protein  contains  1 6  per 
cent,  of  nitrogen.  Hence  the  nitrogen  excreted  x6"25  = 
amount  of  protein  used. 

Proteins  contain  nearly  3|  times  as  much  carbon  as 
nitrogen,  so  for  each  grm.  of  nitrogen  from  protein,  3*4 
grm.  of  carbon  are  excreted  (iig.  132). 

B.  Fats  and  Carbohydrates — All  carbon  excreted,  above 
that  derived  from  proteins,  must  come  from  fats  and  carbo- 
hydrates (fig.  132).       In  the  fasting  animal  it  comes  from 

17 


258 


VETERINARY   PHYSIOLOGY 


fats  alone.  When  food  is  taken,  the  determination  of  the 
extent  to  which  each  of  them  is  being  oxidised  depends  upon 
the  fact  that  carbohydrates  are  rich  in  oxygen,  while  fats  are 
poor  in  oxygen.  Hence,  to  oxidise  a  given  weight  of  fat  to 
CO2  requires  more  oxygen  than  to  oxidise  the  same  weight 
of  carbohydrates. 

For    this    reason    the    amount    of   oxygen    required    to 
produce  say  100  c.c.  of  CO2  is  greater  when  fats  are  being 


Fat 
100 


:u-5 


Protein, 
100 


5'?' 


iNJb 


Fig.  132. — Output  of  Nitrogen  and  Carbon  in  a  fasting  animal  to  show  how  the 
combustion  cf  Proteins  and  Fats  is  calculated  from  it.  A,  the  Protein 
and  Fat  metabolised.  B,  the  Nitrogen  and  Carbon  excreted,  derived 
from  A. 

consumed  than  when  carbohydrates  are  being  used,  and 
hence,  if  the  CO2  output  and  the  O2  intake  are  determined 
and  their  proportion  expressed  as — 


CO., 


^.  ',  the  Respiratory  Quotient. 

This  is  found  to  be,  in  the  case  of  Fats,  0-7,  and  in  the 
case  of  Carbohydrates,  I'O. 

In  the  case  of  proteins,  in  which  the  amount  of  oxygen 
is  intermediate  between  that  in  fats  and  carbohydrates,  the 
respiratory  quotient  is  about  O'S. 

Knowing  the  amount  of  carbon  coming  from  proteins 
we  can  calculate  the  O2  necessary  for  their  combustion, 
and  any  excess  of  CO2  and   of  O2  over  this  is  due  to  com- 


MUSCLE 


259 


bustion  of  fats  and  carbohydrates.  When  this  is  expressed 
as  a  respiratory  quotient,  the  amount  of  fats  and  carbo- 
hydrates respectively  used  can  be  determined. 

Tables    giving     the    significance  of  different    respiratory 
quotients  are  prepared  and  are  used  in  such  investigations. 
The  following  gives  a  rough  indication  of  such  a  table. 
R.Q.  Carbohydrates.  Fats 

I'OO  100  per  cent.  0  per  cent. 

66        „  34 


0-90 
0-80 
0-70 


0 


^8 
100 


Owing  to  the  fact  that  the  amount  of  protein  decomposed 
during  the  relatively  short  period  of  the  collection  of  the 
sample  of  expired  air  is,  as  a  rule,  trivial,  the  calculation 
may  be  based  solely  on  the  combustion  of  carbohydrates 
and  fats. 

Oalorimetry. 
To  determine  the  extent  of  these  exchanges  two  different 
methods  have  been  employed. 

(1.)  Direct  Oalorimetry. — By  this  method  the  amount  of 
•energy  liberated  as  heat  is  directly  measured  by  means  of  a 
Respiratory  Calorimeter.  This  consists  of  an  air-tight  double- 
walled  room,  in  which  an  animal  may  be  kept  for  a  day  or 
longer  at  a  time.  It  is 
provided  (1)  with  an  ar- 
rangement by  which  air 
is  supplied  and  the  amount 
of  air  measured  ;  (2)  with 
An  arrangement  by  which 
the  heating  of  the  air 
and  the  amount  of  water 
vaporised  maybe  measured; 
(3)  with  an  air-tight  double 
window,  through  which  the 
animal    can    be    observed. 


Fig.  133. — Diagram  of  a  Respiration 
Chamber  to  show  the  method  of 
anah'sis  of  the  expired  air  and 
the  renewal  of  the  supply  of 
oxygen. 


With  this  chamber  it  is  possible  to  measure — 

(1.)  The  heat  given  off.  This  is  measured  by  the  extent 
to  which  water  circulating  in  special  coils  is  heated  and 
by  the  amount  of  water  vaporised. 


260 


VETERINARY   PHYSIOLOGY 


(2.)  The  excretion  of  matter.  This  is  determined  by 
analysing  the  air  entering  and  the  air  leaving  the  chamber, 
and  thus  finding  the  amount  of  COg  and  H2O  given  off,  and 
by  analysing  the  other  excretions  for  nitrogen  and  carbon. 
Methane  and  hydrogen  excreted  accumulate  in  the  chamber 
and  connected  parts.  The  amount  is  determined  by  analysis 
at  the  end  of  the  experiment. 

(3.)  The  amount  of  O2  absorbed. 

(4.)  The  composition  and  energy  value  of  the  food. 

By  this  means  an  accurate  measurement  can  be  made  of  the 


Fig.    1.34. — The  Chamber  of  the  Respiratory  Calorimeter.     E  is  the  double- 
doored  opening  fur  the  supply  of  food  and  removal  of  the  excreta. 

intake  of  matter  and  energy  in  the  food  and  the  output  of 
matter  and  energy  from  the  body,  and  thus  the  relationship 
between  them  can  be  determined. 

When  man  is  used  as  the  experimental  animal  the 
chamber  is  provided  with  a  folding-bed,  writing-table  and 
chair,  and  an  ergometer  consisting  of  a  fixed  bicycle  working 
against  a  known  resistance.  The  energy  expended  on  the 
work  done  on  the  bicycle  can  thus  be  measured  in  addition 
to  the  enel-gy  expended  as  heat.  This  apparatus  has  been 
largely  used  to  determine  the  energy  and  material  exchange 
in  work  in  man. 

(2.)  Indirect  Calorimetry. —  By  this  method  an  estimate  is 
made  of  the  O2  consumed  and  of  the  CO2  given  otf,  and 
from  this,  conclusions  may  be  drawn,  as  already  indicated, 
as  to  the  amounts  of  the  proximate  principles  consumed, 
and  of  the  energy  liberated.  The  amount  of  proteins  used 
may  be  determined  by  estimating  the  excretion  of  nitrogen, 


MUSCLE 


261 


Fig.  135. 


-Douglas  bag  for  collection   of 
expired  air. 


but  it  has  been  found  that  under  normal  conditions 
during  experiments  of  short  duration,  their  combustion  is 
so  small  that  it  may  be  neglected. 

The  composition  of  the 
air  taken  in  being  known, 
when  (1)  the  quantity  of 
air  expired  and  (2)  its 
composition  is  ascer- 
tained, it  is  possible  to 
determine  how  much  Og 
has  been  taken  up  and 
how  much  CO,  has  been 
given  off  in  a  definite 
time. 

In  experiments  on  man 
this  is  done  by  collecting 
the  air  in  a  special  rubber 
bag  devised  by  Douglas, 
and  which  can  be  carried 
on  the  back  of  the  subject.  A  mouthpiece  is  fitted 
with  two  valves,  through  one  of  which  air  is  inspired,  and 
through  the  other  of  which  it  is  expired  into  the  bag 
(fig.  135).  The  amount  of  air  breathed  may  be  measured 
by  passing  it  through  a  gas  meter,  and  it  may  be  analysed 
by  means  of  Haldane's  apparatus. 

In  animals  the  exchange  can  be  determined  by  usmg 
a  canula  inserted  into  the  trachea  instead  of  a  mouthpiece 
This  method  has  been  used  by  Zuntz  and  his  co-workers  on 
experiments  on  horses  at  work. 

The  indirect  method  has  been  tested  against  the  direct 
method,  and  has  been  found  to  give  very  concordant  results. 

Zuntz  also  used  another  indirect  method.  He  measured 
the  work  done  on  some  form  of  work-measurer  or  ergometer, 
e.g.  a  wheel  turned  against  a  measured  resistance.  By 
converting  the  work  units  of  the  work  thus  done  into  heat 
units  and  subtracting  this  from  the  total  energy  of  the  food, 
expressed  in  heat  units,  the  energy  lost  as  heat  may  be 
determined,  since  ail  energy  not  used  for  mechanical  work 
is  dissipated  as  heat.     Thus  the  relationship  between  work 


VETERINARY  PHYSIOLOGY 


production  and  heat  production  may  be   ascertained,   as  is 


shown  in  the  following  table  for  a  man  : — 


Food 
Work 

Heat 


Kgm. 
212,500 


Calories. 
3000 
500 

2500 


6.  Results  of  these  Investigations.  —  Investigations  by 
these  methods  have  shown  that  when  there  is  an  abundant 
supply  of  carbohydrates  and  fats,  they  are  used  as  the  chief 
source  of  energy,  the  carbohydrates  being  the  more  readily 
used. 

Only  when  the  work  requires  more  energy  than  can  be 
supplied  from  these  sources  are  the  proteins  used  to  any  great 
extent  (fig.  13 6).      But  in  a  lean  animal,  i.e.  an  animal  with  no 


136. — To  illustiate  the  influence  of  Muscular  Work  upon  the  Excretion 
of  Carbon  dioxide  and  of  Nitrogen — (1)  in  a  fasting  or  underfed 
animal ;  (2)  in  an  animal  fed  on  proteins ;  (3)  in  an  animal  on  a 
normal  diet. 

great  store  of  fats  and  carbohydrates,  fed  on  proteins  the 
energy  for  work  may  be  got  almost  entirely  from  them. 

All  the  three  proximate  principles  of  the  food  are 
available,  but  the  great  use  of  proteins  is  in  the  growth  and 
repair  of  muscular  tissue. 

It  may  therefore  be  concluded  that,  in  a  lean  fasting  animal 
and  in  an  animal  fed  on  proteins,  the  muscles  get  their  energy 
chiefly  from  proteins,  but  that,  in  an  animal  with  an  adequate 
store  of  fat  or  upon  an  ordinary  diet,  the  muscles  get  it  chiefly 
from  the  carbohydrates  of  the  food  or  from  the  fats  of  the  food  and 
of  the  body. 

A  study  of  the  ordinary  diet  of  horses  doing  muscular  work 
corroborates  these  conclusions.      In  this  country  the  diet  of 


MUSCLE  263 

a  draught  horse  consists  of  something  like  the  following 
proportions  of  food  constituents  per  1000  kilos,  of  body 
weight  : — 


Amount 
in  Grams, 

Yielding  Calories. 
Actual.               Per  Cent. 

Proteins     . 

2300 

9500 

14 

Fats  . 

800 

7500 

11 

Carbohydrates    . 

.     12,500 

50,000 

75 

GENERAL    METABOLISM    OF   THE    BODY. 

A.   EXCHANGE   OF  ENERGY. 

As  already  indicated  (p.  254),  the  energy  exchanges  of  the 
body  as  a  whole  are  determined  by  the  chemical  changes 
in  the  muscles.  Further,  the  temperature  of  the  body  must 
be  maintained  at  a  level  at  which  these  chemical  changes 
can  go  on.  For  this  purpose  heat  must  be  produced  to 
compensate  for  cooling.  It  will  be  shown  later  (p.  272) 
that  the  rate  of  heat  production  is  influenced  by  the 
amount  and  nature  of  the  food  taken.  Hence  the  energy 
requirements  depend  upon  : — 

(1)  Muscular  activity  ; 

(2)  The  rate  of  cooling,  i.e.  the  amount  of  heat  that 
must  be  produced  ; 

(8)  The  food  taken. 

Before  considering  the  influence  of  these  factors,  the 
lowest  rate  of  metabolism  compatible  with  the  maintenance 
oi  life  must  be  investigated. 

I.  The  Basal  Metabolism. 

The  basal,  or,  as  it  might  perhaps  better  be  called,  the 
standard  metabolism,  is  the  rate  of  metabolism  necessary  to 
maintain  life  when  the  three  factors  mentioned  above  are 
reduced  to  a  minimum. 

The  factors  of  outstanding  importance  in  determining  the 
basal  metabolism  of  an  animal  are  (1)  size  and  (2)  age. 

1.  Size. — The  variation  in  size  is  in  proportion,  not  to  the 
weight  but  to  the  surface,  i.e.  the  rate  of  cooHng.  This 
is  shown  by  comparing  the  basic  energy  expenditure  of 
animals  of  different  size. 


METABOLIS]\[ 

265 

Calories 
per  kilo. 

Calories  per  square 
metre  of  surface. 

Horse 

.      11-3 

948 

Pig     . 

.      191 

1078 

Goose 

.      66-7 

969 

Mouse 

.   2120 

1188 

It  is  seen  that  this  relative  uniformity  of  the  ratio 
of  basal  metabolism  to  surface  area  is  present  even  in  animals 
of  difterent  species.  This  relationship  may  be  due  to  the 
regulation  of  the  rate  of  metabohsm  by  stimuli  from  the 
skin.  This  is  doubtless  true  so  far  as  heat  production  to 
compensate  for  loss  of  heat  is  concerned,  since  the  rate  of 
cooling  is  directly  proportional  to  surface  area.  It  is  probable, 
however,  that  metabolism  is  also  dependent  upon  the  mass 
of  active  tissue,  i.e.  muscle,  and  that  this  accounts  for  the 
relatively  small  variations  found  in  the  ratio  of  metabolism 
to  surface. 

A   comparison   of    the   metabolism   in   different    animals 
should  thus  be  based  on  the  surface  area.      In  bodies  similar 
in  material  and  shape  but  diifering  in  size,   the  surface  is 
proportional   to    two-thirds   the   power   of  the   weight.      As 
animals  of  the  same  species  are  relatively  constant  in  shape 
and  composition,  the  surface  area  can  be  calculated  from  the 
weight  by  the  following,  known  as  Meeh's  formula  : — 
S  =  k  W* 
where  "  S  "  =  unit  of  surface,  W  =  unit  of  weight, 
"  k  "  =  constant  for  each  species. 

The  value  of  "  k "  for  difterent  species  has  been  deter- 
mined by  direct  measurement  of  the  surface  area  and 
calculating   the  relationship   of   unit   of  surface   to  unit   of 


weight.      Some  of  the  values  found  are  : 

— 

Horse       .... 

.      90 

Pig          ...         . 

.      8-7 

Bullock,  fat       . 

.      7-7 

thin     . 

.      9-9 

Sheep      .... 

.    12-1 

Dog         ...          . 

.     10-3-11-2 

A  direct  determination  of  the  extent  of  body  surface  of 


266  VETERINARY  PHYSIOLOGY 

an  animal  is  a  difficult  matter.  It  can  be  estimated,  how- 
ever, from  the  weight.  Since,  for  animals  of  the  same  species 
and  type,  the  factor  "  k  "  is  constant,  the  basal  metabolism 
is  proportional  to  two-thirds  the  power  of  the  weight  and  can 
be  compared  on  that  basis.  Thus,  for  example,  if  an  animal 
of  1000  lbs.  live  weight  has  a  basal  metaboHsm  of  7000 
Calories,  another  of  the  same  species  and  type,  of  1500  lbs. 
would  have  a  basal  metabolism  of  approximately 
7000  X  (\fUf,  i.e.  9172  Calories. 
2.  Age. — In  the  young  animal  not  only  is  energy  required 
for  growth,  but  the  active  tissues  of  the  body  are  in  greater 
proportion  than  in  the  full-grown  animal.  Actual  measure- 
ments in  the  calorimeter  have  shown  that  the  basal  energy 
requirements  of  a  boy  of  ten  years  of  age  are  about  25  per 
cent,  greater  per  unit  of  surface  area  than  those  of  the  adult. 
A  similar  relative  increase  doubtlessly  exists  in  all  young 
animals.  The  influence  of  age  on  metabolism  and  the 
energy  requirements  of  growth  have  not,  however,  been 
fully  investigated. 

II.  Factors  modifying  the  Metabolism. 
1.  Muscular  Activity. 

Since  the  energy  for  muscular  work  comes  ultimately 
from  the  processes  of  oxidation  in  muscle,  the  rate  of  meta- 
bolism varies  with  the  amount  of  work  done,  and  with  the 
rate  at  which  it  is  done. 

This  is  by  far  the  most  important  factor  determining 
the  extent  of  metabolism  and  the  energy  requirements  of  the 
individual.  Under  the  influence  of  muscular  work  a  tenfold 
increase  has  been  observed  in  man.  In  the  horse,  with  its 
great  muscular  development,  an  even  greater  increase  is 
doubtless  sometimes  obtained. 

The  close  parallelism  between  muscular  activity  and 
energy  expenditure  is  remarkable.  Movements  which  are 
scarcely  noticeable,  and  even  increased  tonus  of  muscle 
apart  from  motion,  are  accompanied  by  an  appreciable  rise 
in  the  rate  of  metabolism.  Thus  a  soldier  standing  rigidly 
at  attention   may  expend    10   per  cent,   more    energy  than 


METABOLISM  267 

when  standing  at  ease,  even  although  in  the  former  case 
there  is  no  visible  movement.  The  lowest  rate  of  meta- 
bolism is  reached  in  sleep  when  the  skeletal  muscles  are 
relaxed  and  the  action  of  the  other  muscles  is  reduced  to  a 
minimum. 

2.  Rate  of  Cooling. 

Since  under  ordinary  conditions  the  temperature  of  the 
surrounding  air  is  lower  than  that  of  the  body,  there  must 
be  a  constant  loss  of  heat  and  consequently  a  constant 
production  of  heat  to  keep  the  temperature  of  the  body 
steady. 

Loss  of  Heat. 

1.  Skin. — Heat  is  lost  from  the  body  by  the  skin  in 
two  ways  : — (a)  by  conduction  and  radiation,  and  (h)  by 
evaporation  of  sweat. 

(rt)  Conduction  and  Radiation. — The  extent  of  this  loss 
depends  upon  the  difference  between  the  temperature  of  the 
body  and  that  of  the  air.  Radiation  plays  the  most  important 
part  when  an  animal  is  at  rest  in  still  air  ;  conduction  when 
the  exchange  of  air  over  the  surface  is  rapid,  as  in  wind. 

The  influence  of  variation  in  the  temperature  of  the  air 
is  minimised  by  the  covering  of  fur  or  feathers,  which  retains 
a  stationary  layer  of  air  of  about  25°  to  30°  C.  over  the 
skin. 

Cold  stimulates  the  growth  of  hair.  On  the  other  hand, 
the  heavy  winter  coat  tends  to  be  shed  when  an  animal  is 
confined  in  a  warm  atmosphere. 

(6)  Evaporation  of  Siueat. — Heat  is  rendered  latent  by 
the  evaporation  of  sweat,  and  is  taken  from  the  body  which 
is  thus  cooled,  just  as  the  hand  may  be  cooled  by  allowing 
ether  to  evaporate  upon  it.  The  extent  of  loss  depends  not 
only  on  the  amount  of  sweat  secreted,  but  also  upon  the 
rapidity  with  which  the  evaporation  goes  on.  This  is  governed 
by  the  dryness  and  temperature  of  the  air,  and  the  rapidity 
of  its  exchange  by  wind  and  other  air  currents. 

The  horse  sweats  easily.  Working  under  ordinary  con- 
ditions of  climate,  it  may  lose  from  5  to  10  litres  of  water 
as  sweat  per  day. 


268  VETERINARY   PHYSIOLOGY 

The  pig  has  sweat  glands  only  on  the  snout ;  the  dog 
only  on  the  muzzle  and  foot  pads.  The  sheep  has  few  sweat 
glands  and  perspires  very  little.  Cattle  also  perspire  little, 
except  on  the  muzzle. 

Moisture  in  the  air  increases  the  conductivity  of  the  body 
covering,  and  consequently  the  loss  of  heat  by  conduction 
and  radiation  in  a  cold  atmosphere  is  greater  in  proportion 
to  the  humidity.  On  the  other  hand,  moisture  in  the  air 
hinders  the  free  evaporation  of  sweat,  so  that  loss  of  heat 
by  evaporation  is  less  rapid  in  a  hot  climate  that  is  moist 
than  in  one  which  is  dry.  Consequently  in  climates  with 
temperatures  above  that  of  the  body  the  heat  is  better  borne 
by  animals  when  the  air  is  dry  and  moving. 

2.  Respiratory  Passages — Evaporation  from  the  respiratory 
passages  and  the  heating  of  the  respired  air  may  account  for 
10  to  20  per  cent,  of  the  heat  lost.  In  the  dog  the  amount 
of  heat  lost  in  this  way  may  be  considerably  greater  Tp.  269). 

3.  Urine  and  Faeces. — A  certain  amount  of  heat  is  lost 
by  raising  the  ingested  food  and  water  to  body  temperature, 
at  which  the  urine  and  fasces  are  voided.  In  the  dog  the 
amount  is  small — something  less  than  2  per  cent.  Where 
the  food  is  bulky — as,  for  example,  in  ruminants  on  a  heavy 
root  diet — the  amount  may  be  as  much  as  10  per  cent. 
of  the  total  heat  lost. 

Heat  Production. 

A.  Muscle. — As  already  indicated,  muscle  is  the  great  heat 
producer  on  account  of  its  great  bulk  and  constant  activity. 
Not  only  may  it  be  demonstrated  that  (1)  the  temperature 
of  muscle  in  action  rises,  but  (2)  it  has  been  found  that  the 
temperature  of  blood  coming  from  the  muscles  is  slightly 
higher  than  that  of  blood  going  to  them.  (3)  Muscular 
exercise  raises  the  temperature  of  the  body.  (4)  Drugs 
which  interfere  Avith  muscular  contraction,  such  as  curare, 
diminish  the  temperature,  and  (6)  young  animals,  before 
their  muscular  tissues  become  active,  have  a  low  temperature 
unless  kept  in  warm  surroundings. 

B.  G-lands. — A   certain   amount   of  heat   is   produced   in 


METABOLISM  269 

glands  and  chiefly,  on  account  of  its  great  size,  in  the  liver. 
During  active  digestion  the  temperature  of  the  blood  coming 
from  the  liver  is  distinctly  higher  than  that  of  the  blood 
going  to  the  organ,  and,  since  the  amount  of  blood  passing 
through  the  organ  is  large,  an  appreciable  amount  of  heat 
is  derived  from  it.  The  production  in  glands,  however,  is 
trivial  when  compared  with  the  production  in  muscle. 

Heat  Regulation. 

In  spite  of  wide  fluctuations  in  rate  of  heat  production 
and  heat  loss,  the  body  temperature  varies  within  very 
narrow  limits.  This  constancy  is  maintained  by  two  means, 
called  respectively  the  j^hysical  regulation,  which  controls 
the  rate  of  loss  of  heat,  and  the  chemical  regulation,  which 
controls  the  rate  of  production. 

Physical  Regulation. — When  heat  production  exceeds  heat 
loss,  the  resultant  rise  in  body  temperature  is  accompanied 
by  dilatation  of  the  cutaneous  vessels,  so  that  more  blood  is 
brought  to  the  surface  and  the  loss  of  heat  increased.  Along 
with  this  an  increased  secretion  of  sweat  occurs. 

Conversely,  when  heat  loss  exceeds  heat  production, 
constriction  of  cutaneous  vessels  and  decrease  of  sweat 
secretion  reduce  the  loss  of  heat. 

These  adjustments  in  the  skin  to  maintain  the  balance 
between  heat  production  and  heat  elimination  are  reflex 
effects. 

In  the  dog,  which  has  no  sweat  glands  on  the  part  of 
the  body  covered  by  hair,  increased  elimination  of  heat  is 
brought  about  by  a  panting  respiration  that  increases  the 
loss  of  heat  by  the  respired  air  and  by  evaporation  from  the 
respiratory  passages.  The  mouth  is  held  open  and  the  tongue 
allowed  to  hang  out,  so  that  as  large  a  moist  surface  as 
possible  may  be  exposed  to  the  air  to  allow  cooling  by 
evaporation.  In  cattle  increased  elimination  of  heat  by 
sweating  is  small  in  amount,  and  their  temperature  is 
therefore  especially  liable  to  rise  with  exercise. 

Chemical  Regulation. — As  the  temperature  of  the  environ- 
ment of  an  animal  falls,  a  point  may  be  reached  at  which 
even  with  loss  of  heat  restricted  to    the  utmost,  heat  pro- 


270 


VETERINARY   PHYSIOLOGY 


duction  may  be  insufficient  to  balance  heat  loss,  and  the 
body  temperature  begins  to  fall.  When  this  occurs  the  rate 
of  metabolism  and  consequently  of  heat  production  is  in- 
creased either  by  increased  muscle  tonus  or  by  muscular 
exercise.     This  increased  muscular  activity  may  consist   of 


Rate  of  MeUbolism 

'*■■*•*—, 

^^;.. 

Temperature 

of  the  Body 

' 

Radiation  and 

/.. 

Conduction 

Physical 

Physiological 

""^•■^•^ 

\^ 

r 

Evaporation 

"^^ 

'•^-< 

Physical 

x  ■ 

^^  ''***^ 

Physiological 

^ 

,>''         , 

:--'' 

Temperature  of 
surroundintr  air 

.  *\ 

in  dfegrees 
centigrade 


60 


SO  0 


Fig.  137. — To  show  the  physical  and  the  chemical  regulation  of  temperature. 
Note  that  roughly  between  20°  and  40°  C.  the  regulation  is  by  changes 
in  the  loss  of  heat,  and  that  below  20°  C.  increased  heat  production 
occurs.    Above  40°  C.  a  hyperthermal  increase  of  heat  production  occurs. 


voluntary  movements,  or,  if  these  be  prevented,  by  a  shivering 
fit  which  is  simply  a  reflex,  involuntarily  bringing  into  action 
a  large  number  of  muscles. 

The  voluntary  movements  for  heat  production  are  seen 
in  the  "  freshness  "  of  a  horse  brought  out  of  a  warm  stable 
into  a  cold  atmosphere.  A  shivering  tit  is  often  seen  in  the 
horse  after  drinkiug  a  large  quantity  of  cold  water,  which 


METABOLISM  271 

abstracts    heat    from    the    body    in    being    raised     to    body 
temperature. 

The  way  in  which  the  rate  of  metaboHsm  rises  as  loss  of 
heat  increases,  with  a  fall  of  temperature,  is  shown  by  the 
following  results  obtained  by  Rubner  on  a  fasting  dog  : — 

_  ^  Rate  of  Metabolism  in 

^e™P-  *-•  Cal.  per  kg.  body  wt. 

35  68-5 

30  56-2 

25  54-2 

20  55-9 

15  63-0 

7  86-4 

In  this  chemical  regulation  energy-yielding  materials, 
usually  carbohydrates  and  fats,  are  metabolised  solely  for  the 
purpose  of  heat  production. 

Wind  has  a  very  marked  effect  upon  the  rate  of  cooling, 
and  consequently  upon  the  rate  of  metabolism,  especially  in 
an  animal  like  the  pig  where  the  hair  is  scanty.  Hill  has 
shown  that  in  man  exposure  to  a  cold  wind  may  nearly 
double  the  energy  expenditure. 

Critical  Temperature. — In  the  case  of  the  fasting  dog 
quoted  above,  the  lowest  rate  of  metabolism  is  reached  in 
an  environment  with  a  temperature  of  about  25°  C.  Below 
this  level  the  rate  of  metabolism  is  increased  for  heat  pro- 
duction by  chemical  regulation.  As  the  temperature  rises 
above  25°  C.  not  only  is  loss  of  heat  decreased,  but  meta- 
bolism is  stimulated  by  the  rise  in  temperature,  so  that  heat 
is  produced  beyond  requirements,  and  its  elimination  is 
hastened  by  physical  regulation.  The  level  of  the  external 
temperature  at  which  chemical  regulation  gives  place  to 
physical,  or  vice  versa,  is  known  as  the  critical  ter)ipera- 
ture.  It  is  the  temperature  of  minimum  metabolism,  and 
consequently  of  lowest  food  requirements. 

Experimental  evidence  is  lacking  to  determine  the  exact 
critical  temperature  for  domestic  animals.  It  is  probably 
about  20°  to  25°  C.  in  the  horse  and  pig,  and  somewhat 
lower  in  ruminants.  In  the  individual  it  varies  with  the 
condition  of  the  coat  and  the  amount  of  subcutaneous  fat. 

The  takinof  of  food  causes  an  increase  in  rate  of  metabolism. 


272  VETERINARY   PHYSIOLOGY 

The    critical    temperature    of    the    fed  animal    is    therefore 
lower  than  that  of  the  fastincf  animal. 


3.  Effect  of  taking  Food  on  Metabolism. 

The  consumption  of  food  by  an  animal  increases  the  basal 
metabolism,  as  is  seen  in  the  following  results  obtained  on 
a  horse  : — 

Calories  per  hour  per  kg. 

Fasting 102 

3^  hours  after  feeding       .  .  .      1'13 

The  increase  is  influenced  by  the  quantity  and  character 
of  the  food.  Each  of  the  proximate  principles  stimulates 
chemical  changes.  Proteins  have  a  special  influence  in  this 
direction,  and,  when  eaten,  metabolism  is  so  increased  that 
something  like  SO  per  cent,  of  the  energy  they  contain  may 
be  liberated  as  heat.  This  is  called  their  specific  dynamic 
action.  Lusk  has  shown  that  it  is  due  to  the  direct  action 
of  certain  of  the  amino-acids  upon  the  metabolism. 

The  increased  metabolism  following  feeding  was  formerly 
attributed  to  (1)  the  muscular  work  involved  in  peristalsis, 
and  (2)  to  the  liberation  of  energy  in  the  disintegration  of 
the  molecules  of  the  food  in  the  process  of  digestion.  Hence 
it  was  said  to  be  due  to  the  "  work  of  digestion,"  an  unfor- 
tunate term  still  widely  used.  Experiments  have  shown 
that  the  energy  expended  in  the  mechanical  work  done  by 
the  intestinal  tract  is  negligible,  and  that  the  chemical 
reactions  involved  in  digestion  are  isothermic. 

It  is  thought  that  the  fermentation  that  takes  place  in 
the  digestive  tract  of  the  ruminant  produces  a  considerable 
amount  of  heat.  To  what  extent  this  occurs  and  what 
increase  follows  feeding  little  is  known. 

4.   Metabolism  in  Prolonged  Fasts, 

When  the  usual  supply  of  energy  in  the  food  is  cut  oft', 
the  animal  gets  the  energy  required  by  oxidising  its  own 
stored  material  and  its  tissues.  This  is  shown  by  the  fact 
that   it   loses  weight   and  goes  on  excreting  carbon  dioxide. 


METABOLISM 


273 


urea,    and  the  other  waste   products  of  the  activity   of  the 
tissues. 

Prolonged  fasts  have  been  borne  by  both  men  and  animals, 
and,  in  one  or  two  of  these  in  man,  careful  observations  have 
been  made  by  physiologists.  It  has  been  found  that  during 
the  first  day  or  two  of  a  fast,  the  organism  draws  most  lavishly 
on  its  store  of  carbohydrates,  and  that  there  is  a  marked 
diminution  in  the  protein  metabolism.  As  soon  as  the 
carbohydrate  depots  are  exhausted,  the  protein  metabolism 
suddenly  increases,  to  fall  slowly  as  the  fast  progresses.  Fat 
metabolism  falls  slowly  throughout  the  fast.  The  follow- 
ing figures  obtained  by  Benedict  on  a  subject  who  fasted 
for  thirty-one  days  clearly  demonstrates  the  effect  of  fasting. 

Amount  of  Proteins,  Fats,  and  Carbohydrates  in  grms. 
cataholised  in  tiventy-four  hours  during  a  fast. 


Day. 

Protein. 

Fat. 

Carbohyr!  rates. 

1 

42-6 

135 

68-8 

5 

62-5 

133 

151 

10 

60-3 

120 

3-9 

15 

50-8 

116 

20 

46-1 

110 

25 

46-9 

109 

31 

41-6 

115 

(1)  It  is  from  the  stored  fats  that  the  energy  is  chiefly 
derived,  and  the  result  of  this  is  that  before  death  the  fats 
of  the  body  are  to  a  great  extent  used  up.  (2)  The  protein- 
containing  tissues  waste  more  slowly  and  waste  at  different 
rates,  the  less  essential  being  used  up  more  rapidly  than  the 
more  essential,  which,  in  fact,  live  upon  the  former.  In  cats, 
deprived  of  food  till  death  supervened,  the  heart  and  central 
nervous  system  scarcely  lost  weight  ;  the  bones,  pancreas, 
lungs,  intestines,  and  skin  each  lost  between  10  and 
20  per  cent,  of  their  weight,  the  kidneys,  blood,  and  muscles 
between  20  and  30,  and  the  liver  and  spleen  between  50 
and  70. 
18 


274 


VETERINARY   PHYSIOLOGY 


The  rate  of  waste  during  a  fast  depends  upon  the  amount 
of  energy  required,  and  it  is  therefore  increased  by  muscular 
work  and  by  exposure  to  cold  (p.  266). 

The  power  of  withstanding  starvation  depends  chiefly 
upon  the  extent  of  the  store  of  fat  in  the  body.  Pigs  and 
have  been  known  to  withstand  starvation  for  60  days 


WEIGHT 


fAT 


PROTEIN 


Fig,  138. — To  show  the  Effects  on  the  Metabolism  of  Proteins  and  Fats  of 
Feeding  a  Fasting  Animal.  The  continuous  hoi-izontal  lines  indicate 
the  amount  of  material  metabolised,  the  broken  horizontal  lines  the 
amount  taken.  The  differences  between  the  levels  of  these  indicate 
the  amount  of  protein  and  of  fats  of  the  animal  body  which  are 
metabolised.  The  first  column  represents  the  condition  in  fasting — 
the  succeeding  columns  the  intake  and  output  each  day  when  food  is 


and  dogs  for  80  days  without  permanent  injury.  In  man 
fasts  of  30  days  have  been  borne  without  injury.  Horses 
and  cattle  succumb  sooner  than  men. 


5.  Metabolism  in  Semi-Starvation. 

When  the  food  supply  of  an  animal  is  inadequate  to 
yield  the  necessary  supply  of  energy,  the  energy  expenditure 
is  reduced  by  the  restriction  of  all  unnecessary  movements. 
Experiments  show  that,  in  ruminants  at  least,  the  digesti- 
bility of  a  ration  is  increased  as  its  amount  is  diminished 


METABOLISM  275 

(p.  3G6),  so  that  a  greater  proportion  of  the  energy  of  the 
food  is  rendered  available  for  metabolism.  The  stimulus 
due  to  taking  food  is  also,  of  course,  decreased  as  the  amount 
is  decreased  (p.  272).  Hence  life  is  maintained  more 
economically  in  an  under-fed  than  in  a  well-fed  animal. 

Short  periods  of  under- feeding  cause  no  damage  beyond 
some  loss  of  weight.  From  the  evidence  in  man  it  is  most 
probable,  however,  that  in  prolonged  periods  of  under- 
nutrition the  power  of  resisting  disease  is  diminished. 

B.   EXCHANGE  OF  MATERIAL. 

Not  only  is  material  necessary  as  a  source  of  energy,  it 
is  also  required  for  the  formation  of  new  tissue  in  growth, 
and  for  repair  in  the  full-grown  animal. 

In  protoplasmic  activity  (p.  21)  there  is  a  continuous 
breaking  down  of  complex  substances  to  simpler  bodies. 
Some  of  these  products  of  katabolism  can  be  re-used  to 
build  up  the  living  protoplasm,  others,  however,  are  excreted 
as  waste.  In  some  cases,  too,  essential  materials  are  carried 
off  from  the  body  in  combination  with  excretory  products 
(p.  280).  New  materials  must  therefore  be  supplied  to 
make  good  the  waste  to  ensure  that  the  destructive  phase  of 
metabolism  shall  be  balanced  by  the  constructive. 

In  addition,  certain  accessory  factors  (p.  281)  are  required 
to  secure  the  harmonious  working  of  the  metabolic  processes 
which  is  necessary  to  health. 

Metabolism  therefore  involves  an  exchange  of  material  as 
well  as  an  exchange  of  energy. 

The  substances  that  take  part  in  metabolism  are  : — 

(1)  Water. 

(2)  Amino-acids. 

(3)  Salts. 

(4)  Accessory  factors. 

Water  is  the  chief  constituent ;  since  it  is  daily  given  oft" 
in  large  quantities  by  the  kidneys,  lungs,  skin,  and  bowels, 
it  must  be  supplied  in  sufficient  amounts,  or  the  chemical 
change  cannot  go  on,  and  death  supervenes. 

Amino-acids. — Nitrogenous  material  is  lost  to  the  body 
in  the  catabolism  of  the  protein  in  the  tissues  and  in  the 


276  VETERINARY  PHYSIOLOGY 

cells  shed  from  the  mucous  membranes  and  from  the  skin 
and  its  coverings.  Material  is  used  up  in  the  production 
of  enzymes  and  of  internal  secretions  (p.  588).  The  new 
nitrogenous  matter  required  for  the  replacement  of  this 
used-up  material  and  also  for  formation  of  new  tissue  in 
growth  must  be  supplied  to  the  tissues  in  the  form  of 
amino-acids. 

The  various  proteins  differ  in  their  amino-acid  build-up 
(p.  16),  and  hence  they  are  of  different  value  in  the  growth 
and  repair  of  the  body.  Their  relative  value  has  chiefly  been 
investigated  by  feeding  young  white  rats  on  a  basal  diet 
consisting  of  protein-free  dried  milk,  since  this  is  known 
to  be  an  adequate  diet  when  suitable  proteins,  such  as  the 
milk  proteins,  caseinogen  and  lactalbumin,  are  added  to  it. 
Various  proteins  or  amino-acids  may  be  added  to  this,  and 
the  effect  upon  the  rate  of  growth  and  the  duration  of  life 
determined. 

In  this  way  it  has  been  shown  that  some  of  the  amino- 
acids,  certainly  glycin,  can  be  formed  from  others,  since 
caseinogen,  which  contains  no  glycin,  has  proved  adequate 
for  normal  growth. 

Other  amino-acids  cannot  be  formed  in  the  body, 
or  cannot  be  formed  in  sufficient  amounts  for  adequate 
nutrition.  Some  proteins  contain  all  the  necessary  amino- 
acids,  some  do  not.  It  has  been  found  that  among  those 
which  are  adequate  to  maintain  normal  growth  when  given 
in  sufficient  amount  are — 

Animal  Origin.  Vegetable  Origin. 

Casein  (milk).  Edestin  (hemp  seed). 

Lactalbumin  (milk).  Glutelin  (maize). 

Ovalbumin  (hen's  egg).  Glutenin  (wheat). 

Ovovitellin  (hen's  egg).  Glycin  (soy  bean). 

While  among  those  which  are  inadequate  are — 
Legumelin  (soy  bean).  Hordein  (barley). 

Gliadin  (wheat).  Zein  (maize). 

Legumin  (pea).  Gelatin. 

The  reason  for  this  failure  is  the  absence  of  essential 
amino-acids. 

Gelatin  lacks  tyrosin  and  tryptophan. 


METABOLISM 


277 


Zein  lacks  the  tryptophan  group  and  the  diamino-acid  lysii 
Ghadin  and  hordein  lack  Ivsin. 


Fig.  139. — Showing  typical  growth  of  rats  on  diets  containing  a  single 
protein.  (1)  On  casein  food  (devoid  of  glj'cin)  satisfactory  growth 
is  shown  ;  (2)  on  gliadin  food  (deficient  in  lysin)  little  more  than 
maintenance  of  body  weight  is  shown  ;  (3)  after  a  mixed  diet  with 
satisfactory  growth,  zein  food  (devoid  of  glyoin,  Ij'sin,  and  trypto- 
phan) even  maintenance  is  impossible.     (Mendel.) 

Chart  1  shows  the  results  of  feeding  rats  on  an  adequate 
and  on  an  inadequate  protein  diet. 


Fig.  140.— To    show    the  eflfect  of  the  addition  of   tryptophan  (A)   and  of 

tryptophan  and  lysin  {B)  to  the  food  of  rats  on  zein  diet.     A  gives 

maintenance    of     weight  alone,     A    and    B    increase     of     growth. 
(Mendel.) 

Chart  2   shows  the  etfect  of  adding  the  lacking  amino- 

acid. 


27i 


VETERINARY   PHYSIOLOGY 


Some  proteins  contain  only  small  amounts  of  certain 
essential  amino-acids,  and  a  larger  supply  must  be  given  to 
supply  a  sufficient  quantity.  This  is  well  seen  in  feeding 
with  casein,  which  is  poor  in  the  sulphur-containing  cystin. 
If  too  small  an  amount  of  casein  be  given  the  rate  of  growth 
is  decreased,  but  it  is  accelerated  when  cystin  is  added 
(Chart  3).      Similarly,  if  too  small  amounts  of  edestin  be  given, 


Fig.  141. — To  show  satisfactory  growth  of  rat  upon  IS  per  cent,  of  casein 
(A)  and  defective  growth  on  9  per  cent,  without  the  addition  of 
cystin,  but  adequate  growth  when  cystin,  in  which  casein  is 
deficient,  was  added  {B).     (Mexdel.) 

the  small  amount  of  lysin  it  contains  renders  it  inadequate, 
and  the  addition  of  lysin  is  required  to  restore  growth. 

A  most  interesting  point  brought  out  in  these  experiments 
is  the  fact  that,  however  long  the  rate  of  growth  has  been 
checked  by  an  insufficient  supply  of  the  constituents  of  the 
food  necessary  for  growth,  when  they  are  again  supplied 
growth  begins  again,  and  proceeds  till  the  normal  size  may 
be  reached. 


METABOLISM 


279 


These  results  explain  why  a  smaller  amount  of  some 
proteins  than  of  others  is  sufficient  to  repair  the  wear  and  tear. 
The  more  nearl\^  the  amino-acid  make-up  of  the  protein 
in  the  food  approaches  that  of  the  tissues,  and  chiefly  of  the 
muscular  tissue,  of  the  animal  consuming  it,  the  smaller  is 
the  quantity  required. 

This  has  been  shown  by  feeding  a  man  on  a  carbohydrate 
and  fat  diet,  yielding  the  energy  required  and  finding  the 
amount  of  nitrogen  excreted,  i.e.  the  amount  of  protein 
oxidised,  and  then  adding  different  proteins.  It  was 
found  that,  while  a  small  quantity  of  some  is  capable  of 
making  good  the  loss  of  nitrogen,  nuich  larger  quantities  of 
others  are  required. 


Protein  of — 

ijrms 
per  Da 

Meat 

30 

Milk 

31 

Rice 

34 

Potato 

38 

Bean 

54 

Bread 

(wheatj     . 

76 

Maize 

102 

In  wheat  the  defective  gliadin  forms  half  the  protein 
content. 

It  has  been  found  that  in  the  dog  the  loss  of  nitrogen  is 
covered  by  the  smallest  protein  intake  when  dog's  flesh  is 
given — a  physiological  justification  of  cannibalism. 

On  account  of  the  different  values  of  different  proteins, 
Lusk  proposes  to  classify  them  into  three  groups  according 
to  their  amino-acid  build-up  and  their  resultant  avail- 
ability for  growth  and  repair  : — 

1st   Class,  e.g.  Caseinogen  of  milk. 
2nd  Class,  e.g.  Gluten  of  wheat  flour. 
3rd  Class,  e.g.  Gelatin. 


Salts. — It  has  been  seen  (p.  218)  that  salts  as  ions  play 
an  essential  part  in  regulating  the  osmotic  pressure  of  the 
body  fluids. 


280  VETERINARY   PHYSIOLOGY 

Salts  are,  however,  continually  being  drained  off  from  the 
body  in  the  urine  and  faeces. 

In  the  katabolism  of  proteins  the  sulphur  and  phosphorus 
which  they  contain  are  oxidised  to  sulphuric  and  phosphoric 
acids.  In  herbiv^ra  hippuric  acid  is  produced  in  consider- 
able amounts.  To  maintain  the  neutrality  of  the  body  fluids, 
the  acids  are  neutralised  by  the  bases  contained  in  the 
carbonates  and  basic  phosphates  of  the  blood,  and  are  then 
excreted.  An  equivalent  amount  of  the  base  need  not  be 
excreted,  as  the  kidney  is  able  to  separate  some  of  the 
phosphoric  acid,  which  is  then  excreted  as  acid  phosphate. 
Certain  amounts  of  bases  are,  however,  lost  to  the  body  in 
this  way.  In  carnivora  these  bases  are  not  so  necessary, 
since  ammonia  is  formed  from  the  nitrogen  of  the  proteins 
in  sufficient  amounts  to  neutralise  the  acids  produced  in 
their  katabolism. 

The  supply  of  bases  for  herbivora  is  obtained  from  the 
sodium  and  potassium  salts  of  citric,  malic,  and  tartaric 
acids,  which  are  abundant  in  green  fodder.  These  are 
oxidised  in  the  tissues  to  carbonates  which  are  alkaline  salts. 

Sodium  chloride  is  the  salt  usually  given  in  largest 
quantities  in  the  diet.  When  not  supplied  in  the  food, 
it  is  retained  in  the  tissues,  and  hence  animals  can,  when 
necessary,  live  on  a  comparatively  small  supply.  One 
purpose  which  it  serves  is  to  supply  the  chlorine  required 
for  the  gastric  secretion. 

Animals,  especially  those  fed  on  certain  kinds  of  hay  rich 
in  potassium  salts,  often  show  a  hunger  for  sodium  chloride. 
Bunge  has  shown  that  this  is  caused  by  the  presence  in  the 
food  of  excessive  amounts  of  potassium,  which  causes  an 
increase  in  the  osmotic  pressure  in  the  body  fluids.  To 
readjust  this,  the  kidney  eliminates  sodium  as  well  as 
potassium,  and  consequently  a  shortage  of  sodium  is 
produced. 

Iodine  is  required  for  the  production  of  the  internal 
secretion  of  the  thyroid  (p.  595).  Iron  is  used  for 
building  up  the  hsemoglobin  of  the  blood.  Calcium  and 
phosphorus  are  especially  essential  for  growing  animals  for 
bone  formation. 


METABOLISM  281 

Accessory  Factors. — By  feeding  young  rats  upon  diets 
containing  an  adequate  amount  of  pure  proteins,  fats, 
carbohydrates,  and  inorganic  salts,  it  has  been  shown  that 
growth  is  arrested  unless  two  substances,  the  chemical 
nature  of  which  is  still  unknown,  are  present. 

(1)  Fat  Soluble  A. — This  usually  occurs  in  close  con- 
nection with  animal  fats.  It  is  particularly  abundant  in 
milk  fats  and  in  cod-liver  oil.    It  is  also  present  in  green  fodder. 

In  rats  its  absence  from  the  diet  not  only  causes  an 
arrest  of  growth,  but  also  a  peculiar  inflammation  round  the 
eyes. 

Rickets  is  a  disease  affecting  young  animals,  notably 
dogs.  It  is  characterised  by  such  constitutional  symptoms 
as  muscular  weakness,  and  by  changes  at  the  epiphyseal 
cartilages,  and  softening  of  the  bones,  either  due  to  loss 
of  lime  salts  or  to  failure  in  calcification.  An  attempt 
has  been  made  to  explain  it  as  a  result  of  a  deficient 
supply  of  the  fat  soluble  A  or  of  some  allied  substance, 
but  no  sufficient  evidence  has  been  adduced  in  proof  of 
this,  and  the  cause  of  the  disease  is  still  unknown. 

(2)  Water  Soluble  B. — (a)  Anti-neuritic. — This  is  present 
abundantly  in  the  germs  of  plants,  but  not  in  the  endosperm, 
in  young  leaves,  and  in  flesh.  Its  presence  is  necessary  for 
growth,  and  it  is  at  least  probable  that  when  the  supply  is 
insufficient  symptoms  of  neuritis  may  develop. 

A  disease  known  as  beri-beri  develops  in  people  living 
too  exclusively  on  polished  rice,  which  is  poor  in  this  sub- 
stance. A  polyneuritis  which  is  substantially  the  same  as 
beri-beri  can  be  similarly  produced  in  animals  and  especially 
in  fowls.  This  condition  can  be  cured  by  adding  substances 
containing  water  soluble  B  in  the  diet.  Hence  it  has  been 
called  an  anti-neuritic  substance. 

(b)  Anti-scorbutic — A  closely-allied  substance,  abundantly 
present  in  orange  and  lemon  juice  and  in  many  fruits  and 
vegetables,  seems  to  be  essential,  and  in  its  absence  scurvy 
is  apt  to  develop,  either  in  men  or  animals.  It  may  be 
called  the  anti-scorbutic  substance. 

Practically  nothing  is  known  about  the  relation  of  these 
substances  to  one  another. 


282  VETERINARY   PHYSIOLOGY 

Accessory  substances  are  destroyed  by  prolonged  heating, 
so  that  they  are  for  the  most  part  absent  in  food  stuffs  that 
have  been  boiled. 

Animals  receiving  green  stuff's  are  not  likely  to  suffer 
from  the  lack  of  these  accessory  factors.  The  exclusive  use 
of  artificially-prepared  feeding  stuffs  which  have  been  heated 
in  the  course  of  their  manufacture  may,  however,  produce 
malnutrition  and  arrest  of  development. 

These  accessory  factors  have  been  given  the  name  of 
"  vitamines,"  an  unfortunate  title,  since  we  do  not  know  if 
they  are  amines  or  what  they  have  to  do  with  life. 


PART    III. 

THE   NUTRITION  OF   THE  TISSUES. 

SECTION   I. 

The  Supply  of  Energy  and  Material  to  the  Body. 

THE    FOOD. 

CONSTITUENTS  OF  THE  FOOD. 

The  nature  of  the  requh^ements  of  the  energy  and  material 
have  been  considered.  The  supply  of  these  in  the  food 
must  now  be  studied. 

The  different  constituents  of  food-stuffs  may  be  classified 
as  : — A.  Those  supplying  energy. 

(1)  Nitrogenous  compounds. 

(2)  Fats. 

(3)  Carbohydrates. 

B.   Those  supplying  no  energy. 

(4)  Ash  (inorganic  elements). 
(.5)  Water. 

(6)  Accessory  factors. 

A.  Food-Stuffs  yielding  Energy. 
1.  Nitrogenous  Compounds. 

Nitrogenous  compounds  are  divided  into  two  classes, 
viz. (rt)  proteins,  and  (6)  soluble  nitrogenous  compounds. 

(a)  Proteins. — The  chemistry  and  nature  of  these  have 
been  already  discussed  (p.  14). 

Occurrence. — Proteins  form    the    chief   and    characteristic 


284  VETERINARY    PHYSIOLOGY 

constituent  of  protoplasm.  They  are  present  to  a  much 
smaller  extent  in  plants  than  in  animals.  While 
carbohydrates  form  the  chief  bulk  of  plant  tissues,  the 
greater  part  of  the  animal  body,  apart  from  bones,  visible  fat, 
and  water,  consists  of  proteins. 

Plant  Proteins. — The  vegetable  proteins  belong  to  three 
groups  of  native  proteins — (a)  the  glohidins ;  (b)  the 
glutelins,  requiring  a  dilute  alkali  for  their  solution  ;  and 
(c)  the  gliadins,  soluble  in  70  to  80  per  cent,  alcohol. 
They  differ  from  the  animal  proteins  in  the  proportion  of 
amino-acids  which  they  contain.  Generally  they  are  poorer 
in  leucin,  but  richer  in  glutamic  acid  (C5H9NO4)  and  in 
arginin  (p.  17).  They  have  a  higher  percentage  of  nitrogen 
than  those  of  animal  origin,  so  that  the  factors  6  2  5  which 
is  used  to  multiply  the  amount  of  total  nitrogen  present  to 
obtain  the  amount  of  protein  is  too  high  in  vegetable 
protein.  The  true  factor  varies  between  5-5  and  6  0  for  the 
different  substances, 

(6)    Soluble   or  Non-Protein  Nitrogen In      addition     to 

proteins,  feeding-stuffs  contain  nitrogenous  substances  of  a 
simple  chemical  structure.  These  are  all  soluble  in  water, 
and  the  term  "  soluble  nitrogen  "  is  advantageously  used  to 
indicate  the  group. 

In  most  cases  they  are  early  stages  in  the  building  up  of 
proteins  from  nitrates  or  ammonium  salts  (diagram,  p,  381). 
In  other  cases  they  are  parts  of  protein  that  have  undergone 
disintegration  to  a  soluble  form,  e.g.  amino-acids  or  peptides, 
for  transportation  in  the  fluids  of  the  plant. 

Amino-acids,  therefore,  constitute  the  most  abundant 
group  forming  usually  from  50  to  70  per  cent,  of  the  whole. 
The  amides  (Appendix),  asparagine  and  glutamine,  are 
usually  present  to  the  extent  of  10  to  20  per  cent.  The 
whole  group  of  soluble  nitrogenous  substances  are  sometimes 
loosely  termed  "  amides,"  an  unfortunate  term  liable  to 
create  confusion. 

For  purposes  of  analysis  this  group  is  differentiated  from 
proteins  by  the  fact  that  they  do  not  coagulate  on  heating 
and  are  not  precipitated  by  certain  reagents  which  pre- 
cipitate proteins,  e.g.  copper  hydrate. 


FOOD  285 

Occurrence. — Non- proteins  are  most  abundant  in  plants 
where  growth  is  most  active,  e.g.  seedhngs.  In  young 
plants  the  amount  of  nitrogen  in  these  compounds  may 
exceed  that  in  proteins.  As  the  plant  reaches  maturity  the 
amount  of  nitrogen  in  non-protein  form  decreases.  Soluble 
nitrogen  compounds  are  especially  abundant  in  roots  and 
tubers.  In  the  potato  the  greater  part  of  the  nitrogen  is  in 
the  non-protein  form. 

2.  Fats  and  Allied  Bodies. 

Nature, — The  chemistry  (p.  41)  and  energy  value  (p.  257) 
of  the  fats  have  been  already  considered. 

In  the  food  they  may  be  substituted  for  an  amount  of 
carbohydrate  of  equal  energy  value,  i.e.  they  are  isodynamic 
with  the  carbohydrates. 

Occurrence. — Fats  are  more  abundant  in  animals  than  in 
plants.  The  amount  present  in  the  animal  body  varies 
within  very  wide  limits.  In  fat  oxen  and  sheep  nearly 
50  per  cent,  of  the  weight  of  the  carcase  may  consist  of  fat. 

In  plants  they  occur  chiefly  in  oil  seeds  where  they  form 
a  concentrated  reserve  material.  Small  quantities  are 
present,  however,  in  all  parts  of  plants. 

In  addition  to  true  fats  there  exist  in  plants  fatty  acids 
and  various  bodies  related  to  fats.  These,  together  with 
resins  and  waxes  which  resemble  fats  in  their  physical 
properties  rather  than  in  their  chemical  composition,  can 
all  be  extracted  with  ether,  which  is  the  conventional  method 
adopted  in  quantitative  analysis.  In  deahng  with  the 
constituents  of  feeding-stuffs,  therefore,  it  is  customary  to 
class  all  these  bodies  soluble  in  ether  as  "  crude  fat,"  or 
more  properly  as  "  ether  extract." 

3.  Carbohydrates. 
Nature. — Carbohydrates  contain  carbon,  hydrogen,  and 
oxygen,  the  carbon  atoms  of  the  molecule  usually  number- 
ing six  or  some  multiple  of  six,  and  the  hydrogen  and 
oxygen  are  in  the  same  proportions  in  which  they  occur  in 
water.  They  are  aldoses  or  ketoses  (Appendix),  and  deriva- 
tives of  these,  of  the  hexatomic  alcohol,  CgHiPg  (Appendix). 


286  VETERINARY   PHYSIOLOGY 

The  simplest  carbohydrates  are  the  monosaccharids,  of 
which  glucose  or  dextrose  or  yrape  sugar  is  the  most 
important.      It  is  the  sugar  of  the  animal  body. 

Closely  allied  to  glucose  in  chemical  composition  is  the 
fructose  or  Icevulose,  a  sugar  which,  instead  of  rotating  the 
plane  of  polarised  light  to  the  right  as  glucose  does,  rotates 
it  to  the  left,  but  which  in  most  other  respects  behaves  like 
dextrose. 

The  other  monosaccharid  of  importance  is  galactose, 
a  sugar  produced  by  the  splitting  of  milk  sugar,  and  also 
found  in  combination  in  the  nerve  tissue  (p.  58).  It  does 
not  occur  free  in  the  body. 

These  monosaccharids  are  easily  tested  for  b}--  boiling 
their  solution  with  Fehling's  solution.  A  reddish  precipitate 
is  formed. 

Under  the  influence  of  yeast  they  split  into  ethyl  alcohol 
and  carbon  dioxide,  galactose,  however,  only  very  slightly. 

By  the  polymerisation  of  two  monosaccharid  molecules 
with  the  loss  of  water,  disaccharids,  or  double  sugars,  are 
formed.  Thus,  two  glucose  molecules  polymerise  to  form 
one  maltose  molecule. 

0,H,,Os  +  CeH,,Oe  =  2(C«H,,0e)  -  H,0 
=  C12H22O11 

Maltose  is  the  sugar  formed  by  the  action  of  malt  and 
other  vegetable  and  animal  enzymes  upon  starch.  By  the 
action  of  dilute  acids  and  other  agents  it  can  be  split  into 
two  dextrose  molecules.  Like  the  monosaccharids  it  reduces 
Fehling's  solution,  and  it  ferments  with  yeast. 

Lactose,  the  sugar  of  milk,  is  a  disaccharid  composed  of  a 
molecule  of  dextrose  united  to  a  molecule  of  galactose  with 
dehydration.  It  readily  splits  into  these  two  monosaccharids. 
It  reduces  Fehling's  solution,  but  it  does  not  ferment  with 
yeast. 

Cane  sugar  or  succose  or  saccharose  consists  of  a  mole- 
cule of  dextrose  united  to  a  molecule  of  lajvulose  with  the 
elimination  of  a  molecule  of  water.  It  does  not  reduce 
Fehling. 

Monosaccharids  anil  disaccharids  which  are  soluble 
and  crystalline  substances  are  usually  called  sugars. 


FOOD  287 

By  further  polymerisation  of  monosaccharids  with  the 
further  loss  of  water,  molecules  of  greater  size  are  produced 
and  form  the  set  of  substances  known  as  the  polysaccharids. 

^•(CeHioOe  -  H„0)  or  x{C,R,,0,). 

The  polysaccharids  are  distinguished  from  the  sugars  by 
being  precipitated  from  their  solutions  by  alcohol.  They 
do  not  reduce  Fehling's  solution,  nor  do  they  ferment  with 
yeast.  In  cold  neutral  or  acid  solution  most  of  them  strike 
a  blue  or  brown  colour  with  iodine.  On  hydrolysis  they 
split  into  monosaccharids.  Some  can  be  split  by  dilute 
mineral  acids,  or  by  enzymes.  Others  can  be  acted  upon 
by  strong  acids.      Most  are  insoluble  in  water, 

Polysaccharids  form  a  series  of  bodies  of  which  the 
following  are  the  most  important  : — 

Starch  is  built  up  of  a  large  number  of  dehydrated  mono- 
saccharid  molecules.  Common  starch  seems  to  have  a  mole- 
cular weight  of  20,000  to  30,000.  On  hydrolysis  it  yields 
glucose. 

Glycogen  occurs  mainly  in  the  livers  of  animals.  On  hydro- 
lysis it  yields  glucose.  It  gives  an  opalescent  solution,  and 
strikes  a  brown  colour  with  iodine. 

Cellulose  is  the  basis  of  the  cell  walls  of  plants.  It  can 
be  hydrolysed  only  by  strong  acids,  and  is  not  acted  upon 
by  the  body  ferments.  There  is  in  plants,  however,  a 
ferment  cytase  which  can  act  upon  it.  On  hydrolysis  it 
yields  glucose.  It  is  attacked  and  disintegrated  by  bacteria 
in  the  first  stomach  of  the  ruminant  and  in  the  colon  of  the 
horse.  On  bacterial  disintegration  it  yields  lower  fatty  acids, 
and  the  gases  methane  and  carbon  dioxide. 

Pentosans. — In  addition  to  the  carbohydrates  described 
above,  which  all  contain  either  six,  or  some  multiple  of  six, 
atoms  of  carbon  in  the  molecule,  there  is  a  group  having  five 
carbon  atoms,  and  hence  called  pentoses.  The  polysac- 
charids of  this  group  are  called  i:)entosa7is.  They  are 
represented  by  gums,  pectins,  mucilages,  and  other  sub- 
stances in  plant  bodies  where  they  occur  in  great  variety. 
It  is  supposed  that  in  digestion  they  are  disintegrated 
by  bacteria,  yielding  the  same  end  products  as  cellulose. 


288  VETERINARY   PHYSIOLOGY 

Occurrence  of  Carbohydrates. — In  animals,  except  in 
carnivora,  carbohydrates  form  the  chief  source  of  energy 
in  the  food,  but  only  small  amounts  are  present  in  the  body. 

Glucose  is  present  in  blood  to  the  extent  of  0*1  to  0*15 
per  cent,  usually.  Lactose  is  present  in  milk  ;  glycogen  in 
the  liver  and  muscles. 

In  plants,  carbohydrates  occur  in  great  variety,  and  form 
the  chief  constituents.  All  the  monosaccharids,  except 
galactose,  and  disaccharids,  except  lactose,  occur  in  solution 
in  plant  juices.  Those  that  are  present  in  greatest  amount 
are  glucose,  fructose,  and  cane  sugar.  Starch  is  found  in 
large  quantities  as  a  reserve  food  in  tubers  and  seeds,  such  as  the 
potato  and  in  the  common  grains,  and  also  in  small  quantities 
in  all  green  plants.  The  cell  walls  which  form  the  frame- 
work or  skeleton  of  the  plant  is  formed  of  cellulose.  In 
the  young  plant,  the  cell  wall  consists  mainly  of  cellulose, 
but  as  the  plants  increase  in  size,  the  framework  is 
made  stronger  by  impregnating  the  cellulose  of  the  cell 
walls  with  hard  tough  substances.  The  chief  of  these  is 
lignin,  which  is  a  typical  constituent  of  the  woody  parts  of 
plants.  The  older  and  larger  the  plant,  the  greater  is  the 
amount  of  this  fibrous  material  present. 

In  the  conventional  analysis  of  food  stuffs  the  carbo- 
hydrates are  divided  into  two  groups. 

(1)  "  Nitrogen  free  extract." — This  consists  of  those  com- 
pounds in  solution,  or  which  can  be  brought  into  solution 
by  the  action  of  dilute  acids  or  alkalies.  This  group 
includes  all  the  monosaccharids  and  disaccharids,  and  also 
those  polysaccharids  like  starch  that  can  be  hydrolysed  to 
the  soluble  form  by  these  reagents. 

(2)  "Crude  Fibre-" — This  includes  all  the  remainder  which 
resist  their  reagents.  Cellulose  and  lignin  are  typical,  and 
form  the  major  part  of  the  group. 

B.  Not  yielding  Energy. 
4.  Ash  (Inorganic  Elements). 

Salts  are  present  in  all  feeding  stuffs,  though  in  different 
amounts   and    proportions.      An    ordinary    mixed    ration    is 


FOOD  289 

likely  to  contain  a  sufficiency  of  all  the  essential  inorganic 
elements  except  sodium,  which  is  usually  added  in  the  form 
of  sodium  chloride.  Calcium  and  phosphorus  are  of  special 
importance  in  the  feeding  of  dairy  cows  and  growing  animals. 
In  the  former  there  is  a  loss  of  these  elements  in  the  milk,  and 
in  the  latter  they  are  required  for  growth  of  bone.  Calcium 
is  abundant  in  leguminous  hays  and  in  animal  products,  such 
as  meat  meal.  It  is  present  only  in  small  quantities  in  grains. 
Maize  is  especially  deficient  in  this  element.  Phosphorus,  as 
phosphates  and  phosphohpins,  e.g.  lecithin  (p.  19),  is  present 
in  relatively  large  amounts  in  feeding  stutfs  that  are  rich  in 
proteins,  such  as  animal  products  and  leguminous  seeds.  It 
is  also  abundant  in  bran,  middlings,  and  oil  seeds.  Roots  and 
straws  are  deficient.  Potassium  is  abundant  in  fodders  where 
it  is  sometimes  present  in  excessive  amounts  (p.  280;. 

The  total  amount  of  ash  of  feeding  stuffs  is  determined 
by  incinerating  a  weighed  example,  and  weighing  the  residue. 

5.  Water. 

The  amount  of  water  present  in  feeding  stuffs  is  very 
variable,  in  difi:erent  foods,  and  in  the  same  material  at  different 
stages  of  growth.  The  following  table  gives  a  rough  idea  of 
the  w^ater  content  of  some  common  feeding  stuffs  : — 


Water  per  cent 

Roots  and  tubers 

80-90 

Green  fodder 

65-80 

Hay  and  Straw     . 

7-15 

Grain            .... 

10-12 

Feeding  cakes  and  meals 

undei 

10 

As  the  water  yields  no  energy  the  percentage  present 
dilutes  the  nutrition  value.  Fermentation  and  the  growth  of 
moulds  are  liable  to  occur  when  the  water  content  exceeds 
17  or  18  per  cent. 

The  percentage  of  water  is  estimated  by  determining 
the  loss  of  weight  on  drying  a  sample  at  a  temperature 
of  100°  C. 

G.  Accessory  Factors. 

These  have  been  dealt  with  (p.  281). 
19 


290 


VETERINARY   PHYSIOLOGY 


Classification  of  Feeding  Stuffs  for  Herbivora. 

Feeding  stuffs  may  be  arranged  in  the  following  groups  : — 
(1)  Green  fodder. — Examples — Grasses,  silage.  These  contain 
a  high  percentage  of  water.  In  the  young  growing  plant 
the  thin  cell  walls  are  full  of  protoplasm.  These  are  therefore 
rich  in  protein  and  easily  digested.  As  the  plant  gets  older 
and  larger  the  percentage  of  protoplasm  decreases,  and  the  cell 
walls  become  impregnated  with  fibrous  matter,  difficult  to 
digest.  On  the  other  hand,  the  seeds  with  their  reserve  store 
of  food  become  more  valuable  as  the  plant  reaches  maturity. 

(2)  Dry  fodder.  —  Examples  —  Hay,  straw.  These  are 
bulky  foods  characterised  by  a  high  percentage  of  crude  fibre. 

(3)  Roots  and  Tubers. —  Examples  —  Potatoes,  turnips. 
These  contain  a  high  percentage  of  water  and  a  small 
amount  of  crude  fibre. 

(4)  Concentrates. — Examples — Grain,  oil  cakes.  A  high 
percentage  of  digestible  material  and  small  amounts  of  crude 
fibre  and  water  are  the  characteristics  of  this  group. 

(.5)  Animal  Products. — Examples — Milk,  fish  meal.  With 
the  exception  of  milk,  these  are  concentrated  feeding  stuffs 
rich  in  proteins. 

The  following  table  shows  the  approximate  results  of 
analysis  of  some  common  feeding  stuffs  : — 


Average  Perce 

ntage   Composition. 

Water. 

Protein. 

Fat. 

Fibre. 

Carbo- 
hydrate. 

Ash. 

Grass 

80 

4 

1 

4 

10 

1 

20  to  30  per  cent, 
of  nitrogen  not 
in  proteins. 

Hay 

15" 

10 

2 

26 

40 

7 

About  10  per  cent, 
of  nitrogen  not 
in  proteins. 

Peas  . 

14 

22 

" 

6 

53 

3 

About  10  percent, 
of  nitrogen  not 
in  proteins. 

Oats 

14 

12 

6 

9 

57 

2 

Less  than   10  per 

(crushed) 

cent,  of  nitrogen 
not  in  protein,?. 

Potatoes     . 

76 

1 

2 

0 

1 

20 

1 

Over  40  per  cent, 
of  nitrogen  not 
in  proteins. 

SECTION    II. 


DIGESTION. 


Digestion     is     the     preparation    of    the    food    in     the    ali- 
mentary    canal     for     absorption     and     utilisation     by     the 


tissues. 


I.  STRUCTURE  OF  THE  ALIMENTARY  CANAL. 

The  anatomy  and  histology  of  the  alimentary  tract  must 
be  studied  practically.  A  mere  outline  of  the  various  struc- 
tures, such  as  will  assist  in  the  comprehension  of  their 
physiology,  is  given  here. 

The  alimentary  canal  (fig.  142)  may  be  divided  into 
the  mouth,  the  o-sophagus  or  gullet,  the  stomach, 
the  small  and  large  intestine,  and  the  following  sup- 
plementary structures — the  salivary  glands,  the  liver,  and 
the  pancreas. 

The  Mouth. — The  lips,  the  tongue,  and  the  teeth  are  the 
organs  of  prehension.  The  lips  of  the  horse  are  strong  and 
mobile,  and  possess  acute  sensation.  Those  of  the  ox  are 
thick  and  immobile.  The  upper  lip  of  the  sheep  is  divided 
into  two  parts,  which  can  move  independently  of  each 
other. 

The  horse's  tongue  has  a  smooth  covering,  and  is  broad 
at  the  apex.  It  is  seldom  protruded.  The  tongue  of  the  ox 
tapers  to  the  apex,  which  is  capable  of  extensive  move- 
ment, and  can  be  easily  protruded.  It  has  a  strong, 
rough  covering,  which  gives  it  a  better  grip,  and  also 
protects  it  from  injury  when  used  for  collecting  the  grass 
in  ofrazincr. 


292 


VETERINARY  PHYSIOLOGY 


The  inside  of  the  mouth  of  the  ox  has  papilla3  sloping 
inwards.      These  help  to  prevent  the  food  from  falling  out. 

In  ruminants  the  incisor 
teeth  are  loosely  fixed, 
and  meet  the  dental  pad 
obliquely.  This  arrangement 
prevents  injury  to  the  dental 
pad. 

The  teeth  of  the  horse  are 
peculiar  in  having  an  invagina- 
tion of  the  enamel  covering  of 
the  crown.  As  the  crown 
wears,  there  comes  to  be  two 
consecutive  rings  of  hard 
enamel,  enclosing  softer 
cement.  This  provides  an 
uneven  surface,  well  suited  to 
the  grinding  of  the  food. 
The  horse  has  incisors  in  both 
the  upper  and  the  lower  jaw, 
and  these  are  used  to  bite 
the    grass     in    feeding.       The 


crops 


closer  than 


horse   thus 
the  ox. 

The  incisor  teeth  of  the 
young  horse  are  vertically 
placed.  With  use  they  gradu- 
ally come  to  assume  an  oblique 
position  and  get  pushed  out 
of  their  sockets,  so  that  the 
fangs  are  reduced  in  length 
and  the  shape  of  the  teeth 
altered.  The  shape  and  slope 
of  the  teeth,  and  the  extent  of  wear  on  the  crowns,  give  an 
indication  of  the  age  of  the  animal. 

The  complex  joint  of  the  upper  and  lower  jaws  in 
herbivora  allows  the  movements  in  mastication  to  be  not 
only  up  and  down,  but  also  lateral,  and  to  some  extent  from 
front  to  rear.      This  freedom  of  movement  is  more  marked  in 


Fig.  142. — Diagram  of  the  Parts  of 
the  Alimentary  Canal,  from 
Mouth  to  Anus.  T.,  tonsils;  Ph., 
pharynx  ;  S.O.,  salivary  glands  ; 
Oe.,  oesophagus  ;  C,  cardiac,  Py., 
pyloric  portion  of  stomach  ;  D., 
duodenum  ;  Li.,  Liver  ;  P.,  pan- 
creas ;  /.,  jejunum  ;  /.,  ileum  ; 
v.,  vermiform  appendix,  present  in 
man  and  in  rabbit  ;  Col.,  colon  ; 
R.,  rectum. 


DIGESTION 


293 


the  ox  than  in  the  horse.  In  the  dog,  movements  other 
than  vertical  are  very  limited.  In  herbivora  the  lower  jaw 
is  narrower  than  the  upper  (fig.  143),  so  that  when  the 
molar  surfaces  of  the  upper  and  lower  teeth  meet  on  one  side 
they  do  not  come  into  contact  on  the  other.  Mastication  is 
therefore  always  unilateral.  In  the  lateral  movement  of 
mastication,  the  outside  of  the  lower  molar  teeth  and  the 
inside  of  the  upper  have  the  greater  amount  of  wear  and 
tear,  and  consequently  the  tables  come  to  be  oblique 
instead  of  horizontal.  In  the  horse  this  sometimes 
produces   long,    sharp,    ragged     edges,    which     prevent    the 


Fig.  143. — Showing  relative  widths  of  lower  and  upper  jaw  in  the  horse. 


proper  mastication  of  the  food  and  consequently  lead  to 
malnutrition. 

Salivary  Glands. — Three  pairs  of  salivary  glands — parotid, 
submaxillary,  and  sublingual — open  into  the  mouth.  These 
are  compound  tubular  glands,  and  are  well  developed  in 
herbivora.  The  acini  are  lined  with  mucus  and  enzyme 
secreting  epithelium  (p.  34),  The  parotid  has  the  most 
copious  secretion,  and  except  in  the  ox,  where  the  submaxillary 
is  developed  to  an  equal  size,  it  is  the  largest  of  the  three  glands. 

The  CEsophagus  is  a  muscular  walled  tube,  lined  by 
squamous  epithelium.  The  muscles  are  in  two  layers — an 
outer    longitudinal    and    an    inner    circular.      This     general 


294  VETERINARY   PHYSIOLOGY 

muscular  arrangement  is  present  in  the  whole  alimentary 
canal,  from  the  oesophagus  to  the  anus.  In  the  horse  the 
lumen  diminishes,  and  the  muscular  wall  becomes  thicker 
just  outside  the  stomach.  In  the  ox  the  lumen  is  wider  and 
more  dilatable  than  in  the  horse. 

In  the  ruminant  there  is  a  series  of  dilatations  near  the 
point  where  the  oesophagus  enters  the  stomach.  These  are 
the  rumen,  the  reticulum,  and  the  omasum,  which  are 
frequently  described  as  parts  of  the  stomach. 

The  Rumen  is  a  great  sac,  containing  in  its  walls  strong 
muscular  bands  that  enable  it  to  contract  on  its  content.     It 


Fig.  144. — Stomach  of  a  Ruminant,    a,  cesophagus ;  I,  rumen ;  c,  reticulum  with 
cesophageal  groove  above  ;  rf,  omasum  ;  e,  abomasum  ;  /,  duodenum. 

is  lined  by  stratified  squamous  epithelium.  It  is  a  temporary 
storehouse  for  the  food. 

The  Reticulum — the  second  reservoir — is  very  nmch 
smaller  than  the  rumen,  The  membrane  lining  its  walls 
is  raised  in  intersecting  ridges,  which  are  arranged  to  form 
polyhedral  cells  resembling  a  honeycomb.  It  communicates 
with  the  omasum,  and  also  with  the  rumen,  whose  overflow 
it  receives. 

The  Omasum — the  third  compartment — is  rather  larger 
than  the  reticulum.  Into  its  interior  are  projected  folds  of 
its  walls,  forming  leaf-like  structures.  There  are  almost  a 
hundred  of  these  of  different  sizes.  They  are  covered  with 
hard  papilla?.  The  omasum  opens  into  the  abomasum,  the 
fourth  compartment,  which  corresponds  to  tlie  stomach  in 
other  animals. 


DIGESTION  295 

The  oesophageal  groove,  formed  of  two  longitudinal 
muscular  folds,  is  a  continuation  of  the  lumen  of  the 
03S0f)hagus,  which  runs  along  the  wall  of  the  first  three 
compartments  and  ends  in  the  abomasum.  By  this 
means  fluid  food  can  pass  directly  from  the  oesophagus  to 
the  abomasum.  When  the  pillars  are  relaxed  the  groove 
communicates  with  the  rumen  and  the  reticulum. 

The  crop  of  the  fowl  is  a  dilatation  of  the  oesophagus  that 
occurs  at  the  root  of  the  neck.  It  corresponds  to  the 
rumen,  reticulum,  and  omasum  of  the  ruminant. 

The  Stomach  is  a  dilatation  of  the  alimentary  canal.  It 
bulges  out  at  the  oesophageal  end — the  fundus.  Towards 
the  outlet  it  tapers  off  to  the  pyloric  canal.  A  strong 
circular  band  of  muscle  between  the  stomach  and  intestine 
controls  the  outflow  of  its  contents.  In  the  dog  the  fundus 
is  separated  from  the  pylorus  by  a  circular  band  of  muscular 
fibres — the  prepyloric  sphincter.  The  mucous  membrane 
is  largely  composed  of  tubular  glands.  Those  at  the 
oesophageal  end  secrete  hydrochloric  acid  and  pepsin,  those 
at  the  pyloric  end  pepsin  only. 

In  the  horse  (fig.  145)  the  stratified  squamous  epithelium 
of  the  oesophagus  is  continued  into  the  stomach,  and  lines 
nearly  one  half  of  the  organ.  Only  the  pyloric  part  and  the 
fundus  are  covered  with  the  glandular  mucous  membrane. 
The  opening  of  the  oesophagus  to  the  stomach  is  small,  and 
partially  occluded  by  folds  of  the  lining  membrane. 

The  Small  Intestine  is  a  long  convoluted  narrow  muscular 
tube  suspended  in  the  folds  of  a  membrane  slung  from  the 
spinal  region  of  the  body  cavity.  The  mucous  membrane  is 
projected  into  the  lumen  of  the  tube  as  a  series  of  delicate 
finger-like  processes — the  villi — w^hich  are  covered  by 
columnar  epithelium. 

There  are  two  kinds  of  glands  that  secrete  the  intestinal 
fluid — the  succus  entericus. 

(1)  Lieberkilhn's  follicles  are  found  throughout  the  whole 
small  intestine.  They  are  simple  test-tube  like  glands 
which  open  between  the  villi. 

(2)  Brunners  glands  are  found  only  in  the  upper  part  of 
the  small  intestine.      They  are  branching  glands   that  pene- 


296 


VETERINARY   PHYSIOLOGY 


trate  the  subuiucous  layer.  The  mucous  membrane  contains 
masses  of  lymph  tissue  scattered  throughout  it.  In  some 
places  these  are  massed  together  and  form  Peyers  iJatches, 
which  are  largest  at  the  lower  part  of  the  small  intestine. 

The  Large  Intestine  (fig.  142). — The  small  intestine  enters 
the  proximal  end  of  the  large  intestine  at  one  side,  and  the 
opening  is  guarded  by  a  fold  of  mucous  membrane  and  a 
ring    of  muscular   fibres   which   form   the   ileo-csecal   valve. 

jSacc.  caec. 


Fi(}.  145. — Stomach  of  the  Horse  to  show — R.  (cs.,  the  cesophageal  part  ;  Fu., 
the  fundus  with  true  gastric  glaiids  ;  R.,  jjyl.,  pyloric  part. 

Above  this  opening  there  is  a  diverticulum — the  caecum 
which  is  very  large  in  herbivora  and  only  vestigial  in 
carnivora.  Below  the  opening  of  the  small  intestine  is  the 
colon  {col).  This  ends  in  the  short  rectum  which  opens  into 
the  anal  canal,  and  this  is  surrounded  by  a  strong  band  of 
muscle — the  internal  sphincter  ani — by  which  it  is  com- 
pressed. An  external  sphincter  ani  composed  of  striped 
muscular  fibres  encircles  the  anal  orifice. 

The  whole  large  intestine  is  lined  with  columnar  epithelium, 
and  is  studded  with  Lieberkilhn's  follicles,  in  which  the  epithe- 
lium is  chiefly  mucus-secreting  in  type.      There  are  no  villi. 


DIGESTION  297 

In  the   horse  (fig.  147)  the  ciecum  is   an  enormous   sac 


with  a  capacity  of  25   to   30   Htres.      Both  extremities  are 
bhnd,  and  the  two  openings  are  only  separated  by  about  two 


298 


VETERINARY   PHYSIOLOGY 


Small 
Colon 


Ventral  Taenia 

Medial  Ttenia 


Left  Dorsal  Colon 


Pelvic  Flexure 


Fig.  147. — The  Colon  and  Caecum  of  the  Horse. 
(From  Bradley's  Topographical  Anatomy  of  the  Abdomen  of  the  Horse.) 


DIGESTION  299 

inches.  That  leading  to  the  colon  is  situated  above  the  ileo- 
csecal  opening,  so  that  the  contents  are  passed  to  the  colon 
against  gravity. 

The  large  colon  is  more  than  double  the  capacity  of  the 
ca3cum.  The  diameter  near  the  csecum  is  only  two  to  three 
inches,  but  it  increases  rapidly  to  nine  or  ten  in  the  ventral 
portions.  At  the  pelvic  flexure  the  diameter  becomes  reduced 
to  three  or  four  inches  but  rapidly  increases,  to  reach  a 
maximum  of  as  much  as  twenty  inches  in  the  right  dorsal 
colon,  which  narrows  down  like  a  funnel  to  join  the  small 
colon.  The  small  colon  has  a  diameter  of  three  to  four 
inches.  The  ceecum  and  the  ventral  portion  of  the  large 
colon  have  four  longitudinal  bands  of  muscle — teenia — and 
four  rows  of  sacculations.  The  small  colon  has  two  bands 
and  two  rows  of  sacculation.  These  sacculations  increase  the 
surface  area.  According  to  F.  Smith,  irregular  contractions 
of  the  muscular  bands  produce  displacements  and  distorsions 
of  the  colon  which  are  causes  of  "colic"  to  which  the  horse 
is  so  liable. 

Supplementary  Structure. — (1)  The  salivary  glands  have 
been  described  (p.  293). 

(2)  The  Liver  is  a  large  solid  organ,  formed  originally  as  a 
double  outgrowth  from  the  alimentary  canal.  Each  of  these 
outgrowths  branches  repeatedly,  and  the  blood  coming  from 
the  mother  to  the  foetus  flows  in  a  number  of  capillary 
channels  between  the  branches.  Later,  when  the  alimentary 
canal  has  developed,  the  blood  from  it  is  streamed  between 
the  liver  tubules.  The  fibrous  tissue  supporting  the  liver 
cuts  it  up  into  a  number  of  small  divisions,  the  lobules,  each 
lobule  being  composed  of  a  series  of  obliterated  tubules 
arranged  radially  with  blood-vessels  coursing  between 
them. 

The  portal  vein,  which  takes  blood  from  the  stomach, 
intestine,  pancreas,  and  spleen,  breaks  up  in  the  liver,  and 
carries  the  blood  between  the  lobules.  From  the  interlobular 
branches  capillaries  run  inward  and  enter  a  central  vein 
which  carries  the  blood  from  each  lobule,  and  pours  it  into 
the  hepatic  veins,  which  join  the  inferior  vena  cava.  The 
supporting  tissue    of  the  liver   is    supplied  by  the  hepatic 


300  A^ETERINARY   PHYSIOLOGY 

artery,   the    terminal    branches  of  which    have    a   very   free 
communication  with  those  of  the  portal  vein. 

(3)  The  Pancreas  like  the  liver  develops  as  an  outgrowth 
from  the  intestine.  It  is  an  enzyme  secreting  gland.  In 
the  lobules  are  certain  little  masses  of  epithelium-like  cells 
closely  packed  together — the  islets  of  Langerhans  (fig.  148). 

fe..  :>:•.•;,:  ■-•<■5•^ 

5  .^.*-  VeC"-.  ^\7  '4v..'^ 
^^  -;i  sr^  *  -       .      •  .  ^Cs^ 


Fig.  148. — Section  of  Pancreas  to  show  Acini  of  Secreting  Cells  ;  a  large 
duct  (a),  and  in  the  centre  an  Island  of  Langerhans  {b). 

The  Nerve  Supply  of  the  Alimentary  Canal. 

The  muscles  round  the  mouth  are  supplied  by  the  fifth, 
seventh,  and  twelfth  cranial  nerves.  The  nerve  supply  of 
the  saHvary  glands  will  be  considered  later.  The  pharynx 
and  the  oesophagus  are  supplied  by  the  ninth  and  tenth 
cranial  nerves,  and  by  fibres  from  the  sympathetic. 

The  stomach  and  small  intestine  get  their  nerve  fibres 
from  the  vagus  and  the  abdominal  sympathethic  (p.  198). 
The  large  intestine  is  supplied  by  the  abdominal  sympathetic, 
the  various  fibres  passing  through  the  abdominal  sym- 
pathetic ganglia.  The  upper  part  is  also  supplied  from  the 
vagus  and  the  lower  part  from  the  pelvic  nerves.      In  the  wall 


DIGESTION  301 

of  tlie  stomach  and  intestine  these  nerves  end  in  an  interiacing 
set  of  fibres  with  nerve  cells  upon  them,  from  which  fibres 
pass  to  the  muscles  and  glands.  One  of  these  plexuses 
(Auerbach's,  or  the  myenteric  plexus)  is  placed  between  the 
muscular  coats — the  other  (Meissner's)  is  placed  in  the 
submucosa. 

Size  and  Capacity  of  Organs. 
The   following   tables   give   some   idea   of    the   size    and 
capacity  of  the  parts  of  the  alimentary  canal  in  full-grown 
animals  : — 

Average  Length  in  Feet  of  Intestines. 


Small  Inte.stine 
Large  Intestine 

Ox.                Sheep. 

130             80 
35              21 

Capacity. 

Horse. 
70 

25 

15 

Stomach 
Small  Intestine 
Large  Intestine 

Ox.              Sheep. 
Lit.    Gals.       Lit.  Gals. 

182    40         18    4 

77   17       11   2-5 
36     8         7   1-5 

Horse. 
Lit.  Gals. 

16       3'5 

55   12 
159  35 

Pig. 
Lit.  Gals. 

9    2 

11   2-5 
13  2-7: 

In  the  ox  and  sheep  the  figures  given  as  capacity  of 
stomach  include  the  rumen,  the  reticulum,  and  the  omasum. 
The  relative  capacity  of  these  for  the  ox  are: — rumen,  80 
per  cent.  ;  reticulum,  5  per  cent.  ;  omasum,  7  per  cent.  ;  and 
abomasum,  8  per  cent.  The  great  capacity  of  the  large 
intestine  of  the  horse  should  be  noted.  In  this  animal  the 
caecum  is  also  large.  Its  capacity  is  25  to  30  litres  ;  that 
of  the  ox  is  8  to  10. 

II.  PHYSIOLOGY. 

Although  the  nature  of  the  food  is  very  different  in 
different  species  of  animals,  the  essential  features  of  the 
digestive  processes  are  common  to  all.  In  herbivora  there 
are  adaptations  for  dealing  with  bulky  food.  For  the  most 
part,  however,  these  are  developed  after  birth  as  the  animal 
begins  to  change  its  diet  from  milk — an  animal  product — , 
to  the  bulky  vegetable  food.  They  are  merely  modifications 
of  the  simpler  system  of  carnivora,  which  may  be   regarded 


302  VETERINARY   PHYSIOLOGY 

as  the  more  fundamental  type.  It  is  convenient,  therefore, 
to  deal  first  with  digestion  in  carnivora,  and  thereafter 
indicate  the  modifications  that  exist  in  herbivora,  Omnivora, 
e.g.  man  and  pig,  present  such  minor  differences  from  carni- 
vora that  they  can  be  included  with  these.  Indeed,  the 
work  done  to  elucidate  the  physiology  of  human  digestion 
has  been  done  mainly  on  the  dog — a  carnivorous  animal. 

A.  DIGESTION  IN  CARNIVORA  AND   OMNIVORA. 
I.  DIGESTION  IN  THE   MOUTH. 

(«)  Mastication. 

In  the  mouth  the  food  is  broken  up  and  mixed  with 
saliva  in  the  act  of  chewing.  Mastication  is  less  perfectly 
performed  in  carnivora  than  in  herbivora.  The  dog 
masticates  very  imperfectly.  After  a  few  rapid  chews  the 
food  is  swallowed. 

(b)  Insalivation. 

The  saliva  is  formed  by  the  salivary  glands — the  parotid, 
submaxillary,  sublingual,  and  various  small  glands  in  the 
mucous  membrane  of  the  mouth. 

1.  Saliva — (1)  Characters. — The  Saliva  is  a  somewhat 
turbid  fluid  which,  when  allowed  to  stand,  throws  down  a 
white  deposit  consisting  of  shed  epithelial  scales  from  the 
mouth,  leucocytes,  amorphous  calcic  and  magnesic  phos- 
phates, and  generally  numerous  bacteria,  Its  specific  gravity 
is  low — generally  about  1003.  In  reaction  it  is  neutral  or 
faintly  alkaline. 

(2)  Chemistry. — It  is  found  to  contain  a  very  small  propor- 
tion of  solids.  The  saliva  from  the  parotid  gland  contains 
only  about  0"4<  per  cent.,  while  that  from  the  sublingual  may 
contain  from  2  or  3  per  cent.  The  sublingual  and  sub- 
maxillary saliva,  in  man,  is  viscous,  from  the  presence  of 
mucin  formed  in  these  glands,  while  the  parotid  saliva  is 
free  from  mucin.  In  addition  to  mucin,  traces  of  proteins 
are  present,  and  in  certain  animals  an  enzyme — ptyalin  — 
is  associated  with  these  proteins.  Ptyalin  is  present  in  man 
and  in  the  pig.      It  is  absent  in  carnivora. 


DIGESTION  303 

(3)   The  functions  of  the  saliva  are  twofold  : — 

(1)  Mechanical. — Saliva  moistens  the  mouth  and  gullet, 
and  thus  assists  in  chewing  and  swallowing.  Since 
the  salivary  glands  are  absent  from  aquatic  mammals, 
and  since  in  carnivorous  animals  saliva  has  no  chemical 
action,  it  would  appear  that  this  is  the  important 
function. 

(2)  Chemical. — Under  the  action  of  the  ptyalin  of  the 
saliva,  starches  are  broken  down  into  sugar.  The  starch  is 
first  changed  into  the  dextrins,  first  into  erythrodextrins  and 
then  into  achroodextrins,  and  lastly  into  the  disaccharid 
maltose  (see  p.  286),  (Chemical  Physiology).  Like  other 
enzyme  actions,  the  process  requires  the  presence  of  water 
and  a  suitable  temperature,  and  it  is  stopped  by  the  presence 
of  strong  acids  or  alkalies,  by  various  chemical  substances, 
and  by  a  temperature  of  over  60°  C,  while  it  is  temporarily 
inhibited  by  reducing  the  temperature  to  near  the  freezing 
point.  During  the  short  time  the  saliva  acts  on  the  food  in 
the  mouth,  the  conversion  is  by  no  means  complete. 

2.  Physiology  of  Salivary  Secretion. — (1)  The  changes 
which  the  secreting  cells  undergo  during  the  so-called  resting 
state  of  the  gland  and  during  secretion  have  been  already 
considered  (p.  35). 

(2)  The  nervous  mechanism  of  secretion. — In  order  to 
study  the  influence  of  different  factors  upon  salivary  secretion, 
a  cannula  may  be  inserted  into  the  duct  of  one  of  the  glands, 
and  the  rate  of  flow  of  saliva  or  the  pressure  of  secretion  may 
be  thus  measured.  In  this  way,  it  may  be  shown  that  in 
the  dog  the  taking  of  food,  or  simply  the  act  of  chewing, 
or,  in  some  cases,  the  mere  sight  of  food,  causes  a  flow  of 
saliva.  This  shows  that  the  process  of  secretion  is  presided 
over  by  the  central  nervous  system,  a  fact  which  is  further 
illustrated  in  man  by  the  decrease  in  the  secretion  of  saliva 
which  accompanies  some  emotional  conditions. 

The  submaxillary  and  sublingual  glands  are  supplied — 
(1)  By  branches  from  the  lingual  division  of  the  fifth  cranial 
nerve  ;  and  (2)  by  branches  of  the  perivascular  sympathetic 
fibres    coming   from   the    superior    cervical    ganglion.      The 


804 


VETERINARY    PHYSIOLOGY 


parotid  gland  is  supplied  by  the  auriculo-temporal  division 
of  the  fifth  and  by  sympathetic  fibres  (fig.  149). 

(a)  The  influence  of  these  nerves  has  been  chiefly  studied 
on  the  submaxillary  and  sublingual  glands. 

(1)  It  has  been  found  that,  when  the  lingual  nerve  is  cut, 
the  reflex  secretion  of  saliva  still  takes  place,  but  that,  when 
the  chorda  tympani  (Ch.T.),  a  branch  from  the  seventh  nerve, 
which  ioins  the  linorual,  is  cut,  the  reflex  secretion  does  not 


Fig.  149. — Nervous  Supjjly  of  the  Salivai-y  Glands.  Par.,  parotid,  and  S.id. 
and  S.L.,  the  submaxillary  and  sublingual  glands  ;  T//.,  the  seventh 
cranial  nerve,  with  Ch.T.,  the  chorda  tympani  nerve,  passing  to  L., 
the  lingual  branch  of  V.,  the  fifth  nerve,  to  supplj'  the  glands  below 
the  tongue,  T.  ;  IX.,  the  glossopharyngeal  giving  oft'/.iV.,  Jacobson's 
nerve,  to  0.,  the  otic  ganglion,  to  supply  the  parotid  gland  through 
Aur.T.,  the  auriculo-temporal  nerve. 

occur.  Stimulation  of  the  chorda  tympani  causes  a  copious 
flow  of  watery  saliva,  and  a  dilatation  of  the  blood-vessels  of 
the  glands.  If  atropine  has  been  first  administered,  the 
dilatation  of  the  vessels  occurs  without  the  flow  of  saliva. 
This  indicates  that  the  two  processes  are  independent  of  one 
another. 

The  secreting  fibres  all  undergo  interruption  before  the 
glands  are  reached  ;  the  fibres  to  the  sublingual  gland  having 
their  cell  station  in  the  submaxillary  ganglion  (S.M.G.),  the 


DIGESTION  305 

fibres  to  the  submaxillary  gland  having  theirs  in  a  little 
ganglion  at  the  hilus  of  the  gland  (S.M.).  This  was  demon- 
strated by  painting  the  two  ganglia  with  nicotine  (p.  198). 
When  applied  to  the  submaxillary  ganglion,  the  drug  does 
not  interfere  with  the  passage  of  impulses  to  the  submaxillary 
gland,  but  stops  those  going  to  the  sublingual. 

If  the  duct  of  the  gland  be  connected  with  a  mercurial 
manometer,  it  is  found  that,  when  the  chorda  tympani  is 
stimulated,  the  pressure  of  secretion  may  exceed  the  blood 
pressure  in  the  carotid,  showing  that  the  saUva  is  not  formed 
by  filtration. 

(2)  When  the  perivascular  sympathetics,  or  when  the 
sympathetic  cord  of  the  neck  is  stimulated,  the  blood-vessels 
of  the  gland  constrict,  and  a  flow  of  very  viscous  saliva 
takes  place. 

Some  time  after  section  of  the  chorda  tympani  nerve  an 
increased  flow  of  saliva  has  been  observed.  It  may  be  due  to 
the  uncontrolled  action  of  the  peripheral  nervous  mechanism. 

(b)  On  the  parotid  gland  (1)  the  auriculo-temporal  nerve 
(Aur.T.)  acts  in  the  same  way  as  the  chorda  tympani  acts 
on  the  other  salivary  glands.  But  stimulation  of  the  tifth 
nerve  above  the  otic  ganglion,  from  which  the  auriculo- 
temporal takes  origin,  fails  to  produce  any  effect.  On  the 
other  hand,  stimulation  of  the  glossopharyngeal  nerve  (IX.) 
as  it  comes  off  from  the  brain,  acts  upon  the  parotid  gland, 
Since  the  glossopharyngeal  is  united  by  Jacobsons  nerve 
(/.iV.)  to  the  small  superficial  petrosal  which  passes  to  the 
otic  ganglion,  it  is  obvious  that  the  parotid  fibres  take  this 
somewhat  roundabout  course  (fig.  149). 

(2)  When  the  sympathetic  fibres  to  the  gland  alone  are 
stimulated,  constriction  of  the  blood-vessels  but  no  flow  of 
saliva  occurs ;  but  if,  when  the  flow  of  watery  saliva  is  being 
produced  by  stimulating  the  glossopharyngeal  or  Jacobson's 
nerve,  the  sympathetic  fibres  are  stimulated,  the  amount 
of  organic  soUds  in  the  parotid  saliva  is  very  markedly 
increased. 

The  nerve  fibres  passing  to  the  salivary  glands  are  pre- 
sided over  by  groups  of  cells,  the  Salivary  Centre,  in  the 
medulla  oblongata  which  may  be  stimulated  reflexly  or 
20 


306  VETERINARY    PHYSIOLOGY 

directly  by  the  condition  of  the  blood.  Stimulation  of  the 
lingual  or  glossopharyngeal  leads  to  a  reflex  flow  of  saliva. 
Other  nerves  may  also  act  on  this  centre.  Thus  gastric 
irritation,  when  it  produces  vomiting,  causes  a  reflex  stimu- 
lation of  salivary  secretion.  In  asphyxia  the  condition  of 
the  blood  may  directly  stimulate  the  centre. 

According  to  the  investigations  of  Pavlov,  the  salivary 
glands  react  appropriately  to  different  kinds  of  stimuli 
through  their  nervous  mechanism.  When  sand  or  bitter 
or  saline  substances  are  put  in  a  dog's  moutb,  a  profuse 
secretion  of  very  watery  saliva  ensues  to  wash  them  out. 
When  flesh  is  given,  a  saliva  rich  in  mucin  is  produced. 
When  dry  food  is  given,  saliva  is  produced  in  greater 
quantity  than  when  moist  food  is  supplied. 

Pavlov  further  states  that  the  sight  of  different  kinds  of 
food  produces  a  flow  of  the  kind  of  saliva  which  their 
presence  in  the  mouth  would  produce.  The  flow  of  saliva 
has  been  used  by  him  for  the  study  of  cerebral,  or 
"conditioned,"  reflexes. 


II.   SWALLOWING. 


1.  Voluntary  Stage. — The  food,  after  being  masticated,  is 
collected  on  the  surface  of  the  tongue  by  the  voluntary 
action  of  the  buccinators  and  other  muscles,  and  then,  the 
point  of  the  tongue  is  pressed  against  the  hard  palate  behind 
the  teeth.  By  a  contraction  passing  backwards  the  parts  of 
the  tongue  behind  the  tip  are  raised,  along  with  the  hyoid 
and  with  the  larynx,  and  the  bolus  of  food  is  thus  pushed 
along  the  palate  and  through  the  pillars  of  the  fauces. 

2.  Reflex  Stage. — Swallowing  now  becomes  a  pure  reflex. 
It  can  be  performed  only  if  something  is  present  to  excite  it, 
and  it  is  abolished  by  paralysing  the  receptors  in  the  pharynx 
by  means  of  cocaine. 

(1)  The  exact  position  of  the  receptor  spots,  stimulation 
of  which  thus  reflexly  causes  swallowing,  varies  in  ditferent 
animals.  In  man  they  are  chiefly  about  the  base  of  the 
tongue   and  the  posterior  pliaryugeal  wall.      Supplementary 


DIGESTION  307 

receptor  spots  of  less  importance  also  exist,  from  which 
swallowing  may  be  elicited  when  food  gets  lodged  upon 
them. 

(2)  The  excito-reflex  nerves  are  the  fifth,  via  the  spheno- 
palatine ganglion,  and  the  vagus.  The  glossopharyngeal  also 
may  contain  fibres  which  act ;  but  section  of  this  nerve 
causes  a  sustained  tonic  contraction  of  the  gullet,  and  stimu- 
lation of  its  central  end  inhibits  swallowing.  It  probably 
acts  chiefly  to  prevent  a  second  act  of  swallowing  occurring 
while  one  is  already  in  progress. 

(3)  The  centre  is  situated  in  the  medulla  oblongata,  and 
swallowing  is  readily  induced  in  a  decerebrated  cat  by  pressing 
a  bolus  of  cotton  wool  through  the  pillars  of  the  fauces  upon 
the  receptor  points  in  the  pharynx. 

(4)  The  effector  mechanism  leads  to  the  following  changes  : 
— (a)  The  hyoid  and  larynx  are  [)ulled  upwards  by  the  muscles 
which  act  upon  these  structures. 

(h)  The  upper  orifice  of  the  larynx  is  closed  by  the  action 
of  the  lateral  crico-arytenoidei,the  arytenoidei,  and  the  thyreo- 
arytenoidei,  which  pull  forward  the  arytenoids  against  the 
posterior  surface  of  the  epiglottis.  The  whole  larynx  is 
pulled  forwards  as  well  as  upwards,  and  thus  the  upper 
part  of  the  oesophagus  is  opened,  and  the  food  slides  over  the 
back  of  the  epiglottis  and  down  the  posterior  aspect  of  the 
larynx  into  the  gullet. 

(c)  The  passage  of  food  into  the  naso-pharynx  is  pre- 
vented by  the  contraction  of  the  glossopharyngeus,  palato- 
pharyngeas,  and  the  levator  and  tensor  palati  muscles, 
which  approximate  the  soft  palate  and  the  back  wall  of  the 
pharynx. 

id)  The  constrictors  of  the  pharynx  contract  from  above 
downwards  and  force  the  bolus  on  into  the  true  oesophagus, 
which  may  be  said  to  begin  at  the  level  of  the  cricoid 
cartilage. 

(5)  The  outgoing  nerves  involved  are  the  hypoglossal,  the 
third  branch  of  the  fifth,  the  glossopharyngeal  to  the  stylo- 
pharyngeus  and  middle  constrictor,  and  the  vagus  which 
supplies  both  the  pharynx  and  the  cesophagus. 

Section  of  the  vagi  nerves  paralyses  the  upper  part  of  the 


308  VETERINARY  PHYSIOLOGY 

gullet.  In  this  condition  food  is  forced  down  entirely  by  the 
pressure  from  the  mouth. 

Fluids  are  normally  shot  right  down  the  relaxed  gullet  by 
the  pressure  from  the  mouth  and  from  the  constrictors.  Less 
fluid  matter  is  passed  down  the  oesophagus  by  a  true  peri- 
stalsis— a  zone  of  relaxation  passing  down  in  front  of  a  zone 
of  contraction.  This  peristalsis  is  stopped  for  some  time  if  the 
vagi  nerves  are  cut  and  as  a  result  food  cannot  be  swallowed. 
The  vagus  is  the  great  efferent  nerve  for  the  reflex  part  of  the 
act  of  swallowing.  But  it  has  been  found  that,  after  the  vagus 
has  been  cut  for  twenty-four  hours  or  more,  distension  of  the 
oesophagus  by  food  may  cause  a  peristalsis  passing  on  to  the 
stomach.  The  peripheral  nerve  plexus  in  the  wall  of  the 
gullet  must  be  capable  of  stimulation  by  such  distension,  and 
it  must  be  able,  in  these  conditions,  to  initiate  and  to  main- 
tain peristalsis. 

Time  of  Swallowing.  —  Observations  made  on  the  human 
subject  by  feeding  with  food  impregnated  with  bismuth  and 
studying  the  changes  in  the  oesophagus  by  X-rays  have  shown 
that  fluids  and  solids,  well  masticated  and  mixed  with  saliva, 
are  passed  rapidly  down  the  gullet,  reaching  the  orifice  of  the 
stomach  in  about  three  seconds.  Here  they  are  delayed, 
and  do  not  pass  into  the  stomach  for  another  period  of 
about  three  seconds.  Dry  solids  take  much  longer  to  pass 
down  the  ofullet,  sometimes  as  much  as  fifteen  minutes. 


III.  DIGESTION  IN  THE  STOMACH. 

Most  important  work  on  digestion  in  the  stomach  has 
been  accomplished  by  Pavlov  on  dogs.  His  method  is 
to  make  a  small  gastric  pouch  opening  on  the  surface, 
and  separated  from  the  rest  of  the  stomach  (fig.  150), 
This  is  done  by  cutting  out  a  V-shaped  piece  along  the 
great  curvature,  the  apex  being  towards  the  pylorus  and 
the  base  being  left  connected  with  the  stomach  wall.  By 
a  series  of  stitches,  the  opening  thus  made  in  the  stomach 
is  closed  up  (top  line  of  XXs  in  tig.  150),  Avhile  the  cut 
edges  of  the  V-shaped  flap   are   stitched  together   to   form 


DIGESTION 


309 


a  tube.  The  one  end  of  this  is  made  to  open  u})on  the  skin 
surface  A,  A,  and,  by  folding  in  the  mucous  membrane,  the 
deep  end  is  isolated  from  the  stomach.  Thus  a  pouch  is 
formed,  still  connected  with  the  muscular  coat  and  with  the 
nerves  and  the  vessels  of  the  stomach,  the  condition  of  which 
represents  the  condition  of  the  whole  stomach. 


Fig.  150. Diagram  of  Pavlov's  Pouch  made  on  the  Stomach  of  a  Dog. 

The  condition  of  the  stomach  is  very  different  in  fasting 
and  after  feeding. 


A.  Stomach  during  Fasting. 

(i.)  The  organ  is  collapsed,  and  the  mucous  membrane  is 
thrown  into  large  ridges,  (ii.)  It  is  pale  in  colour  because 
the  blood-vessels  are  not  dilated.  (iii.)  The  secretion  is 
scanty,  only  a  little  mucus  being  formed  on  the  surface  of 
the  lining  membrane,  (iv.)  Movements  may  be  absent,  or 
rhythmic  contractions  may  occur  at  regular  intervals  of  from 
one-half  to  three  or  four  minutes.  These  movements  are 
generally  associated  with  the  sensation  of  hunger,  and  they 
may  be  called  "  hunger  contractions:'  They  are  best  marked 
when  the  muscular  tone  of  the  stomach  wall  is  good  ;  when 
it  is  decreased  by  section  of  the  vagi,  they  are  less  marked  ; 


310  VETERINARY   PHYSIOLOGY 

when  it  is  increased  by  section  of  the  splanchnic  nerves,  they 
are  more  marked.  But  the}'  persist  after  section  of  both 
nerves,  and  they  must  therefore  be  presided  over  by  the 
local  nervous  mechanism  in  the  wall  of  the  stomach.  They 
are  inhibited  by  taking  food  and  even  by  the  sight  or  smell 
of  food.     They  do  not  stop  during  sleep. 

B.  Stomach  after  Feeding. 

When  food  is  taken,  (1)  the  blood-vessels  dilate,  (2)  a 
secretion  is  poured  out,  and  (3)  movements  of  the  organ 
become  more  marked. 

1.  Vascular  Changes. — The  arterioles  dilate,  and  the 
mucous  membrane  becomes  bright  red  in  colour.  This  is  a 
reflex  vaso-dilator  effect,  impulses  passing  up  the  vagus  to 
a  vaso-dilator  centre  in  the  medulla,  and  coming  down  the 
vagus  from  that  centre.  Section  of  the  vagi  is  said  to  pre- 
vent its  onset. 

2.  Secretion. — There  is  a  free  flow  of  gastric  juice  from 
all  the  glands  in  the  mucous  membrane. 

(1)  Characters  of  Gastric  Secretion. — The  gastric  juice  from 
the  cardiac  end  is  a  clear  watery  fluid,  which  is  markedly 
acid  from  the  presence  of  free  hydrochloric  acid.  In  the  dog 
the  free  acid  may  amount  to  over  0-4  per  cent.,  but  in  the  \ng 
it  is  less  abundant,  and,  when  the  gastric  juice  is  mixed  with 
food,  the  acid  rapidly  combines  with  alkalies  and  with  pro- 
teins and  is  no  longer  free.  In  addition  to  the  HCl,  small 
quantities  of  inorganic  salts  are  present.  Traces  of  proteins 
may  also  be  demonstrated,  and  two  enzymes  are  associated 
with  these — one  a  proteolytic  or  protein-digesting  enzyme, 
pepsin,  the  other  a  milk-curdling  enzyme,  rennin.  The  fact 
that  dilution  may  abolish  the  peptic  activity  while  leaving 
the  curdling  action  intact  seems  to  show  that  these  are  not 
separate  bodies  but  phases  in  the  activity  of  one  body. 

The  secretion  from  the  pyloric  portion  is  alkaline  in 
reaction. 

(2)  Source  of  the  Constituents  of  the  Gastric  Juice.  —  The 
hydrochloric  acid  is  formed  at  the  cardiac  part  of  the 
stomach.  This  may  be  shown  by  making  a  small  stomach 
by  the  method  of  Pavlov.     Since  the  parietal  or  oxyntic  cells 


DIGESTION  311 

are  confined  to  this  portion  of  the  stomach,  it  may  be  con- 
cluded that  they  are  the  producers  of  the  acid.  The  NaCl  of 
the  blood  plasma  must  be  the  source  of  the  HCl. 

Pepsin  and  Rennin  are  produced  in  the  chief  or 
peptic  cells  which  line  the  glands  both  of  the  cardiac  and  of 
the  pyloric  parts  of  the  stomach.  Daring  fasting,  granules  are 
seen  to  accumulate  in  these  cells,  and  when  the  stomach  is 
active  they  are  discharged.  These  granules  are  not  pepsin 
but  the  forerunner  of  pepsin — pepsinogen  (p.  36). 

(3)  Course  of  Gastric  Digestion. —  («)  Amylolytic  Period. — 
The  action  of  the  gastric  juice  does  not  at  once  become 
manifest.  In  the  pig  for  about  two  hours  after  the  food  is 
swallowed,  the  ptyalin  of  the  saliva  goes  on  acting,  and  the 
various  micro-organisms  swallowed  with  the  food  grow  and 
multiply,  and  thus  there  is  a  continuance  of  the  conversion 
of  starch  to  sugar  which  was  started  in  the  mouth,  and,  at 
the  same  time,  the  micro-organisms  go  on  splitting  sugar  to 
form  lactic  acid,  which  may  thus  be  regarded  as  a  normal 
constituent  of  the  oesophageal  end  of  the  pig's  stomach 
during  the  first  two  hours  after  a  meal. 

(6)  Proteolytic  Period. — Before  the  amylolytic  period  is 
completed,  the  gastric  juice  has  begun  its  special  action  on 
Proteins.  This  may  be  readily  studied  by  placing  some 
coagulated  protein  in  gastric  juice,  or  in  an  extract  of  the 
mucous  membrane  of  the  stomach  made  with  dilute  hydro- 
chloric acid,  and  keeping  it  at  the  temperature  of  the  body. 
The  protein  swells,  becomes  transparent,  and  dissolves.  The 
solution  is  coagulated  on  boiling — a  soluble  native  protein 
has  been  formed.  Very  soon  it  is  found  that,  if  the  soluble 
native  protein  is  filtered  off,  the  filtrate  gives  a  precipitate 
on  neutralising,  showing  that  an  acid  compound — a  meta.- 
protein — has  been  produced.  If  the  action  is  allowed  to 
continue  and  the  meta-protein  precipitated  and  filtered  off, 
it  will  be  found  that  the  filtrate  gives  a  precipitate  on  half 
saturation  with  ammonium  sulphate,  showing  that  proto- 
proteo.^es  have  been  formed.  These  differ  somewhat  in  their 
reaction,  and  apparently  differ  in  the  proportion  of  their 
constituent  amino-acids.  On  filtering  off  these  proteoses, 
the  filtrate  yields  a  precipitate  on  saturating  with  ammonium 


312  VETERINARY   PHYSIOLOGY 

sulphate,  indicating  the  formation  of  deutero-proteoses ; 
and,  if  the  filtrate  after  precipitating  this  be  tested,  the 
presence  of  yet  another  set  of  proteins  may  be  demonstrated. 
These  are  Peptones  (p.  18)  {Chemical  Physiology). 

These  changes  may  be  represented  in  the  following 
table : — 

Coagulated  Protein. 

Soluble  Native  Protein. 

I 
Meta-proteins. 

I 
Proto-proteoses. 

I 
Deutero-proteoses. 

I 

Peptones. 

The  process  is  one  of  breaking  down  a  complex  molecule 
into  simpler  molecules,  probably  with  hydration.  It  is  the 
first  step  to  the  more  complete  disintegration  of  the  protein 
to  amino-acids  which  seems  necessary  before  it  can  be  built 
into  the  special  protoplasm  of  the  body  of  the  particular 
animal. 

On  certain  proteins  and  their  derivatives  the  gastric  juice 
has  a  special  action.  On  collagen  the  HCl  acts  slightly  in 
converting  it  to  gelatin.  The  gastric  juice  acts  on  gelatin, 
converting  it  to  a  gelatin  peptone. 

On  nucleo-jjroteins  it  acts  by  digesting  the  protein  part 
and  leaving  the  nuclein  undissolved. 

Hcemoglobin  is  broken  down  into  haematin  and  globin, 
and  the  latter  is  changed  into  peptone.  It  is  the  formation 
of  acid  haematin  (p.  490)  which,  after  a  short  time,  gives  the 
vomited  matter  in  cases  of  haemorrhage  into  the  stomach  a 
brown  colour. 

The  caseinoge7i  calcium  compound  of  milk  (p.  636) 
is  first  coagulated,  and  then  changed  to  peptone.  The 
coagulation  is  brought  about  by  what  is  generally  described 
as  a  second  enzyme  of  the  gastric  juice — rennin.  Very 
probably  this  action  is  merely  a  phase  of  the  action  of  pepsin. 

The  stomach  contains  an  enzyme,  lipase,  which  splits  Fats 


DIGESTION  313 

into  fatty  acids  and  glycerol  if  they  are  in  a  very  line  state  of 
subdivision,  as  in  milk,  but  it  has  no  action  on  fats  not  so 
subdivided.  It  is  probable  that  this  lipase  comes  from 
the  duodenum.  AVhen  fats  are  contained  in  the  protoplasm 
of  cells,  they  are  set  free  by  the  digestion  of  the  protein 
covering. 

On  Carbohydrates  the  free  mineral  acid  of  the  gastric  juice 
has  a  slight  action  at  the  body  temperature,  splitting  the 
polysaccharids  and  disaccharids  into  monosaccharids. 

(4)  Digestion  of  the  Stomach  Wall. — Wlien  the  wall  of  the 
stomach  dies  either  in  whole,  as  after  the  death  of  the  animal, 
or  in  part,  as  when  an  artery  is  occluded  or  ligatured,  the 
dead  part  is  digested  by  the  gastric  juice  and  the  wall  of  the 
stomach  may  be  perforated.  The  typical  gastric  ulcer  which 
so  frequently  occurs  in  man  in  anajmia  is  of  this  nature.  In 
the  normal  condition,  a  substance  may  be  extracted  from 
the  mucous  membrane  which  antagonises  the  action  of 
pepsin  and  may  be  called  antipepsin. 

(5)  Antiseptic  Action  of  the  Gastric  Juice. — In  virtue  of 
the  presence  of  free  HCl,  the  gastric  juice  has  a  marked 
action  in  inhibiting  the  growth  of,  or  in  killing,  bacteria. 
Some  organisms,  while  they  do  not  multiply  in  the  stomach, 
pass  on  alive  to  the  intestine,  where  they  may  again  become 
active. 

(6)  Influence  of  Various  Diets  upon  the  Gastric  Secretion. — 
This  has  been  chiefly  investigated  by  Pavlov  on  dogs  with  a 
gastric  pouch  (p.  309). 

He  found  that — (1)  The  amount  of  secretion  depends 
upon  the  amount  of  food  taken.  (2)  The  amount  and  the 
course  of  secretion  vary  with  the  kind  of  food  taken.  Thus, 
with  flesh  the  secretion  reaches  its  maximum  at  the  end  of 
one  hour,  persists  for  an  hour  and  then  rapidly  falls,  while 
with  bread  it  reaches  its  maximum  at  the  end  of  one  hour, 
rapidly  falls,  but  persists  for  a  much  longer  period  than  in 
the  case  of  flesh.  (3)  The  digestive  activity  of  the  secretions 
was  tested  by  allowing  them  to  act  upon  ca})illary  glass  tubes 
filled  with  egg-white  coagulated  by  heating  (Mett's  tubes).  The 
extent  to  which  this  was  digested  out  of  the  tube  in  unit  of 
time  gave  the  activity  of  the  secretion.      It  varies  with  the 


314 


VETERINARY  PHYSIOLOGY 


kind  of  food  and  with  the  course  of  digestion.  It  is  higher 
and  persists  longer  after  a  diet  of  bread,  which  is  difficult  to 
digest,  than  after  a  diet  of  flesh,  which  is  more  easily 
digested.  (4)  The  percentage  of  acid  does  not  vary 
markedly.  When  more  acid  is  required,  more  gastric  juice 
is  secreted.  (5)  The  work  done  by  the  gastric  glands,  as 
measured    by    the   amount    of   secretion    multiplied    by   its 


AfOSE 


EYE 


MOUTH 


\J    STOMACH 


POUCH 


Fig.  151. — To  show  the  Nervous  Mechanism  of  Gastric  Secretion  and  how  it 
is  reflexly  induced  through  various  ingoing  channels. 


activity,  is  greater  in    the    digestion    of  bread    than    in  the 
diofestion  of  flesh. 

(7)   Nervous  Mechanism  of  Gastric  Secretion. — 
(rt)  Intrinsic. — It  has  been  proved  that  in  the  dog  the 
secretion  of  gastric  juice  can  go  on  after  the  nerves  to  the 
stomach   have  been  divided,  and  this  has  been  ascribed  to  a 
reflex  stimulation  of  the  nerve  plexus  in  the  submucosa. 

(6)  Extrinsic. — Pavlov  found  that,  when  the  vagus  is  cut 


DIGESTION  81& 

below  the  origin  of  the  cardiac  nerves  so  that  the  heart  cannot 
be  inhibited,  stimuLation  of  the  lower  end  of  nerve,  with  a 
slowly  interrupted  induced  current,  causes  a  flow  of  gastric 
juice  after  a  long  latent  period  of  a  minute  or  two. 

This  action  of  the  vagus  may  be  called  into  play,  either 
by  the  contact  of  suitable  food  with  the  mouth  or  by  the- 
sight  of  food.  This  he  demonstrated  by  making  an 
oesophageal  fistula  in  a  dog  with  a  gastric  pouch,  so  tliat 
food  put  in  the  mouth  escaped  from  the  gullet  and  did  not 
pass  into  the  stomach  (fig.  151).  Mere  mechanical  or 
chemical  stimulation  of  the  mouth  produces  no  effect,  but 
the  administration  of  meat  produces  it.  In  a  fasting  dog 
the  sight  of  food  produces,  after  a  latent  period  of  five 
minutes,  a  copious  flow  of  gastric  juice.  Pavlov  calls  thi& 
"  psychic "  stimulation.  It  is  an  example  of  how  the 
"distance  receptor"  in  the  eye  reflexly  brings  about  an 
appropriate  reaction — ^just  as  the  "non-distance  receptor"  in 
the  wall  of  the  stomach,  under  other  stimuli,  brings  about 
an  appropriate  reaction. 

(8)  Chemical  Stimulation. — There  is  some  evidence  that 
the  formation  of  gastric  juice  is  also  influenced  by  the  action 
of  a  chemical  substance  produced  in  the  mucous  membrane 
of  the  pyloric  end  of  the  stomach.  It  has  been  found  that 
the  iojection  into  the  blood-stream  of  an  extract  of  this 
membrane,  made  by  boiling  with  acid  or  peptone,  causes  a 
production  of  gastric  juice.  In  all  probability  the  initial 
secretion  of  gastric  juice  is  dependent  on  the  nervous 
mechanism,  and  the  secondary  secretion,  when  food  is  in  the 
stomach,  on  the  action  of  this  substance.  The  secretion  is- 
also  increased  by  the  presence  in  the  stomach  of  meat 
extracts  and  of  weak  solutions  of  alcohol. 

3.  Movements  of  the  Stomach. — These  have  been  studied 
by  feeding  an  animal  or  a  man  with  food  containing  bismuth, 
and  then  applying  X-rays,  which  are  intercepted  by  the 
coating  of  bismuth,  so  that  a  shadow^  picture  of  the  shape  of 
the  stomach  is  given  (fig.  152). 

1.  Character. — It  is  found  that  soon  after  food  is  taken, 
a    constriction    forms    about    the    angular     incisure    at    the 


316  VETERINARY   PHYSIOLOGY 

middle  of  the  stomach,  due  to  the  contraction  of  the 
prepyloric  sphincter  which  separates  the  cardiac  from, 
the  pyloric  end.  This  contraction  passes  on  towards  the 
pylorus.  Another  contraction  forms  and  follows  the  first, 
and  thus  the  pyloric  part  of  the  stomach  is  set  into  active 
movement. 

The  fundus  acts  as  a  reservoir,  and,  by  a  steady  con- 
traction, presses  the  gastric  contents  into  the  more  active 
pylorus,  so  that,  at  the  end  of  gastric  digestion,  it  is  com- 
pletely emptied. 

While  the  food  is  well  mixed  in   the  p3doric  canal,   no 

A     ^-N  B 


Fig.  152. — Tracings  of  the  Shadows  of  the  Contents  of  the  Stomach  and 
Intestine  of  a  Cat  two  hours  after  feeding  (A)  with  boiled  lean  beef, 
and  (B)  with  boiled  rice  to  show  the  more  rapid  emptying  of  the 
stomach  after  the  carbohydrate  food.  The  waves  of  contraction  in 
the  pyloric  part  of  the  stomach  are  shown.  The  small  divisions  of 
the  food  in  some  of  the  intestinal  loops  represent  the  process  of 
rhythmic  segmentation.      (Cannon.) 

great  mixing  takes  place  in  the  fundus  of  the  stomach, 
and,  by  feeding  with  different  coloured  foods,  its  distribution 
may  clearly  be  seen  (fig.  1 5  3), 

The  pylorus  is  closed  by  the  strong  sphincter  pylori 
muscle,  which,  however,  relaxes  from  time  to  time  during 
gastric  digestion  to  allow  the  escape  of  the  more  fluid 
contents  of  the  stomach  into  the  intestine.  These  openings 
are  at  first  slight  and  transitory,  but,  as  time  goes  on,  they 
become  more  marked  and  more  frequent,  and,  Avhen  gastric 
digestion  is  complete — after  an  ordinary  meal  at  the  end  of 
four  or  five  hours,  the  sphincter  is  completely  relaxed  and 
allows  the  stomach  to  be  emptied.      The  openings  are  regu- 


DIGESTION 


317 


lated  by  a  local  nervous  mechanism  which  is  reilexly  brought 
into  play  by  the  escape  of  the  acid  gastric  contents  into  the 
duodenum.  This  leads  to  an  immediate  closure  of  the 
pylorus,  which  does  not  again  open  till  the  contents  of  the 
duodenum  have  been  neutralised  by  the  alkaline  secretions 
which  are  poured  into  it. 

The  rate  of  passage  from  the  stomach  of  various  kinds  of  food 
has  been  studied  by  feeding  cats  with  equal  amounts  of  different 
kinds  of  food  mixed  with  bismuth,  and  then,  by  X-rays, 
getting  the  outline  of  the  contents  of  the  small  intestine  at 

Prepyloric  Sphincter. 


Pyloric  Sphincter. 


Fig.    153. — Stomach  of  a  Dog  fed  successively  with  three  different  foods 
to  show  the  absence  of  mixing  at  the  cardiac  end. 


different  periods.  Carbohydrates  were  found  to  pass  on 
most  rapidly  and  fats  most  slowly  (fig.  15  2). 

In  man  after  a  moderate  meal  the  stomach  is  usually 
emptied  in  about  four  hours. 

2.   Nervous  Mechanism  of  Gastric  Movements — 

(a)  Intrinsic- — Even  after  the  section  of  all  the  gastric 
nerves,  the  movements  of  the  stomach  may  be  observed  to  go 
on  regularly.  They  are  therefore  due  to  a  mechanism  in  the 
wall  of  the  organ,  and,  in  all  probability,  judging  from  the 
analogy  of  the  small  intestine  (p.  333),  they  are  controlled 
by  the  plexus  of  neurons  in  the  muscular  coat. 

(/j)  Extrinsic. — (i.)  The  vagus  maintains  the  tone  of  the 
muscular  coat  and  augments  the  movements  of  the  pylorus. 
It  also  seems  to  act  upon  the  cardiac  sphincter  to  relax  it. 
(ii.)  The  sympathetic  fibres  decrease  the  tone  and  the 
movements,  but  seem  to  maintain  the  contraction  of  the 
pyloric  sphincter.  Their  terminations  are  stimulated  by 
adrenalin. 


318  VETERINARY  PHYSIOLOGY 

C.  Absorption  from  the  Stomach. 

By  ligaturing  of  the  pyloric  end,  it  has  been  found  that 
the  stomach  plays  a  very  small  part  in  the  absorption  of 
food  ;  water  is  not  absorbed,  altliough  alcohol  and  many 
drugs  are  raj)idly  taken  up.  There  is  a  slight  absorption  of 
peptones  and  of  sugars. 

While  the  stomach  plays  a  certain  part  in  digestion,  its 
action  is  by  no  means  indispensable,  for  it  has  been  removed 
in  animals  and  in  men  without  disturbance  of  the  health. 
It  has  been  shown,  however,  that  tlie  splitting  of 
proteins  is  somewhat  different  if  peptic  precedes  tryptic 
digestion. 

Its  main  function  is  to  act  as  a  reservoir,  and  probablj'  the 
.antiseptic  action  of  its  secretion  is  of  considerable  importance. 

D.  Regurgitation  of  Gastric  Contents. 

1.  Regurgitation  into  the  Gullet. — Normally,  the  contents 
■of  the  stomach  are  prevented  from  passing  back  into  the 
gullet  by  the  cardiac  sphincter.  The  tone  of  this  sphincter 
is  easily  overcome,  and  it  is  relaxed  by  repeated  swallowing, 
so  that  no  sound  is  heard  as  the  contents  pass  into  the 
stomach.  Since  adrenalin  inhibits  it,  it  is  probably 
relaxed  by  the  sympathetic  nerves.  Stimulation  of  the 
vagus  first  inhibits  and  then  causes  it  to  contract.  It  tends 
to  undergo  rhythmic  relaxations,  during  which  the  gastric 
contents  pass  back  into  the  oesophagus,  even  up  to  the 
mouth,  and  then,  by  oesophageal  peristalsis,  are  again 
passed  down.  The  tone  of  the  muscle  is  increased  by  the 
presence  of  hydrochloric  acid  in  the  stomach,  and  thus 
regurgitation  does  not  take  place  in  normal  digestion,  but 
is  associated  witli  a  neutral  reaction  of  the  stomach  con- 
tents which  is  well  marked  in  some  forms  of  atonic 
dys2^epsia  that  occur  in  man. 

2.  Vomiting. — Sometimes  the  stomach  is  emptied  upwards 
through  the  gullet  instead  of  downwards  through  the 
pylorus.  This  act  of  vomiting  is  generally  a  reflex  one, 
resulting^  from  irritation  of  the  sjastric  mucous  membrane. 


DIGESTION  319 

and  more  rarely  from  stimulation  of  other  nerves.  It  is  a 
reaction  to  nocuous  stimuli.  In  some  animals,  as  the  dog, 
it  may  be  voluntarily  induced. 

Usually  vomiting  is  preceded  by  a  free  secretion  of  saliva. 
The  glottis  is  then  closed,  and,  after  a  forced  inspiratory 
effort  by  which  air  is  drawn  down  into  the  gullet,  a  forced 
and  spasmodic  expiration  presses  on  the  stomach,  while  at 
the  same  time  the  cardiac  sphincter  is  relaxed,  and  the  con- 
tents of  the  stomach  are  shot  upwards.  They  are  prevented 
from  passing  into  the  nares  by  the  contraction  of  the  muscles 
of  the  soft  palate.  The  wall  of  the  stomach  also  acts,  the 
pyloric  end  being  firmly  contracted  and  the  cardiac  end  being 
also  in  a  state  of  tonus.  But  its  action  is  non-essential,  since 
vomiting  may  be  produced  in  an  animal  in  which  a  bladder 
has  been  inserted  in  place  of  the  stomach. 

The  centre  which  presides  over  the  act  is  in  the  medulla 
oblongata,  and,  while  it  is  usually  reflexly  called  into  action, 
it  may  be  stimulated  directly  by  such  drugs  as  apomorphine. 


IV.    INTESTINAL  DIGESTION. 

After  being  subjected  to  gastric  digestion  the  food  is 
o-enerally  reduced  to  a  semi-fluid  grey  pultaceous  condition 
of  strongly  acid  reaction  known  as  chyme,  and,  in  this 
condition,  it  enters  the  duodenum. 

Here  it  meets  three  different  secretions  : — 

A.  Pancreatic  secretion. 

B.  Bile. 

C  Intestinal  secretion- 

A.  Pancreatic  Secretion. 

The  secretion  of  the  pancreas  may  be  procured  by  making 
either  a  temporary  or  a  permanent  fistula.  In  the  former 
case,  the  duct  is  exposed,  and  a  cannula  fastened  in  it ;  in 
the  latter,  the  duct  is  made  to  open  on  the  surface  of  tlie 
abdomen,  a  small  piece  of  the  intestinal  wall,  with  the 
mucous  membrane    round   the    opening   of  the   duct,  being 


320  VETERINARY   PHYSIOLOGY 

stitched  to  the  abdominal  opening.     For  experiments  on  pan- 
creatic digestion  extracts  of  the  pancreas  are  generally  used. 

1.  Characters  and  Composition. — When  obtained  immedi- 
ately from  a  temporary  fistula,  the  pancreatic  juice  is  a  clear, 
slimy  fluid,  with  a  specific  gravity  of  about  1015  and  an 
alkaline  reaction.  It  contains  an  abundance  of  a  native 
protein  having  the  characters  of  a  globulin,  and  its  alkalinity 
is  probably  due  to  sodium  carbonate  and  disodium  phosphate. 
From  a  permanent  fistula  a  more  abundant  flow  of  a  more 
watery  secretion  may  be  collected. 

2.  Action. — Closely  associated  with  the  protein,  and  pre- 
cipitated along  with  it  by  alcohol,  are  the  enzymes,  upon 
which  the  action  of  the  pancreatic  juice  depends  {Chemical 
Physiology). 

1st.  A  Proteolytic  Enzyme — Trypsin. — This,  in  a  weakly 
alkaline  or  neutral  fluid,  converts  native  proteins  into 
peptones,  and  then  breaks  these  peptones  into  simpler  non- 
protein bodies. 

The  pancreatic  juice  brings  about  this  breaking  down  of 
proteins  in  stages.  It  does  not  cause  soHd  proteins  to  swell 
up,  but  simply  erodes  them  away.  Fibrin  and  similar  bodies 
first  pass  into  the  condition  of  soluble  native  proteins  and 
then  into  dewtevo-proteose.  The  deutero-proteose  is  then 
changed  into  peptone,  and  part  of  that  peptone  is  split  into 
a  series  of  bodies  which  no  longer  give  the  biuret  test. 
These  consist  chiefly  of  the  component  j^oly peptides,  amino- 
acids,  and  of  ammonia  compounds  (see  p.   18). 

Amino-propionic  acid  linked  to  indol — tryptophan — is 
also  spUt  off,  and,  if  chlorine  water  is  added  to  a  pancreatic 
digest  which  has  proceeded  for  a  long  time,  this  gives  a  rose- 
red  colour. 

On  nucleo- proteins,  trypsin  acts  by  digesting  the  protein 
and  dissolving  the  nucleic  acid  so  that  it  may  be  absorbed. 

On  collagen  and  elastin  trypsin  has  Httle  action  ;  but  on 
gelatin  it  acts  as  upon  proteins. 

2nd.  A  Diastase  or  Amylolytic  Enzyme. — This  acts  in 
the  same  way  as  ptyalin,  but  more  powerfully,  converting   a 


DIGESTION  321 

certain  part  of  the  maltose  into  dextrose.  It  acts  best  in  a 
faintly  acid  medium. 

ovd.  A  Lipase  or  Fat-splitting  Enzyme.— This  is  the 
most  easily  destroyed  and  the  most  difficult  to  separate  of 
the  enzymes.  It  breaks  the  fats  into  their  component  glycerol 
and  fatty  acids.  The  fatty  acids  link  with  the  alkalies  which 
are  present  to  form  soaps,  and  in  this  form,  or  dissolved  as 
free  fatty  acids  in  the  bile,  they  are  absorbed. 

But  the  formation  of  soaps  also  assists  the  digestion  of 
fats  by  reducing  them  to  a  state  of  finely  divided  particles, 
an  emulsion,  upon  which  the  lipase  can  act  more  freely. 
This  process  of  emulsification  is  assisted  by  the  presence  of 
proteins  in  the  pancreatic  juice  and  also  by  the  presence  of 
bile. 

Uh.  It  is  doubtful  whether  the  pancreatic  secretion 
contains  rennin  apart  from  trypsin,  although  it  produces  a 
modified  clotting  of  milk,  under  certain  conditions. 

That  these  enzymes  are  independent  of  one  another  is 
shown  by  the  facts  that  one  may  be  present  without  the 
other,  e.g.  diastase  is  absent  in  man  in  early  childhood  ;  and 
also  that  diastase  may  be  in  an  active  state  while  the  trypsin 
is  in  its  inactive  trypsinogen  state. 

As  to  the  mode  of  production  of  these  enzymes,  it  is  known 
that  trypsin  is  not  formed  as  such  in  the  cells,  for  the 
secretion,  direct  from  the  acini,  has  no  tryptic  action.  A 
forerunner  of  trypsin — trypsinogen — is  produced,  and  this 
changes  into  trypsin  after  it  is  secreted.  The  intestinal 
secretion  contains  a  substance  of  the  nature  of  an  enzyme, 
enterokinase,  which  has  the  power  of  bringing  about  this 
change  of  trypsinogen  to  trypsin,  thus  activating  it  (fig.  154). 

3.  Physiology  of  Pancreatic  Secretion. —  (a)  Chemical 
Control. — The  secretion  of  pancreatic  juice  is  not  constant, 
but  is  induced  when  the  acid  chyme  passes  into  the  duo- 
denum. This  occurs,  even  when  all  the  nerves  to  the 
intestine  have  been  cut,  and  it  appears,  from  the  investiga- 
tions of  Bayliss  and  Starling,  to  be  due  to  the  formation  of 
a  material,  which  has  been  called  secretin,  in  the  epithelium 
21 


322 


VETERINARY   PHYSIOLOGY 


lining  the  intestine,  under  the  influence  of  an  acid.  This  is 
absorbed  into  the  blood  in  which  it  is  carried  round  the 
circulation,  and,  on  reaching  the  pancreas,  stimulates  it  to 
secrete  (fig.  15-i).  It  has  been  shown  that  the  injection 
into  a  vein  of  an  extract,  made  with  dilute  hydrochloric 
acid,  of  the  lining  membrane  of  the  upper  part  of  the  small 
intestine,   leads  to  a  flow  of  pancreatic  juice.      Secretin    is 


I 


TffYPSUI    * 


Fig.  154. — To  show  the  Mode  of  Action  of  Secretin  and  of  the  vagus  nerve 
on  the  secretion  of  the  pancreas  and  the  activation  of  ti'ypsinogen  by 
enterokinase. 

not  destroyed  by  boiling,  and  is  soluble  in  strong  alcohol. 
It  is  therefore  not  of  the  nature  of  an  enzyme. 

(6)  Nervous  Control. — But,  while  secretin  seems  to  play 
so  important  a  role,  it  has  been  found  that  stimulation  of 
the  vagus  nerve,  after  a  latent  period  of  several  minutes, 
increases  pancreatic  secretion,  so  that  it  must  be  concluded 
that  the  process  of  secretion  is,  to  a  certain  extent,  under 
the  control  of  tlie  nervous  system. 

The  influence  of  the  pancreas  in  the  general  metabolism 
will  be  considered  later  (p.  357). 


DIGESTION  823 

B.  Bile. 

1.  Characters  and  Composition. — The  bile  is  the  secretion  of 
the  liver,  and  it  may  be  procured  for  examination — (a)  from 
the  gall  bladdei^,  or  (b)  from  the  bile  passages  by  making  a 
fistula  into  them.  This  may  be  temporary  when  a  tube 
IS  placed  in  the  common  bile  duct,  or  permanent  when  the 
common  bile  duct  is  ligatured,  the  fundus  of  the  gall  bladder 
stitched  to  the  edges  of  the  abdominal  wound  and  an  incision 
then  made  into  it  ;  the  bile  thus  flows  through  the  gall 
bladder  to  the  surface. 

Bile  which  has  been  in  the  gall  bladder  is  richer  in  solids 
than  bile  taken  directly  from  the  ducts,  because  water  is 
absorbed  by  the  w^alls  of  the  bladder,  and  the  bile  thus 
becomes  concentrated. 

Analyses  of  gall  bladder  bile  thus  give  no  information  as 
to  the  composition  of  the  bile  when  formed.  In  several 
cases,  where  surgeons  have  produced  biliary  fistulse,  oppor- 
tunities have  occurred  of  procuring  the  bile  directly  from  the 
ducts  during  life  in  man. 

Such  bile  has  a  somewhat  orange-brown  colour,  and  is 
more  or  less  viscous,  but  not  nearly  so  viscous  as  bile  taken 
from  the  gall  bladder.  It  has  a  specific  gravity  of  almost 
1005,  while  bile  from  this  gall  bladder  has  a  specific  gravity 
of  about  1030.  Its  reaction  is  slightly  alkaline,  and  it  has 
a  characteristic  smell. 

It  contains  about  2  per  cent,  of  solids,  of  which  more 
than  half  are  organic. 

(1)  Bile  Salts  {Chemical  Physiology). — The  most  abundant 
solids  are  the  salts  of  the  bile  acids.  In  man  the  most 
important  is  sodium  glycocholate.  Sodium  taurocholate 
occurs  in  small  amounts.  These  salts  are  readily  prepared 
from  an  alcoholic  solution  of  dried  bile  by  the  addition  of 
water-free  ether,  which  makes  them  separate  out  as  crystals. 

Glycocholic  acid  splits  into  glycin — amino-acetic  acid, 
H0X.CII2.CO.OH— and  a  body  of  unknown  constitution, 
cholalic  acid,  C04H40O5. 

Taurocholic    acid    yields    amino-ethyl    sulphonic    acid    or 


324  VETERINARY    PHYSIOLOGY 

taurin,  H2N.CH2CH2.SO2OH,  which  is  amino-acetic  acid 
linked  to  sulphuric  acid.  This  is  joined  to  cholalic  acid. 
In  man  there  is  very  little  taurocholic  acid. 

Since  both  are  amino-acids,  they  must  be  derived  from 
proteins.  That  they  are  formed  in  the  liver  and  not  merely 
excreted  by  it,  is  shown  by  the  fact  that,  while  they  accumu- 
late in  the  blood  if  the  bile  duct  is  ligatured,  they  do  not 
appear  if  the  liver  is  excluded  from  the  circulation.  The  bile 
salts  manifest  the  following  actions  : — 

(i.)  They  are  solvents  of  lipoids,  and  they  activate 
the  lipase  of  the  pancreatic  secretion.  For  this  reason 
(a)  they  assist  in  the  digestion  and  absorption  of  fats. 
When  bile  is  excluded  from  the  intestines  no  less  than  30 
per  cent,  of  the  fats  of  the  food  may  escape  absorption  and 
appear  in  the  fasces.  When  this  is  the  case,  as  in  jaundice 
in  man  from  obstruction  of  the  bile  duct,  the  faeces  have  a 
characteristic  white  or  grey  appearance  from  the  abundance 
of  fat.  (6)  They  keep  cholesterol  in  solution,  (c)  They 
act  as  powerful  hsemolytic  agents  dissolving  the  lipoid 
capsules  of  the  erythrocytes  and  allowing  the  escape  of 
haemoglobin. 

(ii.)  While  the  salts  have  no  action  on  proteins,  free 
taurocholic  acid  precipitates  native  proteins  and  acid  meta- 
proteins. 

(iii.)  They  lower  the  surface  tension  of  solutions,  and 
in  this  way  they  may  bring  the  fat  and  other  substances 
into  more  intimate  contact  with  the  mucous  membrane. 

(2)  Bile  Pigments. — These  amount  to  only  about  0-2  per 
cent,  of  the  bile.  In  human  bile,  the  chief  pigment  is  an 
orange- brown  iron-free  substance,  hiliruhin,  C32H36N4O6, 
while  in  the  bile  of  herbivora,  biliverdin,  a  green  pigment, 
somewhat  more  oxidised  than  bilirubin,  CgoHggN^Og,  is  more 
abundant.  By  further  oxidation  with  nitrous  acid,  other 
pigments — blue,  red,  and  yellow — are  produced,  and  this  is 
used  as  a  test  for  the  presence  of  bile  pigments  (Gmelin's  test) 
(Chemical  Physiology). 

The  pigments  are  iron-free,  and  they  are  closely  allied  to 
hsematoporphyrin  and  hasmatoidin  (see  p.  491).  They  are 
derived  from  hsemoglobin  by  the  splitting  of  the  hasmatin 


DIGESTION  325 

molecule  into  an  iron-containing  part,  which  is  retained  in 
the  liver,  and  the  iron-free  biliary  pigment.  Their  amount 
is  greatly  increased  when  haemoglobin  is  set  free  or  injected 
into  the  blood.  Old  and  breaking  down  red  cells  are 
scavengered  from  the  blood  by  the  endothehal  cells  of  the 
hepatic  capillaries,  while  free  haemoglobin  is  taken  up 
directly  by  the  liver  cells. 

That  the  pigments  are  formed  in  the  liver  is  shown  by  the 
fact  that,  when  the  liver  is  excluded  from  the  circulation, 
the  injection  of  haemoglobin  is  not  followed  by  their 
appearance  in  the  blood.  But  the  formation  of  haematoidin, 
which  is  practically  identical  with  bilirubin,  apart  from  the 
liver,  indicates  that  other  tissues  have  the  power  of  splitting 
haematin  into  its  iron-containing  and  iron-free  portions. 

The  liver  has  the  property  of  excreting  not  only  these 
pigments  formed  by  itself,  but  also  other  pigments.  Thus, 
the  liver  of  the  dog  can  excrete  the  characteristic  pigment  of 
sheep's  bile  when  this  is  injected  into  his  blood. 

(3)  Cholesterol  is  a  monatomic  alcohol — C26H43OH — 
which  occurs  free  in  small  amounts  in  the  bile.  It  is  very 
insoluble,  and  is  kept  in  solution  by  the  salts  of  the  bile  acids. 
It  readily  crystallises  in  rhombic  plates,  generally  with  a 
notch  out  of  the  corner. 

The  significance  of  cholesterol  in  metabolism  and  its 
source  in  the  bile  is  not  definitely  known.  It  is  a 
constituent  of  all  cells,  and  under  various  conditions  its 
amount  in  the  blood  plasma  may  be  increased,  e.g. 
when  the  amount  in  the  food  is  large.  It  then  appears 
in  larger  quantities  in  the  bile,  and  it  must  therefore 
be  concluded  that  it  is  excreted  by  the  liver.  Possibly 
it  is  derived,  at  least  in  part,  from  the  stroma  of  the 
erythrocytes. 

(4)  Fats  and  Lecithin. — The  true  fats  and  the  phosphorus- 
containing  lecithin  are  present  in  small  amounts  in  the  bile, 
and  apparently  they  are  derived  from  the  fats  of  the  liver 
cells.  The  fats  may  be  increased  in  amount  by  the  admini- 
stration of  fatty  food. 

(5)  Nucleo-protein  and  Mucin. — The  bile  ow^s  its  viscosity 
to  the   presence  of  a  muciu-like  body,  which,  however,  does 


826  VETERINARY   PHYSIOLOGY 

not  yield  sugar  on  boiling  with  an  acid  and  which  contains 
phosphorus.  It  is  precipitated  by  acetic  acid,  but  the 
precipitate  is  soluble  in  excess.  It  is  therefore  a  nucleo- 
protein.  In  some  animals  a  certain  amount  of  mucin  is  also 
present  {Chemical  Physiology). 

(6)  Inorganic  Constituents. — The  most  abundant  salt  is 
calcium  phosphate.  Phosphate  of  iron  is  present  in  traces. 
Sodium  carbonate,  calcium  carbonate,  and  sodium  chloride 
are  the  other  chief  salts. 

2.  Flow  of  Bile — The  bile,  when  secreted  by  the  liver 
cells,  may  accumulate  in  the  bile  passages  and  gall  bladder, 
and  later  be  expelled  under  the  influence  of  the  contraction 
of  the  muscles  of  the  ducts  or  of  the  pressure  of  the 
abdominal  muscles  upon  the  liver.  The  flow  of  the  bile 
into  the  intestine  thus  depends  upon — 1st,  The  secretion  of 
bile  ;  2nd,  the  expulsion  of  bile  from  the  bile  passages.  It  is 
exceedingly  difficult  to  separate  the  action  of  these  two  factors. 

The  taking  of  food  increases  the  flow  of  bile,  and  the 
extent  to  which  it  is  increased  depends  largely  on  the  kind 
of  food  taken.  In  the  dog,  a  protein  meal  has  the  most 
marked  effect,  a  fatty  meal  a  less  marked  effect,  and  a 
carbohydrate  meal  hardly  any  effect.  The  increased  flow  of 
bile  following  the  taking  of  food  does  not  reach  its 
maximum  till  six  or  nine  hours  after  the  food  is  taken,  and 
some  observers  have  found  that  the  period  of  maximum 
flow  is  even  further  delayed. 

Pavlov  found  in  dogs,  in  which  a  biliary  fistula  had  been 
made  leaving  the  opening  of  the  bile  duct  in  the  mucous 
membrane  of  the  intestine,  that  an  expulsion  of  bile  follows 
the  taking  of  food ;  and  Starling  finds  that  the  flow  of  bile 
is  increased  by  the  injection  of  secretin.  It  thus  tends  to 
run  parallel  with  the  flow  of  pancreatic  juice. 

Influence  of  Nerves  upon  the  Flow  of  Bile. — (a)  Exjoulsion 
of  Bile. — There  is  good  evidence  that  nerve  fibres  pass  to 
the  muscles  of  the  bile  passages  and  that  they  mav  cause  an 
expulsion  of  bile  by  stimulating  them  to  contract. 

(6)  Secretion  of  Bile. — There  is  no  convincing  evidence 
that   nerve  fibres   act    directly    upon   the    secretion   of  bile. 


DIGESTION  327 

This  appears  to  be  governed  by  the  nature  of  the  material 
brought  to  the  liver  by  the  blood,  and  by  the  activity  of  the 
liver  cells.  It  is  an  example  of  function  regulated  by 
chemical  substances  rather  than  by  a  nerve  mechanism  : 
although  it  is  quite  probable  that  these  chemical  substances 
act  through  the  rich  terminal  plexus  of  nerves  which  runs 
throughout  the  liver. 

o.  Mode  of  Secretion  of  Bile. — It  has  been  seen  that  the 
bile  salts  are  actually  formed  in  the  liver-cells,  and  there  is 
good  evidence  that  the  water  of  the  bile  is  not  a  mere  tran- 
sudation but  is  the  product  of  the  living  activity  of  these 
cells.  The  pressure  under  which  bile  is  secreted  may  be 
determined  by  fixing  a  cannula  in  the  bile  duct  or  in  a 
biliary  fistula,  and  connecting  it  with  a  water  manometer. 
In  man  the  pressure  is  as  much  as  20  to  30  mm.  Hg,  while 
the  pressure  in  the  portal  vein  of  the  dog  is  only  7  to  1 6  mm. 
Hg.      Hence  bile  cannot  be  formed  by  a  process  of  filtration. 

4.  Nature  and  Functions  of  Bile. — Bile  is  not  a  secretion  of 
direct  importance  in  digestion — (1)  It  has  practically  no 
action  on  proteins  or  carbohydrates,  and  its  action  on  fats  is 
merely  that  of  a  solvent.  Pavlov  maintains  that  it  activates 
the  lipase  of  the  pancreatic  juice,  and  others  have  found 
that  it  increases  the  activity  of  trypsin  and  possibly  of 
diastase,  while  its  action  on  the  surface  tension  of  the 
intestinal  contents  may  favour  the  absorption  of  fat.  It 
may  thus  be  considered  as  an  adjuvant  to  the  action  of 
pancreatic  juice.  (2)  Its  secretion  in  relationship  to  food 
does  not  indicate  that  it  plays  an  active  part  in  digestion. 
It  is  formed  during  intra-uterine  life  and  during  fasting,  and 
it  is  produced  many  hours  after  food  is  taken,  when  digestive 
secretions  are  no  longer  of  use  in  the  alimentary  canal.  (3) 
Digestion  can  go  on  quite  well  without  the  presence  of  bile 
in  the  intestine,  except  that  the  fats  are  not  so  well  absorbed. 
(4)  The  composition  of  bile  strongly  suggests  that  it  is  a 
waste  product.  The  pigment  is  the  result  of  the  decompo- 
sition of  haemoglobin  and  the  acids  are  the  result  of  protein 
disintesrration. 


328  VETERINARY  PHYSIOLOGY 

All  these  facts  seem  to  indicate  that  bile  is  the  medium 
by  ivhich  the  waste  products  of  hepatic  metabolism  are 
eliminated,  just  as  the  waste  products  of  the  body  generally 
are  eliminated  in  the  urine  by  the  kidneys.  (The  action  of 
the  liver  in  general  metabolism  is  considered  on  p.  353.) 


C  Secretion  of  the  Intestinal  Wall. 
(Succus  Entericus). 

1.  Method  of  Procuring. — This  is  formed  in  the  Lieber- 
kiihn's  follicles  of  the  intestine,  and  it  may  be  procured  by 
cutting  the  intestine  across  at  two  points,  bringing  each  end 
of  the  intermediate  piece  to  the  surface,  and  connecting 
together  the  ends  from  which  this  piece  has  been  taken 
away,  so  as  to  restore  the  continuous  tube  of  the  intestine. 

2.  Characters. — On  mechanically  irritating  the  mucous 
membrane,  a  pale,  yellow,  clear  fluid  is  secreted,  which 
contains  native  proteins  and  mucin,  and  is  alkaline  in  re- 
action from  the  presence  of  sodium  carbonate. 

3.  Action. — The  succus  entericus  contains: — (1)  An 
enzyme  or  enz3^mes  which  split  some  disaccharids,  as 
maltose  and  cane  sugar,  into  monosaccharids,  but  do  not 
seem  to  act  on  lactose.  A  special  lactase  seems  to  be 
present  in  the  intestine  of  young  animals  taking  milk.  (2) 
Lipase  is  also  present.  (3)  Erepsin,  an  enzyme  which  seems 
to  act  more  powerfully  than  trypsin  in  splitting  peptones 
into  their  component  non-protein  crystalline  constituents, 
the  polypeptides  and  amino-acids.  (4)  Enterokinase — a 
zymin  which,  acting  on  trypsinogen,  converts  it  into  active 
trypsin  (p.  321). 

4    Mechanism  of  Secretion The  taking  of  food  leads  to  a 

flow  of  intestinal  secretion  which  reaches  its  maximum  in 
about  three  hours  ;  and  this  flow  is  much  greater  from  the 
upper  part  of  the  bowel  than  from  the  lower.  There  is 
some  evidence  that  the  injection  of  secretin  calls  forth  this 
secretion,  and,  according  to  some  observers,  the  injection  of 
succus  entericus  into  the  circulation   acts  in    the  same  way. 


DIGESTION  329 

Mechanical  stimulation  undoubtedly  causes  a '  secretion, 
probably  through  a  reflex  in  the  nerve  plexuses  in  the  wall 
of  the  gut. 

As  regards  the  action  of  extrinsic  nerves  very  little  is 
known.  It  has  been  found  that,  if  the  intestine  be  ligatured 
in  three  places  so  as  to  form  two  closed  sacs,  and  the  nerves 
to  one  of  these  be  divided,  that  part  becomes  filled  with  a 
clear  fluid  closely  resembling  lymph.  The  dilatation  of  the 
blood-vessels  may,  however,  account  for  this,  without 
secretion  being  implicated. 


D.  Bacterial  Action  in  the  Alimentary  Canal. 

Numerous  micro-organisms  of  very  diverse  character  are 
swallowed  with  the  food  and  saliva.  It  has  been  susfg'ested 
that  the  leucocytes,  formed  in  the  lymphoid  tissue  of  the 
tonsils  and  pharynx,  attack  and  destroy  such  organisms,  but 
so  far,  definite  proof  of  this  is  not  forthcoming. 

When  the  food  is  swallowed,  the  micro-organisms 
multiply  for  some  time  in  the  warm,  moist  stomach,  and 
certain  of  them  form  lactic  and  sometimes  acetic  acid  by 
splitting  sugars.  But,  when  sufficient  gastric  juice  is  poured 
out  for  the  hydrochloric  acid  to  exist  free,  the  growth  of 
micro-organisms  is  inhibited,  and  some  of  them,  at  least,  are 
killed. 

Those  which  are  not  killed  pass  on  into  the  intestine, 
and,  as  the  acid  in  the  chyme  becomes  neutralised,  the  acid- 
forming  organisms  begin  to  grow,  and,  by  splitting  the 
sugars,  form  lactic  or  acetic  acid  and  render  the  contents  of 
the  small  intestine  slightly  acid.  Towards  the  end  of  the 
small  intestine,  and  more  especially  in  the  large  intestine, 
the  alkaline  secretions  have  neutralised  these  acids,  and,  in 
the  alkaline  material  so  produced,  the  putrefactive  organisms 
begin  to  flourish  and  to  attack  any  protein  which  is  not 
hydrolysed  by  the  digestive  enzymes — splitting  it  up  and 
forming  among  other  substances  a  series  of  aromatic  bodies, 
of  which  the  chief  are  indol,  skatol,  and  phenol. 

Aromatic  Bodies. — This  splitting  probably  occurs  through 


330 


VETERINARY  PHYSIOLOGY 


the  liberation  of  tryptophan — in  which  amino-propionic  acid 
is  linked  to  a  pyrrol-benzene. 


CH0.CH.NH.-..CO.OH 


By  the  breaking  down  of  the  amino-propionic  acid,  skatol- 
— CH, 


!         I 


NH 


is    formed,   and,   by   the   removal   of    the   methyl,    indol    is 
produced — 


-H 


NH 


Phenol — 


-0— H 


C«H.. 


is  a  further  stage  of  disintegration. 

Aonines  (Appendix)  may  also  be  formed,  and  some 
of  them  have  a  toxic  action.  Their  absorption  from 
the  gut  may  lead  to  marked  symptoms.  Dale  has 
shown  that  some  amines  are  vaso-constrictors  of  great 
strength. 

Folin  has  suggested  tb.at  ammonia  is  also  formed  in 
the  bacterial  changes,  and  that  this  may  account  for  the 
traces  of  ammonia  which  have  been  found  in  the  portal 
blood. 


DIGESTION 


331 


Bacterial  action  is  not  essential  to  digestion.  By  taking 
embryo  guinea-pigs  at  full  time  from  the  uterus  and  keeping 
them  with  aseptic  precautions,  it  has  been  shown  that  the 
absence  of  micro-organisms  from  the  intestine  does  not  inter- 
fere with  their  nutrition. 


E.  Fate  of  the  Digestive  Secretions. 

1.  Water. — Although  it  is  impossible    to  state  accurately 
the    average    amount    of    the    various    digestive    secretions 


MOCTH. 


>MALL 

Intestine. 


Large 
Intestine. 


Am  LO  LYTIC 

I  1 1 1  Alkaline 


-^OTEOLYTIC 


--Lipolytic 
-Putrefactive 


Fig.  155. — A  Synopsis  of  the  Conditions  and  Processes  in  the  Different 
Divisions  of  the  Alimentary  Canal  in  the  pig  and  in  man.  The  nature 
of  the  control — nervous  or  chemical — is  indicated  in  the  top  line. 


poured  into  the  alimentary  canal  each  day,  it  must  be  very 
considerable,  probably  more  than  one-half  of  the  whole 
volume  of  the  blood.  Only  a  small  amount  of  this  is 
given  off  in  the  faeces,  and  hence  the  greater  part  must  be 
re-absorbed.  There  is  thus  a  constant  circulation  between 
the  blood  and  the  alimentary  canal,  or  what  may  be  called 
an  entero-haemal  circulation.  One  portion  of  this,  the  entero- 
hepatic,  is  particular!}-  important.  The  blood-vessels  of  the 
intestine  pass  to  the  liver,  and  many  substances,  wlien 
absorbed  into  the  blood-stream,  are  again  excreted  in  the 
bile    and    are    thus    prevented    from    reaching    the    general 


332  VETERINARY   PHYSIOLOGY 

circulation.  Among  these  substances  are  the  salts  of  the 
bile  acids  and  their  derivatives,  many  alkaloids  such  as 
curarine,  and  in  all  probability  the  amines  formed  by 
putrefactive  decomposition  of  proteins  in  the  gut.  The 
liver  thus  forms  a  protective  barrier  to  the  ingress  of  certain 
poisons. 

2.  Enzymes — Ptyalin  appears  to  be  destroyed  in  the 
stomach  by  the  hydrochloric  acid.  Pepsin  is  probably 
partly  destroyed  in  the  intestine,  but  a  proteolytic  enzyme 
acting  in  an  acid  medium  is  present  in  the  urine,  and  this 
may  be  absorbed  pepsin.  Trypsin  appears  to  be  destroyed 
in  the  alimentary  canal ;  but  the  fate  of  the  other  pancreatic 
enzymes  and  of  the  enzymes  of  the  succus  entericus  is 
unknown. 

3.  Bile  Constituents. —  1.  The  bile  salts  are  partly  re- 
absorbed from  special  parts  of  the  small  intestine — sodium 
glycocholate  being  taken  up  in  the  jejunum  and  taurocholate 
in  the  ileum.  The  acids  of  these  salts  are  also  partly  broken 
up.  The  glycocholic  acid  yields  amino-acetic  acid,  which 
is  absorbed  and  passes  to  the  liver  to  be  excreted  as  urea  ; 
while  the  taurocholic  acid  yields  amino-isethionic  acid,  which 
goes  to  the  liver,  and  yields  urea  and  probably  sulphuric 
acid.  The  fate  of  the  cholalic  acid  is  not  known,  but  it  is 
supposed  to  be  excreted  in  the  faeces.  2.  The  pigments 
undergo  a  change  and  lose  their  power  of  giving  Gmelin's 
reaction.  They  appear  in  the  fseces  as  stercobilin.  It  is 
probably  formed  by  reduction  of  bilirubin  in  the  intestines 
as  the  result  of  the  action  of  micro-organisms.  3.  The 
cholesterol  is  passed  out  in  the  fa3ces  in  a  modified  form  as 
coprosterol. 

F.  Movements  of  the  Intestine. 
1.   The  Small  Intestine. 

These  are  of  two  kinds — segmental  and  peristaltic. 

1.  The  segmental  movements  consist  in  the  formation  of 
local  constrictions,  which  divide  the  gut  up  into  little 
segments  or  compartments.  A  constriction  next  forms  in 
the  middle  of  each  of  these,  and  the  former  constriction  is 


DIGESTION  333 

relaxed,  and  its  site  becomes  the  centre  of  another  com- 
partment. This  process  goes  on  repeating  itself,  and  thus 
the  contents  of  the  gut  are  thoroughly  mixed  and  churned. 
This  may  be  seen  by  feeding  with  food  mixed  with  bismuth 
and  employing  X-rays.  These  movements  occur  when  all 
the  nerves  have  been  divided.  How  far  they  are  dependent 
upon  the  action  of  the  myenteric  plexus  is  not  definitely 
established. 

2.  The  peristaltic  movements  are  much  more  complex 
and  powerful.  They  consist  of  a  constriction  of  the  muscles, 
which  seems  to  be  excited  by  the  distension  caused  by  the 
contents,   and  they  may  be  caused  by  inserting  a  bolus  of 


^^Qyy^l^lu^  ^'^ 


A 

i 


Fig.  156. — Skiagrams  to  show  Segmentation  of  tlie  Small  Intestine.     (Hertz.) 

cotton-wool  covered  with  vaseline.  Starting  at  some  point 
of  the  intestine,  the  wave  passes  slowly  downwards  and 
gradually  dies  away.  In  front  of  the  contraction,  the 
muscular  fibres  are  relaxed,  and  thus  the  contracting  part 
drives  its  contents  into  the  relaxed  part  below.  This  move- 
ment of  the  contents  of  the  intestine,  when  they  are 
mixed  with  gases  produced  by  fermentation,  frequently 
produces  gurgling  noises. 

The  peristaltic  movements  go  on  after  the  nerves  to  the 
gut  are  cut,  but  they  are  stopped  when  the  ganglia  in  the 
wall  of  the  intestine  are  poisoned  by  nicotine.  The 
myenteric  plexus  undoubtedly  forms  a  local  reflex  mechanism 
which  is  stimulated  by  the  presence  of  the  residue  of  the 
food    in    the    intestine     and    which    brings    about    the    co- 


334  VETERINARY   PHYSIOLOGY 

ordinated  contraction    and    relaxation,    which  together    con- 
stitute a  true  j^eristalsis. 

But,  while  peristalsis  is  thus  independent  of  the  central 
nervous  system,  it  is  nevertheless  controlled  by  it.  The 
splanchnic  nerves  inhibit,  while  the  vagus,  to  the  small 
intestine  and  possibly  the  first  part  of  the  large  gut,  and 
the  nervi  erigenfes  or  pelvic  nerves  to  the  greater  part 
of  the  large  gut  are  augmentor  nerves,  increasing  the 
peristalsis.  Stimulation  of  the  splanchnic  fibres,  which 
inhibit  peristalsis,  causes  contraction  of  the  ileo-Ciecal 
sphincter. 

The  movements  of  the  intestine  may  be  inhibited  either 
by  interference  with  the  local  nervous  mechanism  or  by 
reflex  action  through  the  central  nervous  system.  Thus,  it 
has  been  found  that  roughly  handling  the  gut  will  lead  to  a 
prolonged  inhibition,  even  after  the  extrinsic  nerves  have 
been  cut.  Other  stimuli — such  as  crushing  the  testis  under 
an  anaesthetic — lead  to  a  reflex  inhibition,  which  is  manifest 
only  if  the  splanchnic  nerves  are  intact.  When  a  cat, 
under  observation  with  X-rays,  manifests  signs  of  anger,  the 
gastric  movements  and  movements  of  the  small  intestine 
are  checked.  Under  these  conditions  the  movements 
of  the  large  intestine  may  be  increased  and  the  foeces  voided. 

The  Ileo-Caecal  Sphincter  and  Valve.— This  sphincter  pre- 
vents the  free  passage  of  the  contents  of  the  small  into  the 
large  intestine,  and  causes  a  stasis  in  the  ileum — possibly  to 
allow  of  complete  absorption  of  all  the  nutrient  constituents. 
It  relaxes  from  time  to  time,  and  allows  the  contents  to  be 
forced  into  the  large  gut.  The  valve  is  formed  by  a  sleeve- 
like projection  of  the  end  of  the  ileum  into  the  cascum,  but 
it  does  not  completely  prevent  the  backward  passage  of  the 
contents  of  the  large  into  the  small  intestine. 

Usually  the  contents  of  the  small  intestine  travel  down 
at  about  1-5  metres  per  hour.  In  abnormal  conditions  the 
rate  of  passage  may  be  greatly  retarded  or  accelerated, 

2.  The  Large  Intestine. 
In  the  large  intestine  peristaltic  waves  like   those  of  the 
small  intestine  are  not  prominent,  and  the  segmental  move- 


DIGESTION  335 

ments  seem  to  be  replaced  by  rhythmic  contraction  of  the 
saccules.  In  the  cat  peristalsis  in  a  backward  direction— 
an  anti-peristalsis — starts  in  the  middle  of  the  colon  and 
passes  to  the  caecum. 

On  several  occasions,  usually  after  taking  food,  a  rapid 
movement  of  the  contents  downwards  in  mass  has  been 
observed.  This  must  be  due  to  the  rapid  passage  of  a  very 
powerful  contraction,  and  it  appears  to  be  originated  as  a 
reflex  from  the  stomach — a  gastro-colic  reflex. 

The  result  of  the  movement  of  the  large  intestine  is  to 
pass  the  contents  onwards  towards  the  rectum.  In  this 
passage  the  contents  become  less  fluid  from  the  absorption 
of  water. 

3.  Defaecation. 

By  the  peristalsis  of  the  intestine  the  matter  not  absorbed 
from  the  wall  of  the  gut  is  forced  down  and  accumulated  in 
the  rectum.  It  is  prevented  from  escaping  by  two  sphincter 
muscles,  viz.  the  internal  sphincter,  which  is  merely  a 
thickening  of  the  circular  muscular  coat  of  the  colon,  and 
the  external  sphincter,  which  is  of  skeletal  muscle. 

(1)  The  mechanism  of  the  act  of  defascation  is  a 
local  reflex,  similar  to  that  controlling  the  rest  of  the  in- 
testine. In  a  dog,  with  all  the  lower  part  of  the  spinal  cord 
removed,  defsecation  can,  after  some  time,  take  place 
normally. 

(2)  The  peripheral  mechanism  is  controlled  by  a  centre 
in  the  lumbar  enlargement  of  the  spinal  cord.  When  this  is 
destroyed,  the  sphincters  are  for  a  time  relaxed,  and  faeces 
are  passed  whenever  they  are  driven  down  by  intestinal 
contractions.  The  centre  thus  seems  to  exercise  a  tonic 
influence  on  the  sphincters.  Its  action  is  inhibited  by 
impulses  sent  up  the  sacral  nerves  from  the  distended 
rectum. 

(3)  The  lumbo-spinal  centre  is  dominated  by  higher  centres 
in  the  cerebrum,  by  the  action  of  which  defaecation  may  be 
inhibited  for  a  time  or  the  reflex  may  be  liberated. 

The  local  centre  is  stimulated  when  distension  of  the 
rectum    is    produced    by    the    accumulation    of   material — 


336  VETERINARY   PHYSIOLOGY 

undigested  constituents  of  the  food  and  excretory  products. 
The  undigested  material  usually  forms  the  greater  part  of 
the  accumulation,  especially  in  herbivora.  It  has  been 
shown  that  rabbits  on  a  diet  freed  from  cellulose  cease  to 
defecate  and  die  of  intestinal  obstruction. 

The  higher  centres  are  stimulated  by  excitement,  which, 
especially  in  timid  animals,  may  produce  diarrhosa.  Muscular 
exercise  helps  to  induce  the  reflex,  probably  partly  by  a 
sympathetic  increased  muscular  action  of  the  colon,  and 
partly  by  the  contraction  of  the  abdominal  muscles  forcing 
material  into  the  lower  bowel.  The  influence  of  exercise  is 
well  seen  in  dogs,  which  often  defsecate  after  being  released 
and  allowed  a  run. 

When  defecation  takes  place,  all  the  various  muscles 
which  can  increase  the  pressure  in  the  abdomen  are  called 
into  play.  A  deep  inspiration  is  taken,  and  then,  with  the 
glottis  closed,  expiratory  efforts  are  made.  The  levator  ani 
which  supports  the  anus  is  relaxed,  and  the  upper  part  of 
the  rectum  is  brought  more  into  line  with  the  lower  part  of 
the  pelvic  colon.  Faeces  may  thus  be  forced  into  the 
rectum,  and  the  reflex  act  of  defa3cation  with  contraction  of 
the  colon  and  relaxation  of  the  sphincters  is  eflected.  The 
act  is  completed  by  the  contraction  of  the  internal  sphincter 
from  above  downwards  and  by  the  contraction  of  the  levator 
ani  and  external  sphincter. 


B.  DIGESTION  IN  HERBIVORA. 

The  food  of  herbivora  is  characterised  by  its  bulk,  and 
by  the  fact  that  the  digestive  material  is  for  the  most  part 
enclosed  in  cells  whose  cellulose  walls  are  not  dissolved  by 
the  enzymes  of  the  digestive  tract.  The  distinctive  feature 
about  digestion  in  herbivora  is  therefore  the  provision  for 
detaining  a  large  quantity  of  food  in  a  specially  developed 
part  of  the  alimentary  canal,  where,  under  the  influence  of 
bacteria,  the  cellulose  is  dissolved  and  the  enclosed  digestible 
material  liberated. 


DIGESTION  337 


1.  Digestion  in  Ruminants. 

Prehension. — In  the  ox,  in  grazing,  the  mobile  tongue 
curls  round  the  grass  and  pulls  it  into  the  mouth,  when 
it  is  cut  off  by  the  incisor  teeth  against  the  dental  pad. 
The  papillae  in  the  inside  of  the  mouth  (p.  292)  assist  in 
preventing  the  food  from  dropping  out.  The  divided  upper 
lip  of  the  sheep  allow  the  teeth  and  dental  pad  to  bite 
closer  to  the  ground  than  in  the  case  of  cattle. 

In  drinking  the  lips  are  closed  except  for  a  small  orifice 
which  is  put  below  the  surface  of  the  water.  The  tongue 
acts  like  the  piston  of  a  pump  and  the  water  is  sucked  in. 

Mastication  and  Insalivation. — Mastication  in  ruminants  is 
chiefly  a  side-to-side  movement  by  which  the  food  is  ground 
between  the  molar  teeth.  It  goes  on  usually  for  several 
minutes  in  one  direction  and  then  changes  to  the  opposite 
direction.  The  parotid  gland  on  the  side  on  which  the 
animal  is  chewing  secretes  much  more  actively  than  that  of 
the  opposite  side.  The  sublingual  and  submaxillary  glands 
secrete  equally  on  both  sides.  The  quantity  of  saliva  poured 
into  the  mouth  is  very  large.  When  dry  food  is  eaten 
between  50  and  60  litres  may  be  secreted  in  twenty-four 
hours.  The  specific  gravity  of  the  saliva  is  high,  nearly 
1010. 

It  is  doubtful  whether  the  saliva  of  ruminants  contains 
any  ptyalin.      If  it  is  present  it  is  in  very  small  amounts. 

The  food  is  swallowed  after  a  preliminary  incomplete 
chewing.  It  is  returned  to  the  mouth  for  more  complete 
mastication  during  rumination  which  takes  place  after  feed- 
ing has  ceased.  Ruminants  can  therefore  eat  food  about 
three  times  as  fast  as  the  horse,  which  completes  mastication 
before  swallowing. 

Rumination. — This  complication  of  the  digestive  process 
is  peculiar  to  ruminants.  It  consists  essentially  of  a  re- 
mastication  of  the  food  after  a  preliminary  storage. 

(1)  Mechanism. — The  food,  when  first  swallowed,  may  enter 
any  of  the  compartments  with  which  the  oesophageal  groove 
is  connected  (p.  294).     Liquids  for  the  most  part  pass  on 


338  VETERINARY  PHYSIOLOGY 

direct  to  the  abomasum  or  true  stomach.  But  when  solid 
food  is  taken  the  pillars  of  the  oesophageal  groove  relax,  and 
the  oesophagus  then  communicates  with  the  rumen  and 
reticulum  to  which  the  food  passes.  The  fluid  part  tends  to 
accumulate  in  the  reticulum.  After  feeding,  if  the  animal 
be  comfortable  and  undisturbed,  rumination  or  "  chewing  the 
cud  "  begins.  By  fixation  of  the  diaphragm  in  the  position 
of  inspiration,  and  the  contraction  of  the  muscular  walls  of 
the  rumen  and  reticulum  and  of  the  abdomen,  some  of  the 
contents  of  the  rumen  accompanied  by  fluid  from  the 
reticulum  is  passed  into  the  oesophagus.  A  bolus  is  cut  off 
by  contraction  of  the  cardiac  end  of  the  oesophagus,  and  by 
a  reversed  peristalsis,  is  carried  to  the  mouth.  The  fluid  is 
immediately  squeezed  out  and  reswallowed,  passing  along  the 
oesophageal  groove  to  the  omasum  and  thence  to  the  true 
stomach.  The  mass  left  in  the  mouth  undergoes  a  second 
process  of  mastication  and  insalivation.  The  finely  com- 
minuted pasty  material  is  then  reswallowed..  and  by  a  con- 
traction of  the  pillars  of  the  oesophageal  groove  the  omasum 
is  drawn  towards  the  oesophagus,  and  receives  the  material. 

In  the  omasum  it  is  reduced  to  a  still  finer  state  of  division 
by  the  grinding  action  of  the  hard  leaves,  between  which  it 
filters  through  to  the  true  stomach.  Even  after  remastica- 
tion  the  food,  if  not  in  a  fine  enough  state  of  division,  may 
pass  again  to  the  rumen  instead  of  to  the  omasum. 

A  certain  degree  of  distension  of  the  rumen  is  necessary 
to  make  regurgitation  possible.  This  is  maintained  by  the 
constant  activity  of  the  parotid  glands,  whose  secretion 
constitutes  a  considerable  proportion  of  the  contents  of  the 
rumen. 

The  flow  of  saliva  and  the  process  of  rumination 
cease  in  disease.  Under  these  conditions  the  food  may 
become  dry  and  caked  and  set  up  inflammatory  changes. 
Impaction  may  occur,  especially  in  the  omasum.  The  moist 
mouth  indicating  a  flow  of  saliva,  and  the  commencement  of 
rumination  are  of  great  value  in  prognosis. 

The  ox  spends  about  seven  hours  out  of  the  twenty-four 
ruminating.  A  bolus  of  about  100  grams  is  regurgitated, 
remasticated,  and  reswallowed  in  rather  less  than  one  minute. 


DIGESTION  339 

Rumination  is  a  reflex  act.  The  centre  has  not  been 
located  with  certainty.  It  is  probably  situated  in  the 
medulla.  The  two  chief  nerves  involved  are  the  phrenic 
and  the  vagus.  If  the  former  be  cut  the  diaphragm  cannot 
be  contracted,  but  the  food  can  still  be  regurgitated  by  a 
more  powerful  contraction  of  the  walls  of  the  cavity  and  of  the 
abdomen.  If  the  vagus  be  cut,  the  walls  of  the  cavities  are 
paralysed  and  the  process  ceases. 

It  is  a  remarkable  fact  that,  though  boluses  can  be 
returned  from  the  rumen  to  the  mouth,  the  ox  does  not 
vomit,  even  when  distension  of  the  rumen  causes  distress. 
Why  vomiting  does  not  occur  is  unknown.  It  has  been 
suggested  that  the  vomiting  centre  in  the  medulla  is  un- 
developed. 

(2)  Digestive  Changes  in  the  Rumen. — The  contents  of  the 
rumen  are  subjected  to  a  churning  by  the  contraction  of  the 
powerful  muscular  bands  in  the  wall  of  the  cavity.  Newly 
added  food  is  therefore  mixed  with  the  previous  contents. 
No  digestive  juice  is  secreted,  the  only  fluid  added  to  the 
food  being  the  alkaline  saliva  from  the  mouth. 

In  this  warm  alkaline  mass  the  fibrous  substances  become 
softened  and  prepared  for  the  further  digestive  processes.  It 
is  probable  that  finely- divided  material  may  pass  direct  from 
the  rumen  to  the  omasum,  though  doubtless  the  greater  bulk 
is  remasticated. 

The  conditions  in  the  rumen  where  the  contents  are 
warm  and  alkaline  favour  the  conversion  of  starch  to  maltose 
by  the  enzyme  ptyalin.  Whether  ptyalin  is  present  in 
ruminants,  however,  is  doubtful,  and  the  extent  to  which 
the  conversion  takes  place  is  unknown. 

Certain  enzymes  contained  in  the  food  may  act.  Cytase 
has  a  feeble  action  on  cellulose.  Proteolytic  enzymes  may 
act  on  protein,  and  amolytic  enzymes  on  starches.  These 
changes  due  to  enzymes  contained  in  the  food  are,  however, 
of  minor  importance. 

Fats  are  freed  by  the  disintegration  of  enclosing 
substances,  but  in  this  compartment  they  undergo  no 
chemical  changes. 

The    rumen    swarms    with    bacteria    which     attack     the 


340  VETERINARY  PHYSIOLOGY 

cellulose  and  probably  also  the  pentosans,  breaking  them 
down  to  various  organic  acids,  chiefly  acetic  and  butyric. 
These  combine  with  the  bases  of  the  alkaline  saliva.  The 
resulting  salts  are  absorbed  from  the  intestine  and  are  sources 
of  energy.  In  the  upper  part  of  the  rumen  the  contents 
may  be  acid  from  the  accumulation  of  these  organic  acids. 
The  gases,  methane,  carbon  dioxide,  and  in  small  quantities 
hydrogen,  are  produced  and  excreted  in  the  breath.  The 
process  is  therefore  largely  a  destructive  fermentation.  It  is 
estimated  that  about  60  per  cent,  of  the  cellulose  of  the  food 
is  disintregated  in  the  rumen.  As  the  cellulose  is  broken 
down  the  cell  contents  are  liberated  and  rendered  accessible  to 
the  digestive  juices  of  the  following  parts  of  the  digestive  tract. 

In  addition  to  cellulose,  starch  and  sugars  undergo 
destructive  fermentation.  It  has  been  shown  that  the 
addition  of  starch  to  a  fixed  diet  leads  to  a  corresponding 
increase  in  the  excretion  of  methane. 

Nitrogenous  material  is  also  broken  down  by  bacteria, 
and  used  to  build  up  the  proteins  of  their  own  protoplasm. 
It  seems  that  the  soluble  non-protein  compounds  are  more 
readily  utilised  than  the  proteins.  When  a  plentiful  supply 
of  soluble  nitrogen  is  available,  the  multiplication  and  activity 
of  bacteria  is  increased,  and  conseqtiently  there  occurs  a  more 
extensive  disintegration  of  cellulose. 

It  has  been  suggested  that  the  protoplasm  of  bacteria, 
which  are  carried  on  into  the  stomach,  is  digested,  and  the 
resulting  products  absorbed,  and  that  it  is  by  the  bacteria 
consuming  the  non-protein  nitrogenous  material  and  then 
being  themselves  digested  that  non-protein  nitrogen  is  made 
available.  Whether  bacterial  protein  can  be  hydrolysed  by 
the  digestive  enzymes  is  disputed.  As  the  non- protein 
nitrogenous  material  consists  chiefly  of  amino-acids  and 
amides — the  normal  cleavage  products  of  digestion  (p.  320) 
— it  seems  unnecessary  to  involve  the  aid  of  bacteria  for  their 
utilisation. 

Stomach. — After  being  triturated  between  the  leaves  of 
the  omasum,  the  food  enters  the  abomasum  or  true  stomach. 
The  course  of  events  in  the  stomach  has  been  studied  by 
making  a  Pavlov's  pouch  in  the  goat. 


DIGESTION  341 

The  stomach  contents  for  some  time  after  being  received 
from  the  omasum  are  alkaUne.  Micro-organisms  flourish  and 
brealc  down  sugars  to  form  hictic  acid.  An  amoh-tic  enzyme 
converting  starch  to  sugar  seems  to  be  produced. 

Before  this  amolytic  period  is  completed  pepsin  and 
hydrochloric  acid  are  secreted  in  sufficient  amounts  to  make 
the  contents  acid  and  enable  peptic  digestion  to  begin.  The 
concentration  of  hydrochloric  acid,  however,  is  never  so  great 
as  in  carnivora. 

Intestines. — In  the  small  intestine,  so  far  as  is  known,  the 
secretions  and  digestive  processes  are  the  same  as  have  been 
described  for  carnivora.  But  a  smaller  proportion  of  the 
food  is  digested  and  absorbed  so  that  a  bulky  residue  reaches 
the  ccecum  and  colon.  Here  the  destructive  fermentation  of 
cellulose  by  bacteria  is  resumed,  and  digestive  processes  are 
continued  by  enzymes  that  have  been  carried  on  from  the 
small  intestine.  The  large  intestine  is  therefore  a  more 
important  structure  in  herbivora  than  in  carnivora,  where  its 
main  function  is  the  absorption  of  water  and  the  storage  of 
food  residues  and  excretory  products  prior  to  expulsion  in 
the  faeces. 

In  ruminants  where  important  changes  go  on  in  the 
rumen  before  the  food  passes  through  the  stomach  and  small 
intestine  digestion  in  the  caecum  and  colon  are  much  less 
important  than  in  the  horse  (p.  344). 

Faeces. — The  faeces  which  consist  chiefly  of  undigested 
residues  of  the  food  are  more  fluid  in  the  ox  than  in  the 
sheep.  The  amount  varies  with  the  food.  In  the  ox  the 
average  weight  per  diem  is  about  30  kilos.  The  composition 
of  the  faeces  is  dealt  with  later  (p.  861). 

2.  Digestion  in  the  Horse. 

Prehension. — In  grazing,  the  Hps  of  the  horse  are  drawn 
back  to  allow  the  teeth  free  access  to  the  grass.  If  the 
nerves  supplying  the  lips  cut,  it  becomes  impossible  for  the 
horse  to  graze.  In  manger-feeding  the  lips  are  used  to 
gather  the  food. 

In  drinking,  as  in  the  ruminant  (p. 3  3 7;,  the  tongue  acts  like 


342  VETERINARY  PHYSIOLOGY 

the  piston  of  a  pump  sucking  in  the  water.  If  an  opening  be 
made  in  the  cheek  above  the  level  of  the  water  so  that  air 
gets  in  water  cannot  be  sucked  up.  When  drinking,  the 
head  is  extended,  and  there  is  a  forward  movement  of  the 
ears  as  each  gulp  is  swallowed.  The  reason  for  this  peculiar 
backward  and  forward  movement  of  the  ears  in  the  horse 
when  drinking  is  unknown. 

Mastication  and  Insalivation. — The  process  of  mastication 
is  very  completely  performed,  the  animal  taking  about  five 
to  ten  minutes  to  eat  a  pound  of  corn  and  about  fifteen  to 
twenty  minutes  to  eat  the  same  amount  of  hay.  As  in  the 
ruminant,  mastication  is  chiefly  a  side-to-side  movement  and 
is  unilateral,  the  parotid  gland  on  the  chewing  side  being 
the  more  active. 

The  parotid  gland  is  relatively  large  in  the  horse,  being 
about  twice  as  large  as  that  of  the  ox.  Unlike  the  ruminant 
where  the  parotid  secretion  never  ceases  in  health  the  gland 
is  only  active  during  mastication.  The  saliva  probably  con- 
tains ptyalin. 

The  quantity  of  saliva  secreted  has  been  measured  by 
making  an  oesophageal  fistula  and  collecting  the  boluses  of 
food  which  are  swallowed,  and  so  flnding  the  amount  of  fluid 
which  has  been  secreted  in  the  mouth.  About  40  to  50 
litres  may  be  produced  in  a  day.  The  amount  is  determined 
by  the  dryness  of  the  food.  Dry  fodder  absorbs  about  four 
times  its  weight  of  saliva  ;  green  fodder  about  half  its  weight. 

In  abdominal  pain  there  is  complete  cessation  of  all  the 
salivary  glands,  and  the  mouth  and  tongue  become  dry. 

Stomach. — In  the  horse  the  process  of  gastric  digestion 
differs  from  that  of  carnivora  in  the  following  particulars. 

In  the  first  place,  the  horse  has  to  eat  a  very  large  quantity 
of  food  in  proportion  to  the  size  of  its  stomach,  and  it  is  found 
that  part  of  the  food  begins  to  pass  very  rapidly  through  the 
stomach  into  the  intestine.  Colin  found,  when  he  killed  a 
horse  which  in  two  hours  had  eaten  2500  grms.  of  hay,  that 
the  stomach  contained  only  1000  grms.  But  while  this  is 
the  case,  a  small  residue  of  the  meal  remains  for  a  very  long 
time  in  the  stomach,  and  passes  out  only  when  the  next  meal 
is  taken. 


DIGESTION 


343 


The  churning  action  of  the  stomach  is  less  complete  in 
the  horse  than  in  the  dog,  and  hence  when  the  animal  has 
received  hay,  followed  by  oats,  these  are  found  lying  more  or 
less  separate.  Even  when  the  animal  has  taken  water  the 
contents  are  not  much  disturbed  (p.  367). 

In  the  horse,  the  amyolytic  period  is  well  marked,  and 
the  percentage  of  hydrochloric  acid  is  never  so  high  as  in 
the  dog.     Lactic  acid  is  always  formed  from  the  carbohydrate 


Fig.  157.— Stomach  of  Horse  fed  successively  on  four  dififerently  coloured 
foods  to  show  the  distribution  of  the  various  foods  in  the  viscus. 

of  the  food,  and  on  a  diet  of  hay  it  may  exceed  the  hydro- 
chloric acid. 

The  proteolytic  action  of  the  gastric  juice  of  the  horse  is 
slower  than  that  of  carnivora,  but  it  is  very  marked,  and 
peptones  are  found  abundantly  in  the  stomach  at  the  end  of 
digestion.  In  the  stomach  of  the  horse  the  cellulose  of  the 
food  is  partly  decomposed,  probably  by  the  action  of  an 
enzyme  in  the  grain. 

Intestines. — In  the  small  intestine  the  processes  that  go 
on  are  much  the  same  as  in  carnivora. 


344  VETERINARY   PHYSIOLOGY 

In  the  large  intestine  the  caecum  and  double  colon 
perform  much  the  same  function  as  the  oesophageal  diverticula 
of  the  ruminant  (p.  389). 

The  Caecum  acts  as  a  reservoir.  The  contents  of  the  small 
intestine  pass  through  it  to  reach  the  colon.  Water  drunk 
passes  very  rapidly  to  the  caecum.  The  contents  are  always 
fluid,  varying  from  a  pea-soup-iike  consistency  to  a  quite 
watery  liquid,  with  particles  of  undissolved  food  floating 
throughout  it.  Some  of  the  food  may  remain  for  as  long  as 
twenty-four  hours  in  the  caecum.  On  the  other  hand,  some 
may  pass  rapidly  through  to  the  colon.  Food  has  been 
found  in  the  colon  four  hours  after  being  eaten. 

The  outlet  to  the  colon  is  above  the  level  of  the  inlet — 
the  ileo-caecal  valve.  The  contents  therefore  are  emptied 
against  gravity  by  the  contraction  of  the  four  longitudinal 
muscular  bands  in  the  walls  of  the  organ. 

The  contents  of  the  large  colon  and  of  the  first  foot  or 
so  of  the  small  colon  resemble  those  of  the  caecum.  There- 
after by  the  absorption  of  water  the  contents  rapidly  become 
inspissated,  and  by  the  sacculation  of  the  colon,  formed  into 
balls  of  faeces,  ready  for  expulsion. 

The  digestive  change  that  takes  place  in  the  ca3cum  and 
large  colon  are  much  the  same  as  those  that  occur  in  the 
rumen  of  the  ox.  The  contents  are  alkaline  in  reaction  and 
swarm  with  bacteria.  Fibrous  material  that  has  resisted  the 
action  of  the  stomach  and  small  intestine  becomes  macerated. 
Cellulose  is  attacked  by  bacteria  and  broken  down,  yielding 
the  same  products  as  the  disintegration  of  cellulose  in  the 
rumen  (p.  340).  The  gases  appear  as  little  bubbles  scattered 
throughout  the  fermenting  mass. 

As  the  cellulose  envelope  is  broken  down,  the  contents 
that  were  protected  from  the  action  of  the  digestive  secretion 
of  the  stomach  and  small  intestine  are  hydrolysed  by  the 
enzymes  that  have  been  carried  into  the  large  intestine. 

Proteins  that  have  not  been  hydrolysed  by  the  digestive 
enzymes  are  disintegrated  by  putrefactive  organisms  giving 
rise  to  a  series  of  aromatic  bodies  (p.  329),  which  are  absorbed. 
Some  of  these  are  toxic  and  affect  the  health  of  the  animal. 

Absorption  of  the  products  of  digestion  takes  place  in  the 


DIGESTION 


345 


cfficum  and  colon.  So  rapid  is  absorption  in  the  lower 
bowel  that  the  animal  may  be  easily  anaesthetised  by  giving 
ether  per  rectum.  The  rapid  absorption  allows  of  life  being 
maintained  by  nutrient  enemata. 

Some  of  the  movements  of  the  alimentary  tract  are  of 
special  interest  in  the  horse. 

In  neither  the  small  nor  the  large  intestine  have  segmental 
movements  been  observed,  but  an  anti-peristaltic  movement 
has  been  described.     As  the  contents  of  the  intestine  are  Kquid 


Fig.  158. — Mesial  Section  through  the  Head  of  a  Horse,  to  show  the  long 
soft  palate,/,  lying  against  the  front  of  the  epiglottis,  i  ;  c,  the  tongue ; 
I,  the  arytenoids.     (Ellenberger.) 

seo-mentation  is  unnecessary.  Anti-peristaltic  movements 
serve  by  mixing  the  contents  to  bring  the  food  into  more 
intimate  contact  with  the  intestinal  secretions,  and  also 
prevent  the  too  rapid  emptying  of  the  small  intestine  into 
the  csecum. 

In  defsecation  the  contraction  of  the  rectum  is  so  powerful 
that  the  act  can  be  performed  without  fixation  of  the  dia- 
phragm or  closing  of  the  glottis,  though  these  usually  occur 
when  the  animal  is  at  rest.  The  crouching  attitude  common 
to  nearly  all  animals  is  not  adapted.  The  horse  can  there- 
fore defsecate  when  trotting. 


346  VETERINARY   PHYSIOLOGY 

Vomiting  in  the  horse  seldom  occurs.  The  oesophagus 
joins  the  stomach  very  obliquely  and  folds  of  mucous  mem- 
brane of  the  stomach  tend  to  prevent  the  passage  of  stomach 
contents  back  into  the  oesophagus.  Further,  the  lumen  of 
the  oesophagus  is  diminished  at  the  cardiac  end,  and  there 
the  circular  coat  is  thicker  and  firmer.  On  the  few  occasions 
when  vomiting  does  occur  the  vomited  material,  which  is 
prevented  from  entering  the  mouth  by  the  long  soft  palate, 
escapes  from  the  nostrils  (fig.  158). 


SECTION    III. 
ABSORPTION    OF   FOOD 

1.   State  in  which  Food  leaves  the  Alimentary  Canal- 

(1)  The  carbohydrates  generally  leave  the  alimentary  canal 
as  inonosaccharids  ;  but  some  resist  the  action  of  digestion 
more  than  others.  Lactose  seems  to  be  broken  down  in  the 
intestine  only  when  the  special  enzyme,  lactase,  is  present  in 
the  succus  entericus,  but  in  all  cases  it  is  broken  down  before 
it  reaches  the  liver.  Cane  sugar,  when  taken  in  large  excess, 
may  also  be  absorbed  unchanged,  and  it  is  then  excreted  by 
the  kidneys, 

(2)  The  proteins  are  absorbed  as  amino-acids,  formed  by  the 
action  of  trypsin  and  erepsin  (pp.  320  and  328).  Native  pro- 
teins may  be  absorbed  to  a  small  extent  unchanged,  as  is  shown 
by  the  fact  that  the  administration  of  very  large  amounts  of 
egg  albumin  may  cause  its  appearance  in  the  urine.  Egg- 
white,  when  injected  into  the  pelvic  colon  isolated  from  the 
rest  of  the  intestine,  and  hence  free  of  proteolytic  enzymes, 
may  disappear,  probably  as  the  result  of  bacterial  action  ; 
but  in  carnivora  the  amount  absorbed,  as  indicated  by  the 
increased  excretion  of  nitrogen,  is  trivial.  In  the  horse 
absorption  in  the  colon  is  more  complete  (p.  344). 

(3)  Non-protein  nitrogenous  material  may  be  absorbed 
as  amino-acids,  the  form  in  which  it  is  largely  present  in  the 
food,  or  may  be  acted  upon  by  bacteria  prior  to  absorption 
(p.  340). 

(4)  The  fats  are  chiefly  absorbed  as  soaps  and  as  fatty 
acids. 

(5)  The  results  of  the  digestion  of  cellulose  are  absorbed 
as  salts  of  organic  acids  (p.  340). 


348  VETERINARY   PHYSIOLOGY 

2.  Mode  of  Absorption  of  Food. — That  absorption  is  not 
due  merely  to  a  process  of  ordinary  diffusion  is  clearly 
indicated  by  many  facts. 

(1)  Heidenbain  has  shown  that  absorption  of  water  from 
the  intestine  takes  place  much  more  rapidly  than  diffusion 
through  a  dead  membrane. 

(2)  The  relative  rate  of  absorption  of  different  substances 
does  not  follow  the  laws  of  diffusion.  Griibler's  peptone 
passes  more  easily  through  the  intestine  than  the  more 
diffusible  glucose,  while  sodium  sulphate,  which  is  more 
diffusible  than  glucose,  is  absorbed  much  less  readily. 
Again,  an  animal  can  absorb  its  own  serum  under  conditions 
in  which  filtration  into  blood  capillaries  or  lacteals  is 
excluded. 

(8)  Absorption  is  stopped  or  diminished  when  the 
epithelium  is  removed  or  injured,  or  poisoned  with  fluoride 
of  sodium,  in  spite  of  the  fact  that  this  must  increase  the 
facilities  for  diffusion. 

(4)  During  absorption,  the  oxygen  consumption  by  the 
wall  of  the  gut  is  increased. 

8.  Channels  of  Absorption. — There  are  two  channels  of 
absorption  from  the  ahnientary  canal  (see  fig.  162,  p.  388) — 
(1)  the  veins,  which  run  together  to  form  the  portal  vein  of 
the  liver,  and  (2)  the  lymphatics,  which  run  in  the 
mesentery  and,  after  passing  through  some  lymph  glands, 
enter  the  receptaculum  chyli  in  front  of  the  vertebral  column. 
From  this,  the  great  lymph  vessel,  the  thoracic  duct,  leads 
up  to  the  junction  of  the  subclavian  and  innominate 
veins,  and  pours  its  contents  into  the  blood  stream.  The 
lymph  formed  in  the  liver  also  passes  into  the  thoracic 
duct. 

(1)  Proteins. — (1)  During  the  digestion  of  proteins  the 
number  of  leucocytes  in  the  blood  is  enormously  increased, 
sometimes  to  more  than  twice  their  previous  number.  This 
is  due  to  an  emigration  from  the  red  marrow  of  bone.  The 
digestion  leucocytosis  passes  off  in  a  few  hours,  but  what 
becomes  of  the  leucocytes  is  not  known.      Possibly  they  are- 


I 


ABSORPTION  349 

the  carriers  of  the  amino-acids  which  are  formed  in  digestion  ; 
but,  since  it  has  been  found  possible  to  dialyse  these  from 
the  blood,  the  leucocytes  must  either  break  down  in  the 
blood  stream  or  give  up  the  amino-acids  before  the  tissues 
are  reached. 

(2)  The  amount  of  amino-acids  is  increased  in  the 
blood.  That  they  are  absorbed  by  the  blood-vessels  and  not 
by  the  lymphatics  is  indicated  by  the  fact  that  ligature  of 
the  thoracic  duct  does  not  interfere  with  the  absorption  of 
the  nitrogen  of  the  proteins. 

(8)  The  amino-acids  are  rapidly  removed  from  the  blood 
by  the  tissues,  and  chiefly  by  the  liver.      Their  concentration 

'NON;.^TOGEN0US 
Liver 


Intestine 


Fig.  159. — To  show  the  Splitting  of  the  Amino-aeid  Part  of  Proteins  into  a 
nitrogenous  part,  which  is  changed  to  urea  in  the  liver  and  excreted 
by  the  kidneys,  and  into  a  non-nitrogenous  part  yielding  sugar,  which 
is  sent  into  the  muscles. 


in  this  organ  may  be  three  or  four  times  that  of  the  blood. 
Apparently,  any  surplus  over  that  immediately  required  by 
the  tissues,  accumulates  in  the  liver  and  is  so  prevented 
from  exercising  a  toxic  action  on  the  heart.  In  the  liver 
the  stored  amino-acids  are  rapidly  split  up  and  the  amidogen 
portion  converted  to  urea  and  excreted  by  the  kidneys, 
while  the  non-nitrogenous  part  is  converted  into  carbo- 
hydrates to  a  greater  or  less  extent  (fig.  159).  The  muscles 
and  other  tissues  accumulate  these  amino-acids  to  a  much 
smaller    extent    and    hold    them    longer,    probably    for    the 


350  VETERINARY  PHYSIOLOGY 

synthesis  of  their  proteins,  and  possibly  in  order  to  use  their 
non-nitrogenous  part  as  a  source  of  energy. 

(2)  Carbohydrates. — These  are  absorbed  as  monosaccharids 
in  solution,  and  are  carried  away  in  the  blood  of  the  portal 
vein.  Any  surplus,  over  that  required  by  the  body,  may  be 
stored  in  the  liver  and  subsequently  sent  to  the  tissues  (p.  3  5  4). 

(3)  Fats. — After  being  split  up  into  their  component 
acids  and  glycerol,  fats  pass,  as  soluble  soaps  or  as  fatty  acids 
soluble  in  the  bile,  through  the  borders  of  the  intestinal 
epithelium.  Here  they  ap])ear  to  be  again  converted  into 
fats  by  a  synthesis  of  the  acid  with  glycerol.  Fine  fatty 
particles  are  found  to  make  their  appearance  in  the  cells  at 
some  distance  from  the  free  margin  and  to  increase  in  size. 
A  similar  synthesis  occurs  even  when  free  fatty  acids  alone 
are  given,  so  the  cells  must  be  capable  of  producing  the 
necessary  glycerol  to  combine  with  the  acids.  The  fats  are 
sent  on  from  the  cells,  through  the  lymph  tissue  of  the  villi, 
into  the  central  lymph  vessels,  and  thus  on,  through  the 
thoracic  duct,  to  the  blood  stream.  Unlike  the  proteins  and 
carbohydrates,  they  are  not  carried  directly  to  the  liver.  In 
some  animals  they  are  stored  in  the  fatty  tissues,  in  others 
to  a  certain  extent  in  the  liver. 

Since  neither  the  character  nor  the  amount  of  food 
consumed  are  determined  by  the  actual  requirements  of  the 
muscular  and  other  tissues,  it  is  of  importance  that  there 
should  be  some  regulator  which  will  control  the  amount  and 
cliaracter  of  the  nourishment  sent  to  the  muscles. 

Such  a  regulator  is  found  in  the  liver. 

When  more  food  is  taken  than  is  at  once  required  by  the 
tissues,  one  of  three  things  may  happen — 

1.  It  may  be  oxidised  with  the  evolution  of  heat. 
This  is  specially  the  case  with  proteins,  the  high  specific 
dynamic  action  of  whicli  markedly  increase  heat  production 
(p.  272). 

2.  It  may  be  excreted,  unchanged  in  the  urine  as  in  the 
case  of  sugar. 

3.  It  may  be  stored  and  sent  to  the  muscles  as  it  is 
required,  and  thus  the  supply  of  energy-yielding  material 
may  be  regulated. 


ABSORPTION  351 


A.  Storage  of  Surplus  Food. 

1.  Fat. — Since,  bulk  for  bulk,  fat  has  more  than  twice  the 
energy  value  of  proteins  or  carbohydrates  (p.  257),  it  is  an 
advantage  to  store  surplus  food  as  Fat.  In  fat  oxen  or  sheep 
the  fat  may  constitute  nearly  50  per  cent,  of  the  weight 
of  the  carcase.  This  stored  fat  is  not  immediately  available. 
It  may  be  regarded  as  invested  capital  which  has  to  be 
placed  at  current  account  before  it  can  be  used. 

This  storage  takes  place  chiefly  in  three  situations  :  (1) 
Fatty  tissue  ;  (2)   Muscle  ;  (3)  Liver. 

(1)  In  Fatty  Tissue- — In  most  mammals  the  chief  storage 
of  surplus  food  is  in  the  fatty  tissues. 

(a)  That  the  fat  of  the  food  can  be  stored  in  them  is 
shown  by  the  fact  that  the  administration  of  large  amounts 
of  fats,  different  from  those  of  the  body,  leads  to  their 
appearance  in  those  tissues.  The  administration  of  erucic 
acid  to  dogs  leads  to  its  appearance  in  the  fat,  and  cows  fed 
on  maize  oil  yield  butter  of  a  low  melting-point.  Fats 
stained  with  Sudan  III.  carry  the  stain  into  the  fatty  tissue  in 
which  they  are  deposited. 

(b)  That  fat  is  formed  from  carbohydrates  was  proved  by 
Laws  and  Gilbert  in  the  feeding  of  young  pigs.  Two  of 
a  litter  were  taken  and  one  was  killed  and  analysed.  The 
other  was  fed  for  weeks  upon  maize,  the  amount  eaten  being 
weighed,  and  the  excretion  of  nitrogen  by  the  pig  being 
determined.  The  animal  was  then  killed  and  analysed,  and 
it  was  found  that  the  fat  gained  was  more  than  could  be 
accounted  for  by  the  fat  and  protein  of  the  food  eaten. 

In  this  process  of  fat  formation  the  respiratory  quotient 
may  rise  above  I'O.     It  is  probably  carried  out  as  follows: — 

6  CeHiA  +  13  O,  =  20  CO,  +  CieHaA  +  20  H,0 
Glucose.  Fatty  Acid. 

The  greater  energy  of  the  fatty  acid  molecule  as  compared 
with  that  of  the  sugar  is  got  by  the  oxidation  of  some  of  the 
sugar  along  with  a  reduction  of  the  rest. 

(c)  The  evidence  that  fats  may  be  formed  from  the 
proteins  of  the  food  is  conflicting.      (1)  In  the  ripening  of 


352  VETERINARY  PHYSIOLOGY 

cheese  it  is  undoubted  that,  under  the  influence  of  micro- 
organisms, proteins  are  changed  to  fats.  (2)  In  all  proba- 
bility the  same  thing  occurs  in  the  formation  of  the  fatty 
adipocere  in  the  muscles  of  the  dead  body  during  putrefaction. 
(3)  At  one  time  it  was  supposed  that,  under  the  influence  of 
such  poisons  as  phosphorus,  the  proteins  of  the  cells  of  the 
mammalian  tissues  are  changed  to  fat.  But  careful  chemical 
examination  has  shown  that  the  so-called  fatty  degeneration 
is  due  to  accumulation  of  already  existing  fats  in  the 
affected  organs.  (4)  If  a  dog  be  fasted  till  all  the  fat  of  the 
body  is  used  up,  and  then  fed  on  lean  beef,  it  will  lay  on  fat. 
But  analysis  of  such  beef  shows  that  it  contains  enough  fat 
and  glycogen  to  yield  all  the  fat  laid  on. 

At  present  we  have  no  direct  evidence  that  the  fats  of 
the  body  are  formed  from  proteins,  although  the  facts  (1) 
that  carbohydrates  are  formed  from  proteins  (p.  354), 
and  (2)  that  fats  are  formed  from  carbohydrates,  make 
it  possible  that  proteins  may  be  a  source  of  fat,  but  that 
their  specific  dynamic  action  prevents  the  fat  from  accumu- 
lating. 

(2)  In  the  Liver. — In  some  animals,  e.g.  the  cod  and  the 
cat,  fats  are  largely  stored  in  the  liver. 

(3)  In  Muscle. — The  salmon  stores  fats  within  its  muscle 
fibres ;  but  in  mammals  such  a  storage  is  very  limited  in 
amount,  although  large  amounts  may  be  deposited  between 
the  bundles  of  fibres  (p,  374). 

2.  Proteins  may,  to  a  small  extent,  be  stored  in  muscle, 
especially  after  a  fast  or  a  prolonged  illness,  and  during  rapid 
growth  a  suckling  animal  ma}^  store  more  than  40  per  cent,  of 
the  protein  of  the  mother's  milk.  But  in  the  normal  mammal 
it  is  difficult  to  induce  such  a  storage,  except  in  athletic 
training,  where  the  muscles  may  be  enormously  increased  by 
the  building  up  of  the  protein-derivatives  of  the  food  into 
their  protoplasm. 

3.  Carbohydrates  are  stored  to  a  small  extent  in  the  liver 
and  in  muscle  (p.  354).  Probably,  at  most  about  10  per 
cent,   of  glycogen  occurs  in   the  liver  and    1    per  cent,   in 


ABSOEPTION  353 

muscle.  The  amount  varies  with  the  diet,  and  in  a  dog 
which  is  not  fasting,  it  may  be  anything  from  5  to  30  grms. 
per  kilo  of  body  weight.  This  small  store  is  rapidly  used  up 
in  fasting  and  is  drawn  upon  in  muscular  exercise.  Glycogen 
may  be  compared  to  money  at  current  account ;  glucose,  like 
money  in  the  pocket,  may  be  used  at  once. 

B.  The  Liver  as  a  Regulator  of  the  Supply  to  Muscles. 

The  liver  develops  as  two  diverticula  from  the  embry- 
onic gat,  and  is  thus  primarily  a  digestive  gland.  In 
invertebrates  it  remains  as  a  part  of  the  intestine  both 
structurally  and  functionally.  But  in  mammals,  early  in 
foetal  life,  it  comes  to  have  important  relationships  with  the 
blood  going  to  nourish  the  body  from  the  placenta  (see 
p.  623).  The  vein,  bringing  the  blood  from  the  mother, 
breaks  up  into  a  series  of  capillaries    in  the  young    liver. 

(1)  Blood  Formation. — The  development  of  the  cells  of  the 
blood  goes  on  for  a  considerable  time  in  these  capillaries. 

(2)  Bile  Secretion. — Soon  the  liver   begins  to  secrete  bile. 

(3)  Glycogenic  Function. — Animal  starch  and  fat  begin  to 
accumulate  in  its  cells. 

Gradually,  the  formation  of  blood  cells  stops,  and  the 
mass  of  liver  cells  becomes  larger  in  proportion  to  the 
capillaries.  As  the  foetal  intestine  develops,  the  vein 
bringing  blood  from  it — the  portal  vein — opens  into  the 
capillary  network  of  the  hver,  so  that,  when  at  birth  the 
supply  of  nourishment  from  the  placenta  is  stopped,  the 
liver  is  still  associated  with  the  blood  which  brings  nutrient 
material  to  the  body,  and  it  performs  an  important  function 
in  regulating  the  supply  of  nourishment  to  the  tissues,  and 
more  especially  to  the  great  energy-liberating  tissue,  muscle. 

1 .  Regulation  of  the  Supply  of  Sugar. — It  has  been  already 
shown  that  sugar  is  an  essential  source  of  energy  in  muscle. 

(1)  Production  of  Sugar. — The  relationship  of  the  liver 
to  the  metabolism  of  sugar  was  discovered  by  Claude 
Bernard  in  the  middle  of  last  century.  Even  in  the  most 
prolonged  fast,  the  liver  continues  to  supply  to  the  blood 
enough  dextrose  to  maintain  the  normal  proportion  of  about 
23 


354  VETERINARY   PHYSIOLOGY 

0  15  per  cent.  In  fasting  the  only  possible  sources  of  this 
sugar  are  the  proteins  and  the  fats  of  the  body,  (a)  That 
proteins  are  a  source  of  sugar  is  shown  by  the  fact  that,  in 
diabetic  patients  and  in  dogs  rendered  diabetic  by  removal 
of  the  pancreas,  i.e.  in  animals  which  are  excreting  and  not 
using  the  sugar  (p.  357),  the  output  of  sugar  is  increased  by 
giving  proteins.  It  has  further  been  found  that  most  of  the 
amino-acids  which  build  up  the  proteins,  undergo  the  same 
change,  the  non-nitrogenous  part  being  to  a  greater  or  less 
extent  converted  to  sugar,  the  nitrogenous  part  being  excreted 
as  urea.  Claude  Bernard  had  discovered  that,  after  feeding 
a  dog,  which  had  fasted  till  all  the  stored  carbohydrates  of 
the  liver  had  disappeared,  on  lean  beef,  glycogen,  the 
precursor  of  sugar,  appeared  in  the  liver. 

(6)  The  question  of  whether  the  liver  can  form  sugar 
from  the  fats  of  the  body  is  more  difficult  to  answer.  The 
argument  in  favour  of  such  a  conversion  is  that  in  many 
cases  of  pancreatic  diabetes  the  amount  of  sugar  formed  is 
more  than  could  be  derived  from  the  proteins  decomposed, 
as  indicated  by  the  nitrogen  excreted.  Hence,  it  would 
seem  that  it  must  be  derived  from  the  fats. 

(2)  Storing  Sugar  as  Glycogen. — The  liver  not  only 
manufactures  sugar  for  the  muscles  when  the  supply  from 
outside  is  cut  off,  but  it  also  has  the  power  of  storing  sugar 
derived  from  an  excess  of  carbohydrates  in  the  food,  or  from 
an  excess  of  proteins.  This  it  does  by  converting  the 
monosaccharid  into  a  polysaccharid — animal  starch,  or 
glycogen.  This  substance  accumulates  in  the  protoplasm  of 
the  cells,  and  its  presence  may  be  demonstrated  by  staining 
with  iodine.  Since  the  same  glycogen  is  derived  from  all 
the  single  sugars,  Isevulose  (a  ketose)  as  well  as  dextrose  (an 
aldose),  the  liver  protoplasm  must  perform  a  chemical  change 
in  the  process  of  synthesising  them  into  glycogen,  from 
which  dextrose  alone  is  formed.  The  storage  of  glycogen 
may  be  very  great,  amounting  in  certain  conditions  to  as 
much  as  10  per  cent,  of  the  weight  of  the  liver. 

(3)  Conversion  of  Glycogen  to  Dextrose. — When  sugar  is 
required  by  the  muscles,  it  is  again  converted  to  glucose, 
and  passes  off  in  the  blood.      This  subsequent  conversion  of 


ABSORPTION  355 

glycogen  to  glucose  is  generally  ascribed  to  the  action  of  an 
hepatic  diastase.  This  conclusion  is  supported  by  the  fact 
that  the  liver  tissue,  after  prolonged  treatment  with  alcohol, 
has  an  active  diastatic  action.  But  (i)  fresh  liver  has  no 
greater  diastatic  power  than  any  other  tissue.  (ii)  While 
the  conversion  of  glycogen  to  glucose  in  the  liver  removed 
from  the  animal  immediately  after  death,  is  at  a  maximum 
during  the  first  few  minutes  and  gradually  decreases,  the 
conversion  of  glycogen  to  glucose,  under  the  influence  of 
liver  tissue  treated  with  alcohol,  gradually  reaches  a 
maximum  in  an  hour  and  gradually  wanes,  (iii)  It  has 
Also  been  shown  that  the  injection  of  methylene  blue,  which 
poisons  protoplasm,  but  does  not  interfere  with  the  action 
of  enzymes,  checks  the  conversion,  and  (iv)  that  stimulating 
the  splanchnic  nerves  going  to  the  liver  increases  the 
conversion  of  glycogen,  without  increasing  the  amylolytic 
enzyme  in  the  liver  and  blood.  It  is  therefore  possible  that 
the  conversion  results  from  chemical  changes  in  the  proto- 
plasm which  are  controlled  by  the  nerves  of  the  liver. 
These  nerves  are  derived  from  the  true  sympathetic 
system. 

Carbohydrate  Tolerance. — If  more  sugar  is  taken  than  the 
liver  can  deal  with,  it  passes  on  into  the  general  circulation, 
and  is  excreted  in  the  urine.  Every  animal  has  a  certain 
power  of  oxidising  or  of  storing  sugar.  If  the  carbohydrates 
are  taken  as  starch  instead  of  sugar,  the  process  of  digestion 
iind  the  slower  absorption  enables  the  liver  to  deal  with  much 
larger  quantities.  The  carbohydrate  tolerance  varies  greatly, 
and  even  in  the  same  animal  it  is  different  under  different 
conditions. 

Glycosuria. — If  the  limit  of  carbohydrate  tolerance  is 
overstepped,  sugar  increases  in  amount  in  the  blood 
(glycsemia)  and  appears  in  the  urine.       Glycosuria  is  produced. 

Glycosuria  may  be  caused  in  several  different  ways. 

1.  By  decreased  carbohydrate  tolerance — alimentary 
glycosuria  (fig.  160). 

2.  ^Yhen  the  glycogen  stored  in  the  liver  is  changed  to 
glucose  more  quickly  than  is  required  by  the  tissues,  the 
glucose    may,    to   a   small   extent,    be   again   stored   in   the 


356 


VETERINARY   PHYSIOLOGY 


muscles  as  glycogen  (p.  352),  or  it  may  accumulate  in  the 
blood  and  be  excreted  in  the  urine.  This  latter  condition 
is  seen  when  large  doses  of  adrenalin,  the  active  principle 
of  the  medullary  part  of  the  suprarenal  bodies  (p.  598),  is 
injected  subcutaneously.  This  substance  has  a  specific 
action  on  the  termination  of  the  true  sympathetics,  and,  in 
all  probability,  it  acts  upon  the  termination  of  the  splanchnic 
nerves  in   the  liver  to  increase  the  conversion  of  glycogen  to 


Fig.    160. — To  show  the  various  waj's  in  which  glycosuria  may 
be  produced  (see  text). 

glucose.  Ergotoxin,  which  checks  its  action  elsewhere 
(p.  592),  also  hmits  its  power  of  causing  glycosuria  (fig.  160). 
3.  The  condition  is  also  caused,  if  the  liver  is  rich  in 
glycogen,  by  puncturing  the  posterior  part  of  the  floor  of  the 
fourth  ventricle  of  the  brain.  Since  this  effect  is  not 
produced  after  the  suprarenals  have  been  removed,  it  has 
been  concluded  that  it  is  due  to  a  stimulation  of  these 
structures  through  the  splanchnic  nerves  by  Avhich  an 
increased  outpouring  of  adrenalin  is  induced.      This  might 


ABSORPTION  357 

act  in  the  same  wa}'  as  the  administration  of  large 
doses  of  adrenalin.  It  is  probable  that  two  elements  are 
involved — (a)  the  stimulation  of  the  suprarenals,  and  (6) 
the  stimulation  of  the  branches  of  the  splanchnic  nerves 
to  the  liver  and  that  the  latter  action  is  merely  facilitated 
or  activated  by  the  former,  since  the  accumulation  of 
adrenalin  in  the  blood  found  in  puncture  diabetes  is  not 
sufficient  in  itself  to  cause  the  glycosuria,  and  since  M'Leod 
found  that  section  of  the  hepatic  nerves  generally  prevents 
its  onset  (fig.  160). 

4.  The  injection  of  phloridzin  and  some  other  substances 
such  as  chrome  salts,  or  even  solutions  of  neutral  sodium 
salts,  also  causes  sugar  to  appear  in  the  urine.  Under  the 
influence  of  these,  the  sugar  in  the  blood  is  not  increased. 
It  must  be  concluded  that  they  act  by  causing  the  kidneys 
to  excrete  glucose  too  rapidly,  so  that  it  is  not  available  for 
the  tissues.  But,  even  when  carbohydrates  are  withheld  or 
cleared  out  of  the  body,  phloridzin  causes  glycosuria.  Hence, 
a  formation  of  glucose  from  the  proteins  of  the  blood  plasma 
must  occur  (fig,  160). 

5.  Removal  of  the  pancreas  also  causes  glyctemia  and 
glycosuria.  This  may  be  prevented  by  transplanting  a 
piece  of  the  pancreas  under  the  skin  if  the  graft  grows 
(p.  601).  The  pancreas  forms  something  which  (i)  checks 
the  conversion  of  glycogen  to  glucose  in  the  liver,  so  that, 
when  it  is  removed,  this  process  goes  on  too  rapidly.  (ii)  At 
the  same  time  the  utilisation  of  sugar  by  the  muscles  seems 
to  be  interfered  with.  This  failure  to  use  sugars  is  indicated 
by  the  fact  that  the  respiratory  quotient  (p.  258)  is  low  in 
diabetes,  indicating  that  proteins  and  fats  are  being  used 
and  not  carbohydrates,  and  that  it  is  not  raised  when  sugar 
is  administered.  The  carbohydrates  of  the  food  are  no 
longer  available  as  a  source  of  energy,  and  the  animal  has 
to  use  proteins  and  fats  alone. 

(i)  But  the  part  of  the  jjroteins  which  is  normally  used 
as  a  source  of  energy  is  the  non-nitrogenous,  and  this  is  not 
available  and  is  simply  excreted  as  sugar.  Hence,  although 
the  animal  decomposes  its  proteins,  the  non-nitrogenous 
part  is  lost  as  sugar,  and  energy  is  not  got  from  them. 


858  VETERINARY  PHYSIOLOGY 

(ii)  Nor  are  the  fats  fully  available,  because  the 
metabolism  of  carbohydrates  is  necessary  for  their  combus- 
tion, and  this  is  in  abeyance.  As  a  result  of  this  incomplete 
metabolism  of  fats,  /S-oxybutyric  acid  is  produced,  which 
leads  to  a  decrease  in  the  alkalinity  of  the  blood  and  tissues, 
to  a  condition  of  acidosis. 

The  /S-oxybutyric  acid  is  not  oxidised  to  CO.,  and  HoO 
as  it  normally  is,  but  is  converted  to  diactic  acid,  and  this 
in  turn  to  acetone,  and  these  bodies  may  be  detected  in  the 
urine  (Chemical  Physiology). 


/3-Oxybutyric  Acid       .      CHg.ClH.OHiCH.CO.OH 
Diacetic  Acid     .  .     CHgCO.CH^jCQ.OHl 

Acetone    .  .  .      CH3CO.CH3. 

Hence,  in  fully-developed  pancreatic  diabetes,  none  of  the 
proximate  principles  of  the  food  yield  the  energy  required, 
and  the  animal  or  man  rapidl}^  becomes  weak,  emaciates 
and  dies. 

2.    Regulation  of  the  Supply  of  Fats — (1)  Storage The  fats 

leave  the  intestine  not  by  the  blood  of  the  portal  veins 
which  goes  straight  to  the  liver,  but  through  the  lymphatics 
which  enter  the  blood  stream,  just  where  it  returns  to  the 
heart,  through  the  thoracic  duct.  They  thus  reach  the 
liver  by  the  arterial  blood.  Nevertheless,  when  taken  in 
excess  of  the  immediate  requirements,  they  are  stored  in 
large  amounts  in  the  liver  of  some  animals — e.g.  the  cod 
among  fish  and  the  cat  among  mammals.  Animals  which 
have  little  power  of  storing  fat  in  the  muscles  and  other 
tissues  seem  to  have  a  marked  capacity  for  accumulating  it 
in  the  liver.  Even  in  starvation,  the  fats  do  not  disappear 
from  the  liver,  and  throughout  all  conditions  of  life  a  fairly 
constant  amount  of  lecithin  (p.  20)  is  present  in  the  liver 
cells.  Lecithin,  in  the  yolk  of  the  egg,  is  an  intermediate 
stage  in  the  formation  of  the  more  complex  nucleins  of  living 
cells ;  and  the  formation  of  lecithin  in  the  liver  by  the 
synthesis  of  glycerol,  fatty  acids,  phosphoric  acid,  and 
cholin  is  probably  a  first  step  in   the  construction  of  these 


ABSORPTION  359 

nucleins.  The  fat  of  the  liver  thus  plays  an  important 
part  in  retaining  and  fixing  phosphorus  in  the  body. 

(2)  Change  in  Liver. — The  fats  of  the  liver  have  a  higher 
iodine  value  than  the  fats  of  adipose  tissue,  i.e.  they  are  less 
saturated,  and  the  theory  has  been  advanced  that  this 
indicates  that  in  the  liver  the  first  stage  in  the  breaking- 
down  of  fats  takes  place,  in  preparation  for  their  use  in  the 
muscles.  When  the  fatty  acid  chain  has  two  hydrogens 
removed  at  any  point  so  that  a  double  link  between  carbon 
atoms  is  formed,  this  becomes  a  weak  point  in  the  chain  at 
which  the  higher  acids  are  apt  to  break  across  with  the 
production  of  lower  acids. 

Lower  fatty  acids,  liberated  by  the  de-aminisation  of  certain 
amino-acids  from  proteins,  e.g.  leucin,  tyrosin,  phenyl-alanin 
are  not  changed  to  glucose  (p.  354)  but  to  /3-oxy butyric  acid, 
which  is  oxidised. 

3.  Regulation  of  the  Supply  of  Proteins. — The  part  played 
by  the  liver  in  the  storage  of  the  surplus  amino-acids  formed 
from  proteins  and  in  their  de-aminisation,  and  the  conversion 
of  the  amidogen  to  urea,  has  already  been  indicated  (p.  349). 
When  the  supply  of  amino-acids  is  too  large,  or  when  the 
liver  is  not  acting  properly  in  grave  hepatic  disease,  this 
conversion  takes  place  imperfectly  and  amino-acids  appear 
in  the  urine. 

Urea  is  the  bi-amide  of  carbonic  acid. 


0 

II 
H— 0— C--0— H 

0 
H^           11             /H 

J\N— C— N< 
H                         \h 

Carbonic  Acid. 

Urea. 

It  contains  4  6 '6  per  cent,  of  nitrogen.  It  is  a  white  sub- 
stance crystallising  in  long  prisms.  It  is  very  soluble  in 
water  and  alcohol — insoluble  in  ether.  With  nitric  and 
oxalic  acids  it  forms  insoluble  crystalline  salts.  It  is  readily 
decomposed  into  nitrogen,  carbon  dioxide  and  water  by 
nitrous  acid  and  by  sodium  hypobromite  in  excess  of  sodium 
hydrate  (Chemical  Physiology). 


360  VETERINARY   PHYSIOLOGY 

Urea  is  chiefly  formed  in  the  Liver. — This  is  indicated — 
(1)  By  the  fact  tliat  when  an  ammonium  salt,  such  as  the 
carbonate,  dissolved  in  blood,  is  streamed  through  the  organ, 
it  is  changed  to  urea  ;  (2)  by  the  evidence  that  the  liver 
stores  the  surplus  amino -acids,  and  that,  as  they  again 
disappear  from  the  liver,  urea  increases  in  the  blood  ;  (3)  by 
the  observation  that,  when  the  liver  is  cut  out  of  the 
circulation,  the  urea  in  the  urine  rapidly  diminishes,  and 
ammonia  and  lactic  acid  take  its  place. 

The  exclusion  of  the  liver  from  the  circulation  in 
mammals  is  difficult,  because,  when  the  portal  vein  is 
ligatured,  the  blood  returning  to  the  heart  tends  to  accumu- 
late in  the  great  veins  of  the  abdomen.  But  this  difficulty 
has  been  overcome  by  Eck,  who  devised  a  method  of  connect- 
ing the  portal  vein  with  the  inferior  vena  cava,  and  afterwards 
occluding  the  portal  vein,  and  of  thus  allowing  the  blood  to 
return  from  the  abdomen  to  the  heart  without  passing 
through  the  liver. 

That  it  is  not  produced  in  the  kidneys  was  first  shown  by 
the  French  chemist  Dumas.  He  found  that  when  these 
organs  are  excised,  urea  accumulates  in  the  blood.  Later 
investigators  found  that  when  ammonium  carbonate  is 
added  to  blood  artificially  circulated  through  the  kidney  of 
an  animal  just  killed,  no  urea  is  formed. 

That  it  is  not  formed  to  any  marked  extent  in  the  muscles 
is  shown — (1)  By  the  absence  of  a  definite  increase  in  urea 
formation  during  muscular  activity ;  (2)  by  the  fact  that 
when  the  blood,  containing  ammonium  carbonate,  is  streamed 
through  the  muscles,  urea  is  not  produced. 

The  Sources  of  Urea. — (1)  The  source  of  urea  from  the 
amino-acids  formed  in  the  digestion  of  proteins  in  the  food 
has  already  been  discussed  (p.  349).  (2)  But  urea  is  also 
formed  during  starvation,  and  it  must  therefore  be  derived 
from  the  proteins  of  the  tissues.  It  has  been  found  that 
in  starvation  there  is  an  increase  of  the  amino-acids  in  such 
tissues  as  muscle,  and  it  would  thus  seem  that  they  are  pro- 
ducts of  the  disintegration  of  the  muscle  proteins,  and  that 
they  are  carried  to  the  liver  to  be  converted  to  urea. 

The  fate  of  haemoglobin  tends  to  show  that   the   whole 


ABSORPTION  361 

process  of  protein  catabolism  ma}'  be  conducted  in  the  liver 
cells.  When  haemoglobin  is  set  free  from  the  corpuscles  in 
moderate  amounts,  the  nitrogen  of  its  protein  part  is  changed 
to  urea,  while  the  pigment  part  is  deprived  of  its  iron  and 
excreted  as  bilirubin. 

The  process  of  urea  formation  from  proteins  may  be 
divided  into  four  stages — (I)  The  liberation  of  the  amino- 
acids.  (2)  The  de-aminisation  of  the  amino-acids.  This  is 
probably  effected  by  de-aminising  enzymes.  (3)  The 
ammonia  set  free  is  probably  linked  to  carbonic  acid ;  and  (4) 
the  carbonate  of  ammonia  is  then  dehydrated  by  other 
enzymes  and  so  changed  into  urea  (p.  5  59). 

The  nitrogen  excreted  is  not  all  in  the  form  of  urea. 
The  other  nitrogen-containing  waste  products  are  dealt  with 
on  p.  559  et  seq. 

Summary  of  the  Functions  of  Liver. — The  functions  of  the 
liver  may  be  briefly  summarised  as  follows  : — (1)  It 
regulates  the  supply  of  glucose  to  the  muscles  (a)  by  manu- 
facturing it  from  proteins  when  the  supply  of  carbohydrates 
is  insufficient,  and  (6)  by  storing  it  as  glycogen  when  the 
supply  of  carbohydrates  is  in  excess,  and  giving  it  off  after- 
wards as  required.  (2)  Along  with  the  intestinal  wall,  it 
regulates  the  supply  of  proteins  to  the  body  by  de-aminising 
any  excess,  conserving  the  non-nitrogenous  part  by  convert- 
ing it  into  glucose  for  use  by  the  muscles,  and  giving  off"  the 
nitrogen  as  urea,  etc.  (3)  It  regulates,  in  many  animals  at 
least,  the  supply  of  fat  to  the  body  by  storing  any  excess  ; 
and  it  probably  plays  an  important  part  in  de-saturating  the 
fatty  acids,  and  thus  making  them  more  available  for  combus- 
tion in  the  tissues.  (4)  It  breaks  down  the  haemoglobin  of  old 
erythrocytes,  and  retains  tlie  iron  for  further  use  (see  p.  491). 
(5)  From  the  part  it  plays  in  the  enterohepatic  circulation, 
it  protects  the  body  against  certain  poisons  by  excreting 
them  in  the  bile  (see  p.  831). 

The  Faeces. 

The  unabsorbed  contents  of  the  alimentary  tract,  whether 
originally  derived  from  the  food  or  from  the  tract  itself,  are 


362  VETERINARY  PHYSIOLOGY 

constantly  being  passed  onward  to  the  lower  part  of  the 
large  intestine.  Here  absorption  of  water  leaves  a  more  or 
less  inspissated  mass  which  collects  and  is  periodically 
voided  as  the  faeces. 

(1)  Carnivora. — (a)  In  fasting  animals,  fseces  are  passed  at 
long  intervals,  and  consist  of  mucin,  shed  epithelium,  the 
various  products  of  the  bile  constituents,  inorganic  salts,  and 
enormous  numbers  of  bacteria. 

(b)  In  feeding  animals  the  amount  and  character  of  the 
faeces  depend  largely  (1)  upon  the  amount  and  character  of 
the  food  ;  and  (2)  upon  the  bacteria  which  are  growing  in  the 
large  intestine.  If  digestion  and  absorption  of  food  are  com- 
plete, the  fasces  are  the  same  on  different  diets,  and  consist  of 
the  intestinal  products,  which  are  increased  in  amount  by  the 
stimulating  action  of  the  food  in  the  alimentary  canal. 

The  solids  of  the  fa?ces  of  a  feeding  animal  consist  of 
the  same  constituents  as  the  fasces  in  a  fasting  animal,  with 
the  addition  of  the  undigested  constituents  of  the  food — 
elastic  and  white  fibrous  tissue,  remains  of  muscle  fibres,  fat, 
and  the  earthy  soaps  of  the  fatty  acids,  the  fat  forming  about 
one-third  of  the  weight  of  dry  fteces.  When  a  vegetable 
diet  is  taken,  the  cellulose  of  the  vegetable  cells,  and  some- 
times starch,  are  present.  The  cellulose,  by  stimulating  the 
intestine,  is  a  valuable  natural  purgative. 

Phosphates,  as  well  as  calcium,  magnesium,  and  iron,  de- 
rived from  the  metabolism  of  the  tissues,  are  largely  excreted 
into  the  large  intestine,  and  are  passed  out  in  the  feces. 

Probably  in  an  ordinary  mixed  diet  some  80  to  40  per 
cent,  of  the  phosphorus,  about  90  per  cent,  of  the  calcium, 
some  70  per  cent,  of  the  magnesium  are  excreted  in  the  faeces. 

The  odour  is  due  to  the  presence  of  many  different  sub- 
stances, and  it  varies  with  the  character  of  the  bacterial 
flora  of  the  large  intestine. 

(2)  Herbivora. — In  these  the  residual  products  of  digestion 
are  very  bulky  and  evacuation  is  more  frequent  than  in  car- 
nivora. The  horse  usually  defascates  about  ten  times  a  day. 
The  amount  passed  in  twenty-four  hours  varies  with  the 
nature  of  the  food.  On  an  ordinary  diet  about  15  kilos  per 
diem  are  passed  by  the  horse,  and  about  20  by  the  ox. 


ABSORPTION  363 

An  idea  of  the  average  amount  of  water  present  is  shown 
in  the  following  table  given  by  Gamgee  : — 

Approximate  percentage  Composition  of  the  Fceces. 


Horse. 

Cow. 

Sheep. 

Pig- 

Water         . 

76 

84 

58 

80 

Organic  Matter  . 

21 

13-6 

36 

17 

Mineral  Matter  . 

3 

2-4 

6 

3 

100  100  100  100 

When  first  passed  fa3ces  usually  float  in  water  owing  to 
the  amount  of  gas  contained  in  them.  They  are  nearly 
always  acid  in  reaction  from  the  presence  of  organic  acids. 

As  in  carnivora  fseces  consist  of  material  derived  from 
two  sources — (1)  food  residues  ;  and  (2)  excretions  by  w\ay  of 
the  intestines. 

Food  Residues. — These  consist  of  indigestible  material  such 
as  lignin  and  waxes,  which  pass  through  the  gut  unchanged, 
and  also  of  material  that  has  escaped  digestion  either  because 
as  in  the  case  of  cellulose  digestion  is  difiicult,  and  only  part 
is  dealt  with,  or  because  it  is  protected  by  an  envelope  that 
is  not  dissolved  by  the  digestive  juices.  This  part  consists 
chiefly  of  the  constituents  of  the  crude  fibre  of  the  diet. 

Excretions. — These  consist  of  the  same  material  as  in  the 
case  of  carnivora.  Inorganic  matter,  e.g.  calcium,  magnesium, 
iron,  and  phosphates  are,  however,  excreted  by  the  bowel  to  a 
much  greater  extent  than  in  carnivora. 

Meconium  is  the  name  given  to  the  first  fasces  passed 
by  the  young  after  birth.  It  is  greenish-black  in  colour, 
and  consists  of  inspissated  bile  and  shed  epithelium  from 
the  intestine. 

Availability  of  Food-Stuffs. 

Only  that  part  of  the  food  which  is  digested  and  absorbed 
is  available  as  a  source  of  either  energy  or  material  to  the 
animal.  In  carnivora  the  indigestible  residue  is  very  small. 
In  herbivora,  however,  a  very  large  proportion  of  the  food  is 
not  digested.  In  comparing  the  value  of  different  food- 
stufts,  therefore,  it  is  necessary  to  know  what  proportion  is 
available. 


364  VETERINARY   PHYSIOLOGY 

In  the  literature  of  aniinul  nutrition  the  term  "digesti- 
bility "  is  used  in  a  specific  sense.  It  denotes  the  percentage 
of  the  food  or  of  any  constituent  of  the  food  that  is  absorbed 
from  the  aUmentary  tract.  Thus  in  a  digestion  experiment 
on  a  horse,  57  per  cent,  of  the  protein  of  hay  was  apparently 
digested  and  absorbeil  as  that  proportion  was  not  recovered 
in  the  fteces.  The  "digestibility"  of  the  protein  of  hay  for 
the  horse  in  this  case  is  said  to  be  57  per  cent.  The  per- 
centage of  digestibility  is  often  termed  the  "  coefficient  of 
digestibility." 

Digestion  Experiments. 

The  availability  of  food-stuffs  is  determined  by  digestion 
experiments.  In  these  the  animal  is  fed  on  a  weighed 
quantity  of  the  food  to  be  tested  for  a  preliminary  period 
of  about  ten  days  to  make  sure  that  the  previous  food  has  com- 
pletely passed  out  of  the  intestinal  tract.  The  feeding  is 
then  continued  for  another  period  of  not  less  than  ten  days, 
during  which  time  twenty-four  hourly  collections  of  fa3ces 
are  made.  The  difference  between  the  amount  of  each 
constituent  of  the  food  eaten  and  that  found  by  analysis  in 
the  fceces  is  regarded  as  the  digested  portion. 

Concentrates  cannot  be  fed  alone  to  ruminants.  Their 
availability  is  determined  by  superimposing  a  weighed 
quantity  upon  a  roughage  diet,  whose  availability  has  been 
previously  determined.  The  increased  amount  of  the  various 
constituents  found  in  the  f^ces  is  retjarded  as  the  undig'ested 
matter  of  the  concentrate  tested. 

Accuracy  of  the  Method. — Digestion  experiments,  though 
of  great  use,  indicate  only  apparent  digestibility.  The  issue 
is  confused  by  (1)  excietory  products  ;  and  (2)  loss  through 
fermentation. 

(1)  Nitrogenous  substances,  ether  soluble  substances,  and 
inorganic  salts  are  present  in  the  faeces  even  if  absent  in  the 
food  (p.  362).  The  smaller  the  amount  of  these  present  in 
the  food  the  greater  is  the  percentage  error  due  to  excretory 
products  ;  with  fats  and  salts  the  error  may  be  so  great  as 
to  make  the  results  of  little  value  as  a  means  of  determining 
the  real  amount  di^^ested. 


ABSORPTION  865 

In  the  case  of  protein  an  attempt  has  been  made  to 
diderentiate  between  excretory  nitrogen  and  undigested 
nitrogen,  by  regarding  nitrogenous  material  which  can  be 
rendered  soluble  by  pe[)sin  and  hydrochloric  acid  as  excretory, 
and  that  which  remains  insoluble  as  undigested  nitrogen. 
It  has  also  been  suggested  that  for  every  100  grams  of  dry 
matter  in  the  food,  4  grams  of  nitrogen  of  the  faeces  should 
be  regarded  as  excretory  nitrogen. 

Even  if  precautions  such  as  those  indicated  be  taken,  the 
results  of  digestion  experiments  on  food-stuffs  with  small 
amount  of  nitrogenous  constituents  should  be  received  with 
caution.  It  is  probable  that  in  the  adult  animal  at  least,  a 
more  accurate  determination  of  the  percentage  digestibility 
of  nitrogenous  material  would  be  obtained  by  regarding  the 
urinary  nitrogen  as  an  index  of  the  amount  digested  and 
absorbed,  instead  of  taking  the  fsecal  nitrogen  as  an  index  of 
the  amount  not  digested  (see  p.  558  ei  seq.). 

(2)  The  part  of  the  crude  fibre  and  soluble  carbohydrate 
that  disappears  in  transit  through  the  alimentary  canal  is  not 
all  digested.  Part  is  lost  through  destructive  fermentation 
(p.  340).  In  estimating  the  availability  of  the  food,  this 
must  be  taken  into  account.  The  excretion  of  methane 
and  hydrogen  gives  an  indication  of  the  extent  of  the 
fermentation.  According  to  Armsby,  the  following  deduc- 
tions should  be  made  for  fermentation 


Factors  for  computing  Fermentation  Losses. 
Per  100  grams  digested  carbohydrates — 


Weight. 

Equivalent 
Energy. 

Grams. 

Cals. 

Ruminants— Methane     . 

4-5 

60-1 

Swine — Methane    . 

0-65 

8-7 

Hydrogen . 

0-07 

2-4 

Total  . 

0-72 

IM 

Per    100    grams    digested    crude 

fibre- 

Horse — Methane    . 

4-7 

62-7 

Hv<h'ogen 

0-2 

7-0 

Total   .         .         .     49 


366  VETERINARY  PHYSIOLOGY 

The  loss  in  the  pig  is  comparatively  small.  In  the  horse 
fermentation  occurs  in  the  caecum  and  colon  after  the  food 
has  passed  through  the  small  intestine.  Consequently,  it  is 
chiefly  the  crude  fibre,  the  constituent  that  has  resisted  the 
digestive  processes  that  is  affected. 

Factors  affecting  Availability — ( 1 )  Species.— Concentrates  are 
equally  well  digested  by  all  farm  animals.  Differences  occur 
in  the  availability  to  digest  fodders,  or  other  food-stuffs 
containing  much  crude  fibre.  Tliese  are  utilised  more  com- 
pletely by  the  ruminant  than  by  the  horse.  The  following 
table  shows  the  average  percentages  of  the  constituents  of  oats 
and  hay  digested  by  the  horse  and  the  sheep  : — 


Hay. 


Nitro2;eDou.s  Ether  Carbohydrates  Crude 

Material.  Extract.      (Nitrogen  Free  Extract).  Fibre. 


Horse  ...  57  21  55  36 

Sheep  ...  57  51  62  56 

Oats. 

Horse.         .         .  86  71  74  21 

Sheep  ...  80  83  76  30 

The  pig  digests  pure  cellulose  well,  but  owing  to  the  lack 
of  a  macerating  compartment  like  the  rumen  of  the  ox  or 
the  Cfecum  and  colon  of  the  horse,  fibre  with  encrusting 
material  in  it  is  very  incompletely  digested.  Of  the  crude 
fibre  of  ordinary  feeding-stuffs  it  digests  little  more  than 
50  per  cent,  of  the  amount  digested  by  the  ruminant. 

(2)  Amount  of  Food. — The  availability  of  the  food  is 
little  affected  by  the  amount  eaten  unless  on  heavy  mixed 
feeding  with  fodder  and  concentrates,  in  which  case  the 
percentage  digested  is  decreased.  The  decrease  is  due  to  the 
too  rapid  passage  of  the  food  through  the  alimentary  tract. 
On  feeding  with  fodder  alone,  however,  the  results  of 
experiments  show  that  the  amount  eaten  makes  no  appre- 
ciable difference  on  the  percentage  digested. 

(3)  Cooking  and  Grinding. — Starchy  foods  like  potatoes 
are  more  completely  digested  after  cooking.  With  this 
exception  cooking  usually  decreases  the  percentage  digested, 
proteins  being  especially  affected. 

For  horses  and  swine  the  digestibility  of  grain  is 
increased    by    crushing,    and    still    more    by    grinding.      In 


ABSOEPTION  367 

ruminants  whole  grain  is  digested  as  completely  as  crushed 
grain. 

(4)  Watering. — It  is  commonly  believed  that  drinking 
after  eating  tends  to  wash  the  food  out  of  the  stomach  and 
decrease  the  amount  digested,  and  that  therefore  animals, 
and  especially  horses,  should  be  watered  before  being  fed. 
According  to  Scheunert,  however,  water  drunk  by  the  horse 
after  feeding,  passes  between  the  wall  of  the  stomach  and  its 
contents  to  the  duodenum  with  very  little  disturbance  or 
dilution  of  the  contents,  and  experiments  by  Tangl  show 
that  less  water  is  drunk  and  the  food  is  not  so  completely 
digested  when  watering  takes  place  prior  to  feeding,  and  no 
water  is  allowed  during  or  after  feeding.  These  results 
agree  with  those  obtained  on  both  dogs  and  men. 

Unless  there  be  some  good  reason  supported  by  experi- 
mental evidence,  the  prevention  of  an  animal  from  drinking 
either  before,  during,  or  after  feeding  according  to  its 
inclination  does  not  seem  to  be  warranted.  It  should  be 
noted,  however,  that  in  watering  and  feeding  any  sudden 
change  in  a  system  that  the  animal  has  been  accustomed  to 
may  lead  to  disturbance  of  the  digestive  functions. 

(5)  Proportion  of  Constituents  of  Food. — Excess  of  carbo- 
hydrates in  the  diet  decreases  the  percentage  availability  of 
all  the  constituents,  but  especially  the  a]jparent  availability 
of  the  nitrogenous  substances.  According  to  Kellner, 
decreased  digestion  occurs  when  the  nutritive  ratio  (p.  368) 
is  wider  than  1-8  in  ruminants  and  1-12  in  pigs. 

Armsby  suggests  that  the  decrease  in  the  amount 
digested  is  caused  by  a  modification  of  the  fermentation 
processes  due  to  the  presence  of  excessive  amounts  of  soluble 
carbohydrates  which  bacteria  attack  in  preference  to  the 
crude  fibre.  The  fibre  consequently  escapes  disintegration 
and  carries  off  the  contained  digestible  constituents.  On 
the  addition  of  protein  to  the  diet  the  digestibility  is 
increased,  the  reason  assigned  being  that  the  increase  of 
nitrogenous  material  stimulates  the  multiplication  and 
activity  of  the  bacteria. 

As  Kellner  has  pointed  out,  the  apparent  decrease  in  the 
percentage  of  protein  digested  is  not  a  true  decrease.      As 


368  VETERINARY   PHYSIOLOGY 

the  undigested  nitrogen  in  the  faeces  decreases  in  amount,  as 
occurs  on  a  low  nitrogen  intake,  the  relative  proportion  of 
excretory  nitrogen  becomes  greater,  so  that  on  a  very  low 
protein  intake  the  apparent  amount  digested  may  become  a 
negative  quantity. 


FOOD  REQUIREMENTS. 

Food  is  the  source  of  (1)  tJie  energy,  and  (2)  the  material 
(p.  283)  which  the  animal  kept  for  profit  transforms  into 
the  products  desired  by  the  feeder.  Thus,  the  horse  changes 
the  chemical  energy  of  oats  and  hay  into  work.  Grass  and 
other  food-stuffs  are  changed  by  the  dairy  cow  to  milk,  by 
the  bullock  to  meat,  by  the  sheep  to  mutton  and  wool.  The 
problem  of  the  feeder  is  to  secure  the  greatest  return  of 
these  products  with  the  most  economical  consumption  of  food. 

Nutritive  Ratio. — Protein  is  of  special  importance  in  the 
food.  This  constituent  can  replace  fats  and  carbohydrates  as 
a  source  of  energy,  but  none  of  the  other  constituents  of  the 
food  can  replace  protein  as  the  source  of  material  for  growth 
and  repair  (p.  275).  In  comparing  the  values  of  different  food- 
stuffs or  combination  of  food -stuffs,  therefore,  it  is  necessary 
to  know  what  proportion  of  the  energy  value  of  the  digestible 
portion  of  the  food  consists  of  protein.  This  is  expressed  by 
the  nutritive  ratio,  sometimes  called  the  "  albuminoid  ratio." 
It  might  be  written — 

Protein  :  fats  +  carboliydrates  =  nutritive  ratio. 

In  calculating  the  ratio  it  must  be  remembered  that  fats 
can  liberate,  weight  for  weight,  about  two  and  a  quarter 
times  as  much  energy  as  carbohydrates  (p.  2.57).  Fats  are 
therefore  multiplied  b}'  2  "2 5.  For  example,  if  a  sample  of 
meadow  hay  contained  the  following  percentage  of  digestible 
nutrients — nitrogen  free  extract  28,  crude  fibre  15,  fats  1-2, 
protein  5 — the  nutritive  ratio  would  be  — 

5  :  28  +  15  +  1-2  X  2-25,  i.e.  5  :  457,  or  1  :  9-14. 

The  ratio  gives  useful  information  as  to  the  suitability  of 
a  food  for  purposes  that  require  much  protein,  e.g.  growth 


FOOD   REQUIREMENTS  369 

or  milk  production,  or  for  purposes  that  require  less  protein, 
e.g.  work,  fat  formation  or  maintenance  without  production. 
As  protein  is  usually  the  most  expensive  energy  yielding 
constituent  of  the  food,  a  knowledge  of  the  nutritive  ratio  is 
of  importance  to  feeders. 

1.  Methods  of  determining  Requirements. 

1 .  Live  Weight  Test. — In  these,  the  value  of  the  food  in 
meeting  the  requirements  of  the  animal  is  determined  by 
alterations  in  live  weight.  When  the  weight  increases,  the 
food  is  providing  more  than  the  maintenance  requirements, 
and  the  excess  is  being  stored  in  proportion  to  the  increase 
in  weight.  When  the  weight  decreases  the  food  is  deficient, 
and  the  animal  is  using  its  own  tissues  as  a  source  of  energy. 
In  the  case  of  dairy  cows,  of  course,  the  milk  secreted  must 
be  taken  into  account. 

Provided  a  large  number  of  animals  be  considered  and 
the  weighing  be  continued  over  a  long  enough  period,  these 
experiments  give  results  of  practical  value.  Normally,  how- 
ever, the  weight  fluctuates  from  day  to  day,  especially  in  the 
case  of  ruminants,  owing  to  the  great  weight  of  the  contents 
of  the  intestinal  tract.  A  bullock  may  vary  from  day  to 
day  as  much  as  20  kilos,  the  change  being  in  the  contents 
of  the  gut,  and  not  in  the  living  tissue  of  the  animal. 

2.  Slaughter  Tests. — In  these  experiments  a  group  of 
animals  as  nearly  identical  as  possible  in  age  and  condition 
are  taken.  Some  are  killed  and  the  carcases  analysed.  The 
others  are  fed  for  varying  periods  before  being  killed  and 
analysed.  The  difference  in  those  analysed  before  feeding 
and  those  after  feeding  is  taken  as  an  indication  of  the  value 
of  the  food. 

In  these  experiments,  which  are  very  laborious  and 
require  a  long  time  to  carry  out,  it  is  assn,med  that  the 
animals  killed  at  the  beginning  of  the  experiment  are 
identical  in  percentage  composition  with  what  the  fed  group 
were  at  the  beginning. 

3.  The  "Balance  Sheet"  Experiment. — An  exact  determina- 
tion of  the  food  requirements  of  an  animal  and  of  the  value 
of  any  food-stuff  in  meeting  these  requirements  can  only  be 

24 


370  VETEKINARY   PHYSIOLOGY 

determined  when  a  complete  detailed  account  is  kept  of  the 
intake  and  output  of  both  energy  and  material.  This  can  be 
done  by  the  use  of  the  calorimeter  (p.  259),  which  registers 
the  amount  of  energy  liberated,  and  also  the  consumption  of 
oxygen  and  the  output  of  carbon  dioxide  in  the  expired  air. 
The  additional  information  obtained  by  analysis  of  the 
collected  urine  and  faeces  give  a  complete  account  of  the 
sum  of  the  changes  in  the  food  taking  place  within  the  body. 
The  energy  and  material  intake  in  the  food  can  be  compared 
with  the  energy  and  material  output  and  the  gain  or  loss 
determined.  The  comparatively  few  complete  balance  ex- 
periments which  have  been  done  on  large  animals  have 
afforded  valuable  information  as  to  food  requirements  and 
the  productive  value  of  the  different  feeding  stuffs  and  their 
constituents. 

Protein  Requirements. — Protein  beyond  the  requirements 
for  construction  and  repair  of  tissues  is  catabolised  yielding 
energy,  the  nitrogen  being  excreted  in  the  urine.  By  sub- 
stituting carbohydrates  and  fats  as  a  supply  of  energy  the 
protein  intake  may  be  reduced.  When  the  intake  is  reduced 
below  the  level  of  the  protein  requirement,  the  protein  of  the 
tissues  is  used  and  consequently  the  nitrogen  of  the  urine 
exceeds  that  in  the  food. 

The  minimum  protein  requirement  therefore  can  be  deter- 
mined by  the  lowest  intake  which  is  just  balanced  by  the 
urinary  nitrogen  plus  a  small  estimated  amount  for  loss  in 
nitrogen  excretions  in  faeces  and  in  hair,  hoofs,  etc. 

In  lactating  animals  account  must  be  taken  of  the  loss  to 
the  body  of  the  protein  of  the  milk.  In  young  animals,  the 
question  is  complicated  by  the  growth  of  new  tissue. 

The  minimum  amount  of  protein  is  not  necessarily  the 
optimum  amount.  It  is  found  in  man,  at  least,  that  when 
reduction  of  protein  in  the  diet  reaches  a  certain  level  more 
than  isodynamic  quantities  of  fats  and  carbohydrates  must  be 
substituted  to  maintain  nitrogenous  equilibrium,  so  that,  on  a 
very  low  protein  intake,  the  total  caloric  intake  is  increased. 
Further,  the  influence  on  the  availabiHty  of  the  food  (p.  367), 
and  also  the  fact  that  all  proteins  are  not  of  equal  value 
(p.    276),   must    be   considered.       It   is   usual    therefore    in 


FOOD   REQUIREMENTS  871 

computing  rations   to  allow  a  margin  of  safety  in  fixing  the 
nutritive  ratio. 

2.  Rations. 

Feeding  tables  are  contained  in  text-books  on  animal 
nutrition.  Owing  to  the  number  of  different  kinds  of 
animals  and  different  kinds  of  production  aimed  at  in  feeding, 
these  tables  are  somewhat  extended.  It  is  only  necessary 
here  to  indicate  the  physiological  principles  on  which  feeding 
standards  should  be  based. 

In  arranging  rations,  the  chief  consideration  is  that 
available  energy  and  material  will  be  supplied  in  sufficient 
amounts  to  yield  the  desired  products,  e.g.  work,  milk,  or 
increased  weight.  It  is  also  usually  necessary  to  arrange  by 
a  combination  of  different  feeding  stuffs  that  the  total  bulk 
will  be  suitable  for  the  animal.  In  practice,  other  considera- 
tions such  as  relative  costs,  manurial  values  (p.  380),  and 
the  labour  involved  in  preparing  the  food,  are  taken  into 
account  in  deciding  what  feeding  stuffs  and  what  combinations 
and  proportions  of  these  are  most  economical. 

1.  Maintenance. — A  maintenance  ration  is  one  that  supplies 
just  sufficient  energy  and  protein  to  sustain  life  without  pro- 
duction and  without  either  gain  or  loss  of  tissue.  This 
ration  is  the  commercial  base  line,  since  it  is  only  what  the 
animal  assimilates  beyond  its  maintenance  requirement  that 
can  be  transformed  to  a  marketable  product. 

The  ration  must  cover  the  basal  metabolism  (p.  264) 
plus  the  increased  metabolism  due  to  the  consumption  of  the 
food  (p.  272).  If  the  environment  be  below  the  critical 
temperature  (p.  271),  a  further  addition  is  necessary  for 
heat  production  to  maintain  the  body  temperature. 

A  horse  of  1000  lbs.  weight  housed  comfortably  and  doing 
no  work  requires  a  daily  ration  the  digestible  nutrients  of 
which  yield  about  -5  kilos  protein  and  15,000  Calories. 

2.  Growth — (1)  Material. — In  the  growing  animal  there  is 
formation  of  new  tissue  and  bone.  The  food  therefore  must 
be  rich  in  protein  and  ash.  The  rate  of  growth  diminishes 
from  birth  onward,  and,  therefore,  the  younger  the  animal  the 
higher  is  the  proportion  of  protein  and  ash  required.    Experi- 


372  VETERINARY  PHYSIOLOGY 

mental  results  obtained  by  Bull  and  Emraett  show  how  the 
protein  requirement  relative  to  weight  decreases  with  age. 


Age  (Lamb) 

Protein  requirement  in  food 

Months. 

per  1000  lbs.  live  weight. 

5 

0-32 

7 

0-27 

9 

0-23 

15 

0-17 

The  milk  of  the  mother  contains  the  material  necessary 
for  growth  in  the  requisite  proportions.  Bunge  has  shown 
that  the  proportion  of  protein  and  ash  in  milk  varies  with 
the  rapidity  of  growth. 


Time  in  days  to 

Milk  contains 

double  weight. 

Protein.                 Ash. 

Horse 

.     60 

2-0                  -4 

Calf 

.     47 

3-5                 -7 

Dog 

.       8 

7-1               1-3 

The  accessory  factors  (p.  281)  are  of  importance  in 
growing  animals  in  confinement  after  the  sucking  is  stopped, 
especially  when  the  food  is  cooked.  It  is  probable  that 
certain  diseases  to  which  young  pigs  are  liable  are  really 
nutritional  disorders  caused  by  the  absence  of  these. 

A  deficiency  of  calcium  and  phosphorus  limits  bone 
formation.  In  pigs  it  is  usual  to  allow  free  access  to  a 
mineral  mixture  containing  these. 

(2)  Energy. — The  available  energy  of  the  food  must  be 
sufficient  (1)  to  meet  the  maintenance  requirement,  i.e.  the 
amount  liberated  as  heat,  and  (2)  to  supply  an  amount  of 
energy  equal  to  that  contained  in  the  new  tissue  formed. 

The  maintenance  requirement  can  be  determined  by  the 
calorimeter  (p.  259).  It  is  greater  per  unit  of  surface  in 
the  growing  than  in  the  full-grown  animal.  It  is  certain 
that  energy  is  expended  in  the  structural  organisation  of 
growing  tissue,  but  to  what  extent  is  not  known. 

The  energy  content  of  the  new  tissue  is  determined  by 
slaughter  tests  (p.  369).  It  is  found  to  vary  with  age,  being 
less  in  very  young  animals,  where  the  increase  is  chiefly  in 
protein  tissue,  which  contains  about  75  per  cent,  of  water, 
and  greater  as  the  animal  approaches  maturity,  when  a 
higher  proportion  of  the  increase  is  fat. 


Protein 
Lbs. 

Digestible 

Nutvieuts, 

Calories. 

■63 

9,300 

•70 

10,974 

•78 

12,^276 

•85 

13,950 

•93 

15,^252 

FOOD   REQUIREMENTS  373 

Rations  vary  for  different  species.  According  to  Murray, 
the  food  for  crrowinsf  cattle  should  contain  as  follov/s  : — 

Live  Weight. 
Lbs. 

300 

400 
500 
600 

700 

3.  Fattening.  —  In  the  full-grown  animal  there  is  no  growth 
of  muscle  tissue  ;  the  process  of  fattening  therefore  consists 
essentially  of  changing  to  fat  the  food  absorbed  beyond  what 
is  utilised  by  the  animal  for  its  maintenance.  It  is  estimated 
that  when  the  maintenance  energy  requirements  have  been 
met,  about  40  to  50  per  cent,  of  the  excess  energy  of  the 
digestible  nutrients  of  the  food  can  be  stored  in  fat. 

Ordinary  food-stuffs  contain  very  little  fat.  Carbohydrates 
form  by  far  the  most  important  source  of  the  fat  deposited 
during  fattening  (p.  351).  It  is  doubtful  whether  proteins 
can  form  fat  except  indirectly  by  first  being  changed  to 
carbohydrates  (p.  854),  and  only  certain  of  the  amino-acids 
yield  these.  On  the  average  about  50  per  cent,  of  protein 
is  convertible  to  carbohydrates,  the  deaminised  portion  of 
the  remainder  being  catabolised,  yielding  heat  (p.  272).  In 
cold  weather  proteins  may  replace  carbohydrates  in  producing 
heat — an  uneconomical  process,  as  protein  is  the  most 
expensive  constituent  of  the  food.  In  full-grown  animals 
there  is  no  need  for  the  nutritive  ratio  to  be  higher  for 
fattening  than  for  maintenance.  If  the  ratio  be  kept 
constant,  the  increased  amount  of  food  will  supply  all  the 
extra  protein  that  the  animal  can  utilise  in  the  process  of 
fattening.  In  practice,  the  percentage  of  protein  is  often 
higher,  because  many  of  the  concentrated  foods  used  con- 
tain relatively  large  proportions  of  this  constituent.  It  is 
probable  that  on  a  diet  with  a  high  percentage  of  protein  the 
return  on  the  protein  in  fattening  is  in  increased  manurial 
value  rather  than  in  increased  fat  formation. 

As  the  object  in  fattening  is  to  get  the  greatest  increase 
in  weight  in  the  shortest  time,  the  animal  is  fed  to  the  fullest 


374  VETERINARY  PHYSIOLOGY 

capacity  of  its  digestive  powers,  and  concentrated  foodstuffs 
are  added  to  the  ration  to  increase  its  energy  value.  A 
bullock  should  increase  in  weight  about  two  pounds  per  day 
when  receivinsr  a  ration  containing  dis^estible  nutrients  vieldinsf 
30,000  Calories  and  -75  to  1'5  lbs.  of  protein. 

4,  Meat  Production. — In  meat  production,  the  animal  before 
it  reaches  maturity  is  confined  and  subjected  to  intensive 
feeding.  The  process  of  fattening  is  thus  superimposed 
upon  the  process  of  growth,  and  so  muscle  formation  and 
deposition  of  fat  proceed  simultaneously.  Some  of  the  fat 
is  deposited  between  the  muscle  bundles,  producing  the 
so-called  "marbling"  of  the  meat  upon  which  its  quality 
largely  depends. 

As  in  fattening  in  the  adult  animal,  the  object  desired  is 
to  get  as  large  an  increase  in  weight  per  day  as  possible.  The 
animal  therefore  is  allowed  to  eat  as  much  as  it  can  digest, 
and  concentrated  foods  are  given  to  increase  the  energy  value 
of  the  ration.  The  total  energy  of  the  food  given  is  usually 
about  twice  the  maintenance  requirement.  The  proportion 
of  protein  varies  with  the  rate  of  growth,  being  greater  the 
younger  the  animal.  According  to  Haecker's  standard,  the 
ration  of  meat-producing  bullocks  should  be  as  follows  : — 

Live  Weight. 
Lbs. 

200 

500 

1000 

5.  Milk  Production. — The  food  requirement  in  dairy  cows 
depends  upon  the  amount  of  milk  produced.  According  to 
Kellner's  experiments,  after  the  maintenance  requirement  is 
met,  from  60  to  70  per  cent,  of  the  excess  energy  of  the 
digested  food  is  recovered  in  the  energy  values  of  the 
constituents  of  the  milk.  The  protein  is  much  better 
utilised  for  milk  production  than  for  fat  formation.  In  some 
experiments  nearly  100  per  cent,  of  the  excess  above  main- 
tenance requirement  has  been  recovered  in  the  milk.  The 
value  of  protein  for  milk  production  varies  with  the  amino- 
acid  content  (p.  276).  Casein  stands  highest.  The  proteins 
of  maize  and  wheat  are  of  much  less  value. 


Dige.stible  Nutrients  per  1000  lbs 
Protein.                 Carbohvdrates. 
Lbs.                              Lte. 

3-05                    11-6 

live  weight. 
Lbs.' 

0-55 

1-90 

IM 

0-60 

1-64 

9 '5 

0-48 

FOOD   REQUIREMENTS  375 

Haecker's  standard  for  dairy  cows  allows  for  maintenance 
for  a  1000  lbs.  cow  0*7  lbs.  of  protein,  7-0  lbs.  of  carbo- 
hydrates, and  -1  lb.  of  fat.  The  additional  food  requirement 
for  the  production  of  the  milk  varies  with  the  composition 
of  the  milk,  which  is  judged  by  its  fat  content. 


^at  as  Milk. 

Digestible  Nutrients  required  per  lb. 

Milk. 

Per  Cent. 

Protein. 

Carbohydrates. 

Fat. 

2-5 

■0446 

•176 

•0151 

3-0 

•0469 

•199 

•0170 

3-5 

■0492 

■221 

•0189 

4-0 

■0539 

■242 

•0208 

4-5 

■0572 

■264 

•0226 

6.  Work. — The  efficiency  of  the  horse  and  its  capacity 
for  mechanical  work  have  been  considered  (p.  252).  (a)  The 
energy  of  the  food  must  cover  the  maintenance  requirement 
(p.  371).  If  the  efficiency  be  taken  as  88 i  per  cent.,  the 
food  must  in  addition  supply  3 J  times  the  energy  expended 
in  mechanical  work.  It  is  estimated  that  a  horse  of 
1000  lbs.  weight,  working  a  full  day  of  eight  hours,  will  do 
work  equivalent  to  between  4000  and  5000  Calories.  The 
food  therefore  should  contain  between  12,000  and  15,000 
Calories,  in  addition  to  the  maintenance  requirement. 

(6)  The  protein  metabolism  is  little  atfected  by  work,  so 
long  as  a  sufficient  supply  of  carbohydrates  and  fats  are 
available  as  a  source  of  energy  (p.  262).  There  is  therefore 
no  need  for  any  marked  increase  of  the  protein  above  the 
maintenance  requirement. 

The  food  requirement  for  a  working  horse,  doing  a  full 
eight-hour  day's  work,  should  yield  about  1  to  2  lbs.  protein  and 
80,000  Calories,  i.e.  nearly  double  the  maintenance  require- 
ment. 

3.  Feeding  Standards. 

Various  feeding  standards  have  been  proposed  as  guides 
to  feeders.  In  these,  the  food  required  for  different  animals 
and  for  different  kinds  of  production  are  usually  stated  in 
lbs.  for  an  animal  of  a  fixed  weight — 1000  lbs.  live  weight 
for  large  animals.  The  ration  for  any  individual  animal  can 
be  calculated  according  to  its  weight  (p.  265). 

Three  standards  which  are  commonly  used  may  be  briefly 


376  VETERINARY   PHYSIOLOGY 

indicated.      They  illustrate  three  different  methods  used  to 
compare  the  values  of  different  feeding  stuffs. 

(1)  The  Wolff-Lehmann  standards  were  first  published  by- 
Wolff  in  1864,  and  were  the  basis  of  further  work  done  on 
the  subject.  They  were  modified  later  by  Lehmann.  The 
requirements  are  stated  in  terms  of  lbs.  of  total  dry  sub- 
stances, and  of  digestible  protein,  carbohydrate,  and  fat. 
The  nutritive  ratio  is  also  given. 

These  standards  were  originally  based  on  the  conception 
that  protein  was  especially  valuable  for  fat  formation  and  for 
work.  The  protein  requirement  therefore  as  stated  is  too 
high.  The  part  of  the  food-stuff  taken  as  digestible  is  the 
apparent  digestible  portion  (p.  3  6  4).  This  is  not  a  certain  indi- 
cation of  its  energy  value  to  the  animal.  From  this  should 
be  deducted — (1)  the  loss  of  energy  in  fermentation,  and 
(2)  the  energy  of  certain  non- nitrogenous  substances  that 
appear  in  variable  amounts  in  the  urine  of  herbivora). 
These  substances  are  chiefly  derived  from  the  fodders,  and 
consequently  the  energy  value  of  fodders  as  calculated  from 
the  apparent  digestibility  is  less  than  the  real  energy  value. 
The  productive  value  of  a  food  therefore  is  not  always  the 
same  as  the  value  calculated  from  the  digestible  nutrients. 

(2)  Kellner's  Standards. — In  these  it  is  recognised  that 
the  apparent  digestibility  of  a  food  is  not  a  reliable  indication 
of  its  productive  value.  Foods  are  therefore  compared 
according  to  their  fat-forming  value.  Starch  is  taken  as  the 
standard,  and  it  is  estimated  by  Kellner  that  the  fat-forming 
power  of  the  digestible  nutrients  of  the  food  bear  the 
following  relationship  to  starch  : — 

1  part  protein  .  .  ^0-94  parts  starch  equivalent. 

1  part  fat,  in  coarse  fodders, 

chaff,  and  roots      .         .  =1-91  ,,  ,, 

1  part  fat,  in  grain              .  =2-12  „  „ 

1  part  fat,  in  oil  seeds        .  =2-41  ,,  ,, 
1  part  nitrogen-free  extract 

and  crude  fibre      .          .  =rOO  .,  „ 

The  value  of  a  food  is  determined  by  multiplying  the 


FOOD   REQUIREMENTS  377 

percentage  of  the  respective  digestible  constituents  by  these 
factors  and  then  deducting  a  certain  percentage  of  the  total. 
The  deduction  is  supposed  to  represent  the  loss  of  fat- 
forming  value  due  to  energy  expended  in  "  the  work  of 
mastication  and  of  digestion."  It  includes  loss  in  fer- 
mentation and  in  the  non-nitrogenous  substances  in  the 
urine.  The  percentage  to  be  deducted  is  indicated  in 
Kellner's  tables,  where  each  food- stuff  is  assigned  a  "  value  " 
which  varies  roughly,  inversely  with  the  amount  of  crude 
fibre  in  the  food.  The  lower  the  value  the  greater  is  the 
percentage  to  be  deducted.  It  is  taken  as  100  minus  the 
value  number.  The  method  of  calculation  is  shown  by  the 
following  example  given  for  rape  cake  : — 

24-8%  digestible  proteins  x  0-94  =  23-37^  starch  equivalent. 
6-3%  digestible  fat  x  2-41  =  15-27^ 

20-5%  digestible  nitrogen 

— free    extract  + 

crude  fibre  x  1  '00  =  20-57^ 

Total    .  59-07o  starch  equivalent. 

The  "value"  of  rape  cake  is  95,  so  5  per  cent,  is 
deducted.  The  final  result,  56  per  cent.,  is  termed  the 
"staTch  value,"  or  "starch  equivalent"  of  rape  cake.  It 
means  that  100  lbs.  of  rape  cake  is  equal  in  value  to  56 
lbs.  of  starch. 

Certain  modifications  of  the  method  of  calculation  are 
advised  in  the  case  of  food  stuffs  containing  much  crude 
fibre. 

In  Kellner's  standards,  food  requirements  are  stated  in 
lbs.  of  protein  and  lbs.  of  starch.  In  the  tables  showing  the 
composition  and  digestibility  of  the  various  food-stuffs,  a  starch 
value  is  assigned  to  each.  The  object  aimed  at  in  the  system 
is  to  give  a  common  basis  for  comparing  the  productive  value 
of  the  various  feeding  stuffs. 

This  starch  equivalent  method  of  comparing  the  values  of 
food-stuff  is  widely  used  in  this  country.  It  is  of  doubtful 
value.      The    use    of   a    pound    of   starch    as    the    unit    for 


378  VETERINARY   PHYSIOLOGY 

measuring  what  is  essentially  the  energy  of  a  food  leads  to 
confusion.  The  logical  as  well  as  the  simplest  unit  for  this 
is  the  Calorie.  The  method  of  calculating  the  starch  value  is 
open  to  question.  One  gram  of  protein  is  almost  certainly  not 
equal  to  0-94  grams  of  starch  for  fat  formation  on  account 
of  its  specific  dynamic  action  (p.  272).  In  foodstuffs  rich 
in  protein  the  error  must  be  considerable.  Even  if  the 
relative  starch  values  be  taken  as  accurate,  the  values  are 
for  fat  formation  and  are  not  applicable  without  error  for 
other  purposes. 

(3)  Armsby's  Standard. — In  these,  food  requirements  are 
stated  in  terms  of  pounds  of  digestible  protein  and  "therms" 
of  "net  energy,"  and  the  accompanying  tables  of  food  values 
give  the  lbs.  of  protein  and  therms  of  net  energy  per  100  lbs. 
of  the  different  food-stuffs.  A  therm  is  1000  calories. 
"  Net  energy  "  is  available  energy  minus  the  energy  liberated 
as  heat  in  the  increased  metabolism  due  to  food  con- 
sumption. It  is  regarded  as  the  "  productive  energy  value  " 
of  the  food. 

The  following  extracts  illustrate  the  way  in  which  the 
food  requirements  and  food  values  are  stated  : — 


Maintenance  requirements  for  Cattle. 


Live  weight. 

Digestible  Protein. 
Lbs. 

Net  energy 
Therms. 

500 

0-25 

3-78 

750 

0-38 

4-95 

1000 

0-50 

6-00 

Table  of  Food  Values. 


VALUES 

PER  100  LBS. 

FOR  RUMINANT. 

Dry 

Matter. 

Digestible. 

Crude 
Fibre. 

True 
Protein. 

Net  Energy 
Value. 

Grains. 
Cereal  Grains. 

Lbs. 

Lbs. 

Lbs. 

Therms. 

Corn  (maize)  meal   . 

88-7 

6-9 

64 

85-20 

Oats       . 

90-8 

9-7 

8-7 

67-56 

Oatmeal 

92-1 

12-8 

11-5 

86-20 

FOOD   REQUIREMENTS  379 

In  calculating  the  ration  "  true  protein "  is  taken  as 
"  digestible  protein."  Separate  tables  of  food  values  are 
given  for  ruminants,  horses,  and  swine. 

Armsby's  system  is  less  open  to  objection  than  Kellner's. 
If  the  net  energy  values  which  he  assigns  to  the  different 
feeding  stuffs  be  used  only  in  conjunction  with  his  feeding 
standards  no  error  is  likely  to  arise,  since  the  requirements 
are  estimated  in  accordance  with,  and  stated  in  terms  of, 
these  values. 

While  a  unit  of  productive  value  enables  the  feeder 
to  compare  different  food  -  stuffs  on  a  common  basis, 
its  use  is  apt  to  focus  attention  too  exclusively  on 
the  food  to  the  neglect  of  other  factors  that  affect  the 
productive  value  of  rations.  Two  of  these  may  be  briefly 
indicated. 

(1)  For  maintenance,  the  energy  value  for  food  depends 
upon  the  energy  that  can  be  liberated  by  the  animal,  and, 
consequently,  foods  can  be  compared  according  to  their 
caloric  values  (p.  257).  For  productive  purposes,  however, 
the  energy  value  of  different  foods  cannot  be  compared  on 
the  basis  of  any  common  unit.  For  example,  in  fattening, 
after  the  maintenance  requirements  are  satisfied,  of  the  surplus 
food  absorbed,  about  87-5  per  cent,  of  the  energy  of  the 
starch  can  be  transformed  to  energy  in  deposited  fat,  while 
only  a  doubtful  proportion,  certainly  less  than  50  per  cent, 
of  the  energy  of  the  surplus  protein,  can  be  so  transformed. 
On  the  other  hand,  if  protein  tissue  be  the  form  of  produc- 
tion, as  in  growth,  nearly  100  per  cent,  of  the  energy  of  the 
surplus  protein  may  be  deposited  in  the  growing  tissue, 
whereas  for  tissue  formation  starch  has  only  an  indirect 
value  as  a  protein  saver. 

(2)  In  both  Kellner's  and  Armsby's  systems,  in  estimating 
the  productive  value,  the  heat  liberated  in  increased  meta- 
bolism due  to  taking  food  is  regarded  as  waste  and  deducted. 
Whether  or  not  it  is  waste  depends  upon  the  external  tem- 
perature. Below  the  critical  temperature  (p.  271),  it  serves  to 
maintain  body  temperature,  and  so  saves  food  of  an  equiva- 


380  VETERINARY   PHYSIOLOGY 

lent  energy  value  from  being  catabolised  merely  as  fuel  for 
beat  production.  At  low  temperatures  more  than  the  amount 
of  heat  liberated  by  increased  metabolism  due  to  taking  food 
may  be  necessary,  and  an  amount  of  food  varying  with  the 
temperature  must  be  consumed  to  yield  heat,  leaving  a 
variable  surplus  for  production.  The  productive  value  of  a 
ration  therefore  is  not  a  fixed  quantity  applicable  to  all 
conditions. 

Whatever  feeding  standard  or  unit  ot  production  value  is 
used,  it  should  constantly  be  kept  in  mind  that  a  food-stuff 
has  only  two  real  energy  values: — (1)  The  amount  that  can 
be  transformed  to  heat  by  complete  combustion  in  a  bomb 
calorimeter  (p.  256) — the  j^hy sic al  energy;  and  (2)  the  amount 
that  can  be  transformed  by  the  animal  body — t\\Q  'physiological 
energy.  What  proportion  of  the  physiological  energy  is 
available  for  production  depends  upon — 

(1)  The  maintenance  requirement  of  the  animal,  which 
is  partly  determined  by  an  external  condition — the  tempera- 
ture. 

(2)  The  nature  of  the  product,  e.g.  milk,  fat,  meat,  or 
work. 

These  factors  are  not  qualities  of  the  food,  and  con- 
sequently the  absolute  productive  value  of  a  food  cannot  be 
determined  by  any  system  of  calculation  based  solely  on  per- 
centages of  digestible  nutrients.  The  productive  values 
given  are  merely  hypothetical,  and  error  arises  unless  they 
are  regarded  as  such. 

4.  Manurial  Values. 

In  farm  animals  the  further  question  of  the  manurial 
value  of  the  food  has  to  be  considered.  The  constituents  of 
the  excreta  which  are  of  special  manurial  vahie  are  nitrogen, 
phosphorus,  and  potash.  Since  so  little  nitrogen  is  fixed  in 
the  body  of  the  adult  animal  as  proteins,  the  greater  quantity 
of  the  nitrogen  of  the  food  is  recoverable  in  the  urine  and 
fa?ces.      In  ruminants  the  phosphorus  is  chiefly  excreted  in 


FOOD   REQUIREMENTS 


381 


the  faeces.  Hence,  by  careful  storage  of  the  excreta  and  its 
proper  disposal,  a  large  return  to  the  land  may  be  made. 
This  cycle  between  animal  and  plant  may  be  illustrated  by 
the  following  diasfram  : — 


BACTERIA 


A  MM  ON/A 
COMPOUNDS 


BACTERIA 


VEGETABLE 
PROTEJO 


Fig.  161. — Diagram  to  illustrate  Nitrogen  Cycle. 


The  nutritive  products  of  digestion  reach  the  blood  and 
have  to  be  distributed  to  the  muscles  and  other  tissues. 
The  oxygen  of  the  air  has  also  to  be  conveyed  to  the  tissues, 
while  the  products  of  combustion  must  be  removed. 

The  way  in  which  the  circulation  of  the  blood  is  carried 
out  must  next  be  considered. 

SECTION   IV. 

A.  The  Manner  in  which  the  Nourishing  Fluids  are 
Brought  to  the  Tissues. 

THE    CIRCULATION. 

I    GENERAL    CONSIDERATIONS. 

The  arrangement  by  which  the  blood  and  lymph  are  dis- 
tributed to  the  tissues  may  be  compared  to  a  great  irrigation 
system. 

It  consists  of  a  central  force  pump — the  systemic  heart 
(fig.  162,  S.H.) — from  which  passes  a  series  of  conducting 
tubes — the  arteries — leading  otf  to  every  part  of  the  body, 
and  ending  in  innumerable  fine  irrigation  channels — the 
capillaries  {Gap.) — in  the  substance  of  the  tissues.  From 
these  some  of  the  blood  constituents  are  passed  into  the 
spaces  between  the  cells  as  lymi^h.  From  these  spaces  the 
fluid  either  passes  back  into  the  capillaries,  or  flows  away  in 
a  series  of  lymph  vessels,  which  carry  it  through  lymph 
glands  (Ly.),  from  which  it  gains  certain  necessary  con- 
stituents, and  finally  bring  it  back  to  the  central  pump. 

The  fluid,  which  has  not  passed  out  of  the  capillaries  into 
the  tissues,  has  been  deprived  of  many  of  its  constituents, 
and  this  withdrawal  of  nutrient  material  by  the  tissues  is 
made  good  by  some  of  the  blood  being  sent  through  the 
walls  of  the  stomach  and  intestine  {Al.C),  in  which  the 
nutrient  material  of  the  food  is  taken  up  and  added  to  the 


CIRCULATION 


383 


blood  returning  to  the  heart.  At  the  same  time,  the  waste 
materials  added  to  the  blood  by  the  tissues  are  partly  got 
rid  of  by  some  of  the  blood  being  sent  through  the  liver  and 
kidneys  {Liv.  and  Kid.). 

The  blood  is  then  poured  back  through  the  veins  into  a 
subsidiary  pump — the  pulmonic  heart  (P.-ff.) — ^by  which  it  is 
pumped  through  the  lungs,  there  to  obtain  a  fresh  supply 
of  oxygen,  and  to  get  rid  of  the  carbon  dioxide  excreted 
into  it  by  the  tissues.  Finally  the  blood,  with  its  fresh 
supply  of  oxygen  from  the  lungs,  and    of  nourishing  sub- 


CAF 


Fig.  162. — Scheme  of  the  Circulation.  S.H.,  systemic  heart  sending  blood 
to  the  capillaries  in  the  tissues,  Cap.  The  blood  brought  back  by 
veins,  and  the  exuded  lymph  by  lymphatics,  Ly.,  passing  through 
glands  ;  blood  sent  to  the  alimentary  canal,  Al.C,  and  from  that  to 
the  liver,  Liv.  ;  blood  also  sent  to  the  kidneys,  Kid.  ;  the  blood  before 
again  being  sent  to  the  body  is  passed  through  the  lungs  by  the 
pulmonic  heart,  P.H. 

stances  from  the  alimentary  canal,  is  poured  into  the  great 
systemic  pump — the  left  side  of  the  heart — again  to  be 
distributed  to  the  tissues. 

Thus  the  circulation  is  arranged  so  that  the  blood, 
exhausted  of  its  nourishing  material  by  the  tissues,  is 
replenished  in  the  body  before  being  again  supplied  to  the 
tissues. 


The  sectional  area  of  the  vascular  system  varies 
enormously.  The  aorta  leaving  the  heart  has  a  compara- 
tively small  channel.  If  all  the  arteries  of  the  size  of  the 
radial  were  cut  across  and  put  together,  their  sectional  area 


384  VETERINARY    PHYSIOLOGY 

would  be  many  times  the  sectional  area  of  the  aorta.  And 
if  all  the  capillary  vessels  were  cut  across  and  placed 
together,  the  sectional  area  would  be  about  700  times  that 
of  the  aorta. 

From  the  capillaries,  the  sectional  area  of  the  veins  and 
lymphatics  steadily  diminishes  as  the  smaller  branches  join 
with  one  another  to  form  the  larger  veins  and  lymphatics  ; 
but,  even  at  the  entrance  to  the  heart,  the  sectional  area  of 
the  returning  tubes,  the  veins,  is  about  twice  as  great  as  that 
of  the  aorta. 

The  circulatory  system  may  thus  be  compared  to  a 
stream  which  flows  from  a  narrow  deep  channel,  the  aorta, 
into  a  gradually  broadening  bed,  the  greatest  breadth  of  the 
channel  being  reached  in  the  capillaries.  From  this  point 
the  channel  gradually  narrows  until  the  heart  is  reached. 

Hence  the  blood  stream  is  very  rapid  in  the  arteries 
where  the  channel  is  narrow,  and  very  sluggish  in  the 
capillaries  where  the  channel  is  wide,  so  that  in  them  plenty 
of  time  is  allowed  for  exchanges  between  the  blood  and  the 
tissues. 


II.   THE  CENTRAL  PUMP— THE  HEART. 

A.  Structure. 

1 .  Myocardium.— The  heart  in  the  early  embryo  is  a  simple 
tube  which  undergoes  regular  rhythmic  contractions.  These 
start  from  the  venous  end  and  pass  along  the  tube  to  the 
other  end,  and  thus  force  the  blood  from  the  veins  to  the 
arteries. 

As  development  advances,  a  receiving  chamber — the 
auricle,  and  an  expelling  chamber — the  ventricle,  grow  out 
from  the  primitive  tube,  and  thus  break  up  the  continuous 
sheath  of  primitive  tissue  which  constitutes  the  embryonic 
heart  (fig.  163). 

In  fish  this  primitive  tissue  is  found  as  a  ring  round  the 
entrance  of  the  veins  into  the  heart  with  an  extension,  as  a 
narrow  band  over  the  auricles  to  the  ventricles  (S.  V.). 

In  the  mammal,  where  the  heart  is  double,  one  part  of 


HEART 


385 


the  primitive  tissue — the  sino-auricular  node — is  found  at  the 
junction  of  the  superior  vena  cava  and  the  right  auricle,  the 
other — the  auriculo-ventricular  node — extends  from  a  point 
just  below  and  to  the  left  of  the  coronary  sinus  to  the  upper 
part  of  the  inter- ventricular 
septum  to  form  the  aur- 
iculo  -  ventricular  band. 
Here  it  divides  into  a  right 
and  left  bundle  which  pass 
down  under  the  lining  of 
the  heart  (the  endocardium) 
to  end  in  tine  ramifications 
in  the  papillary  muscles 
and  in  the  ventricular  wall. 
A  continuation  of  the  primi- 
tive tissue,  from  the  former 
to  the  latter  of  these  nodes, 
along  the  posterior  walls  of 
the  auricles,  has  been  de- 
scribed. 

The  essential  features  of 
cardiac  muscle  are  (i)  that  the  fibres  are  continuous  with 
one  another,  and  form  a  syncytial  network  ;  (ii)  that  the 
network  contains  fibrils  which  are  cross  striped  (p.  205)  ; 
(iii)  that  there  are  nuclei  placed  deeply  in  the  sarcoplasm  ; 
and  (iv)  that  a  sarcolemma  is  absent. 

In  the  nodal  tissue  and  auriculo-ventricular  band,  the 
fibres  have  a  larger  quantity  of  sarcoplasm  between  the 
fibrils  and  round  the  nuclei. 

The  muscular  fibres  of  auricles  and  ventricles  take  origin 
from  three  fibrous  rings — the  auriculo-ventricular  rings — 
(1)  one  encircling  the  opening  between  the  right  auricle  and 
ventricle,  and  crescentic  in  shape  ;  (2)  one,  more  circular  in 
shape,  encircling  in  common  the  left  auriculo-ventricular  and 
the  aortic  orifice,  and  (3)  one  encircling  the  pulmonary 
opening.  The  auricles  are  attached  to  the  auriculo- 
ventricular  rings  above,  the  ventricles  are  attached  below, 
while  the  valves  of  the  heart  are  also  connected  with 
them. 

25 


Fig.  163. — To  show  the  Development  of 
Auricle,  Ventricle,  and  Bulbus  on  the 
Primitive  Tube  of  the  Heart. 


386  VETERINARY  PHYSIOLOGY 

The  muscular  fibres  of  the  auricles  are  arranged  in  two 
badly-defined  layers — 

1st.  An  outer  layer  running  horizontally  round  both  auricles. 

2nd.  An  inner  layer  arching  over  each  auricle,  and  con- 
nected with  the  auriculo- ventricular  rings.  The  inmost  fibres 
are  raised  into  longitudinal  ridges — -the  pectinate  muscles. 

Contraction  of  the  first  layer  diminishes  the  capacity  of 
the  auricles  from  side  to  side.  Contraction  of  the  second 
set  with  the  pectinate  muscles  draws  the  ventricles  upwards 
towards  the  auricles,  and  thus  over  the  blood  .that  is  being 


Fig.  164. — Cross  Section  through  the  Ventricles  of  the  Heart  looking  towards 
Auricles,  to  show  the  right  Ventricle  placed  on  the  Central  Core  of  the 
left  Ventricle.  The  Cusps  of  the  Auriculo- ventricular  Valves  are  also 
shown. 

expelled  from  the  auricles,  and  also  pulls  the  auricles  down- 
wards towards  the  ventricles,  and  thus  diminishes  their 
capacity  from  above  downwards. 

The  peculiar  striped  muscle  fibres  of  the  auricular  wall 
extend  for  some  distance  along  the  great  veins  which  open 
into  these  chambers. 

The  left  ventricle  forms  the  cylindrical  core  to  the  heart, 
and  the  right  ventricle  is  attached  along  one  side  of  it.  The 
septum  between  the  ventricles  is  the  right  wall  of  the  left 
ventricle,  and  it  bulges  into  the  right  ventricle  with  a  double 
convexitv  from  above  downwards  and  from  before  backwards 
(fig.  164). 


HEART 


387 


The  muscle  fibres  of  the  ventricles  are  arranged  essentially 
in  three  layers  : — 

(1)  The  outmost  layer  takes  origin  from  the  auriculo- 
ventricular  and  pulmonic  rings,  and  passes  downwards  and 
to  the  left  till  it  reaches  the  apex  of  the  heart.  Here  it, 
turns  inwards,  forming  a  sort  of  vortex,  and  becomes  con- 
tinuous with  the  inmost  layer. 

(2)  The  middle  layer  is  composed  of  fibres  running  hori- 
zontally round  each  ventricle.      It  is  the  thickest  layer  of 


Tig. 


165. — The  Right  Ventricle  and  Tricuspid  Valve  to  show  the  relationship 
of  the  Papillary  Muscles  and  Chordse  Tendineae  to  the  Cusps  of  the 
Valve.     (See  text.) 


the   heart,    and   in    contracting   it    pulls    the   walls    of   the 
ventricles  towards  the  septum  ventriculi. 

(8)  The  inmost  layer  is  continuous  with  the  outmost 
layer,  as  it  turns  in  at  the  apex,  and  it  is  mixed  with 
primitive  fibres  of  the  auriculo-ventricular  band.  It  may  be 
considered  as  composed  of  two  parts — (a)  A  layer  of  fibres 
running  longitudinally  along  the  inside  of  each  ventricle 
from  the  apex  upwards  to  the  auriculo-ventricular  ring. 
These  fibres  are  raised  into  fleshy  columns,  the  columnse  carnese. 
(b)  A  set  of  fibres,  constituting  the  papillary  muscles  (fig.  165, 
F.M.),  which,  taking  origin  generally  from  the  apical  part  of 


388  VETERINARY  PHYSIOLOGY 

the  ventricles,  extend  freely  upwards  to  terminate  in  a  series 
of  tendinous  cords  (the  chordae  tendineae),  which  are  inserted 
partly  into  the  auriculo-ventricular  valves,  presently  to  be 
described,  and  partly  into  the  auriculo-ventricular  rings. 
The  papillary  muscles  are  merely  specially  modified  columni© 
carnese.  In  many  cases,  actual  muscular  processes  extend 
from  the  apex  of  the  papillary  muscles  to  the  auriculo- 
ventricular  ring. 

In  the  left  ventricle  there  are  two  papillary  muscles,  or 
groups  of  papillary  muscles,  one  in  connection  with  the 
anterior  wall  of  the  ventricle,  and  one  in  connection  with 
the  posterior  wall. 

In  the  right  ventricle  there  are — (1)  One  or  more  small 
horizontally  running  papillary  muscles  just  under  the  pul- 
monary orifice,  their  apices  pointing  backwards — (fig.  165, 
S.P.M.).  (2)  A  large  papillary  muscle  taking  origin  from 
the  mass  of  fleshy  columns  at  the  apex  of  the  ventricle 
{A.P.M.).  (3)  One  or  more  papillary  muscles  of  varying 
size  arising  from  the  posterior  part  of  the  apical  portion  of 
the  ventricle  {P. P.M.).  (4)  A  number  of  small  septal 
papillary  muscles  arising  from  the  septum. 

The  distribution  of  the  chordas  from  these  muscles  will  be 
considered  in  connection  with  the  auriculo-ventricular  valves. 

In  contraction,  the  outmost  and  inmost  layers  of  the 
ventricles  tend  to  approximate  the  apex  to  the  base  of  the 
ventricles,  but  this  is  resisted  by  the  contracting  middle 
layer.  The  apex  tends  to  be  tilted  towards  the  right,  the 
papillary  muscles  shorten,  the  columnse  carnese  by  their 
shortening  and  thickening  encroach  upon  the  ventricular 
cavity,  and  help  to  abolish  it,  while  the  auriculo-ventricular 
rings  are  drawn  inwards  towards  the  septum  and  downwards. 

2.  The  Distribution  of  Neurons  in  the  Heart. — (1)  In  the 
frog's  heart  the  nervous  structures  are  generally  described  as 
distributed  in  three  groups  or  ganglia,  but  the  separation 
of  these  from  one  another  is  artificial  (fig,  166). 

(a)  In  the  wall  of  the  sinus  venosus  a  plexus  of  nerve 
cells  and  nerve  fibres  constitutes  the  ganglion  of  the  sinus 
(Remak's  ganglion). 


HEART  389 

(6)  In  the  inter-auricular  septum  a  similar  plexus  consti- 
tutes the  ganglion  of  the  auricular  septum. 

(c)  In  the  auriculo-ventricuiar  groove  a  plexus  forms 
the  auriculo-ventricuiar  ganglion  (Bidder's  ganglion). 

With  these  intra-cardiac  ganglia  the  terminations  of  the 
cardiac  branch  of  the  vagus  nerve  to  the  heart  form  definite 
synapses. 

(2)  In  the  mammalian  heart,  on  the  right  side  an 
interrupted  chain  of  nerve  cells  extends  from  near  the  sino- 
auricular  node  to  the  posterior  part  of  the  inter-auricular 
septum  and  down  to  the  auriculo-ventricuiar  node.      On  the 


Fig.    166. — Scheme  of  the  Various  Chambers  of  the  Frog's  Heart  and  of  the 
Distribution  of  the  Intracardiac  Nervous  Mechanism, 

left  side,  a  similar  chain  of  cells  begins  rather  lower  on  the 
back  of  the  left  auricle  and  extends  inwards  to  the  septum 
to  end  near  the  auriculo-ventricuiar  node.  Through  all  the 
primitive  tissue  there  is  a  dense  plexus  of  nerve  fibres,  and 
this,  according  to  Dogiel  and  Bethe,  also  extends  between 
the  ordinary  fibres  of  auricles  and  ventricles.  Bodies  like 
muscle  spindles  have  been  described  among  the  fibres. 

3.  Pericardium. — The  myocardium  is  covered  by  a  layer 
of  fibrous  tissue  with  endothelium  on  its  surface.  Tliis  is 
the  visceral  pericardium.  The  parietal  pericardium,  in  which 
the  heart  lies,  is  a  fibrous  sac  lined  by  endothelium,  firmly 


390  VETERINARY  PHYSIOLOGY 

attached  to  the  great  vessels  above  and  to  the  diaphragm 
below. 

4.  Endocardium. — This  is  a  thin  layer  of  hbrous  tissue 
lined  by  endothelium  extending  from  the  vessels  over  the 
inner  aspect  of  auricles  and  ventricles.  At  certain  points 
flaps  of  this  endocardium  are  developed  to  form  the  valves 
of  the  heart. 

In  the  heart,  valves  are  situated  at  the  entrance  to  and 
at  the  exit  from  the  expeUing  cavities — the  ventricles. 
There  is  thus  on  each  side  of  the  heart  a  valve  between  the 
auricles  and  the  ventricles,  and  a  valve  between  the  ventricles 
and  the  great  arteries. 

(1)  Auriculo-ventricular  Valves. — On  each  side  of  the 
heart  the  auriculo-ventricular  valve  is  formed  by  flaps  of 
endocardium,  which  hang  downwards  from  the  auriculo- 
ventricular  ring  like  a  funnel  into  the  ventricular  cavity, 
and  which  are  attached  to  the  apices  of  the  papillary  muscles 
by  the  chordae  tendineag  (figs.  165  and  167). 

(i.)  On  the  left  side  of  the  heart  there  are  two  main 
cusps,  forming  the  mitral  valve  (fig.  167) — 

(a)  An  anterior  or  right  cusp,  which  takes  origin  from, 
and  is  continuous  with,  the  right  posterior  wall  of  the  aorta. 
It  hangs  down  into  the  ventricle  between  the  aortic  and 
auriculo-ventricular  orifices.  This  cusp  is  very  strong,  and 
the  chordae  are  inserted  chiefly  along  its  edges. 

(6)  The  posterior  or  left  cusp  is  smaller.  It  takes  origin 
from  the  back  part  of  the  auriculo-ventricular  ring.  The 
chordae  tendinese  are  not  only  inserted  into  its  edge,  but  run 
up  along  its  posterior  aspect  to  be  inserted  into  the  auriculo- 
ventricular  ring. 

When  the  papillary  muscles  contract,  the  cusps  are 
drawn  together.  The  edge  of  each  cusp  thins  out  to  form  a 
delicate  border,  which,  when  the  cusps  are  approximated, 
completely  seals  the  aperture. 

(ii.)  On  the  right  side  of  the  heart  the  auriculo- 
ventricular  orifice  is  separate  from  the  pulmonary  opening, 
and  the  three  cusps  of  the  tricuspid  valve  are  developed  in 
connection   with   the   crescentic    opening   from    the    auricle 


HEART  391 

(fig.  165).  One  rises  from  the  auriculo-ventricular  ring 
above  the  septum,  and  hangs  down  into  the  ventricle  upon 
the  septum.  The  two  larger  outer  cusps  are  held  down  by 
chordse  tendinete  rising  from  the  papillary  muscles  near  the 
apex  of  the  ventricle.  When  these  two  sets  of  papillary 
muscles  contract,  the  outer  cusps  are  drawn  flat  against  the 
septal  cusp  lying  upon  the  bulging  septum. 


Fig.  167. — Vertical  Mesial  Section  through  Heart  to  show  Aortic  and  Mitral 
Valves.  B.V.,  right  ventricle;  L.V.,  left  ventricle  with  papillary 
muscle;  L.A.,  left  auricle  with  the  mitral  valve  extending  into  the 
left  ventricle  ;  Ao.,  aorta  witli  anterior  cusp  on  top  of  septum. 

(2)  Semilunar  Valves. — The  valves  situated  at  the  opening 
of  the  ventricles  into  the  great  arteries  are  also  formed  as 
special  developments  of  the  endocardium. 

Each  is  composed  of  three  half-moon-shaped  membran- 
ous pouches  attached  along  their  curved  margin  to  the  walls 
of  the  artery  and  upper  part  of  the  ventricle,  and  with  their 
concavities  directed  away  from  the  ventricle.  In  the  centre 
of  the  free  margin  is  a  fibrous  thickened  nodule,  the  corpus 


392 


VETERINARY   PHYSIOLOGY 


Arantii,  from  which  a  very  thin  piece  of  membrane,  the 
lunule,  extends  to  the  attached  margin  of  the  edges.  A 
jDOUch,  the  sinus  of  Valsalva,  lies  behind  each  cusp. 

The  arrangements  of  these  various  cusps  is  of  import- 
ance in  connection  with  their  action  (fig.  167). 

(i.)  Aortic  Valve. — The  anterior  cusp  is  largest,  and  lies 
somewhat  deeper  in  the  heart  than  the  others.  At  each  side 
it  is  attached  to  the  aortic  wall,  but  below  it  is  attached  co 
the  upper  part  of  the  septum  ventriculi,  so  that  the  base  of 
the  sinus  of  Valsalva  is  formed  by  the  upper  part  of  the 


Fig.  168. — Relations  of  the  Thoracic  Viscera  in  the  Horse.  C,  heart ;  Bp. 
the  diaphragm;  exup.,  in  expiration;  insp.,  in  inspiration;  Vtntr, 
stomach;  /Sp/.,  spleen,     (i'rom  Ellenberger.) 


septum.  At  a  somewhat  higher  level  is  a  cusp  which  is 
partly  attached  to  the  upper  part  of  the  septum,  partly  to 
the  posterior  wall  of  the  aorta,  where  this  becomes  continuous 
with  the  anterior  cusp  of  the  mitral.  The  third  cusp  is  still 
higher,  and  is  attached  to  the  aortic  wall,  where  it  becomes 
continuous  with  the  anterior  cusp  of  the  mitral. 

(ii.)  Pulmonary  Valve. — The  posterior  cusp  is  mounted  on 
the  top  of  the  septum  ventriculi,  and  is  at  a  somewhat  lower 
level  than  the  other  two. 


HEART  393 

Thus,  in  each  valve  the  cusp  placed  lowest  is  mounted 
on  a  muscular  cushion,  the  use  of  which  will  afterwards  be 
considered. 


B.  Attachments  and  Relations  of  the  Heart. 

The  heart  is  attached,  by  the  great  vessels  coming  from 
it,  to  the  dorsal  wall  of  the  chest. 

In  the  horse  the  heart  hangs  downwards  from  the  vertebral 
column,  and  the  apex  is  in  relation  to  the  posterior  end  of 
the  sternum  and  a  little  to  the  left  (fig.  168). 

Behind,  the  heart  is  in  relation  to  the  tendon  of  the 
diaphragm, 

All  round  it  are  the  lungs,  completely  filling  up  the  rest 
of  the  thorax. 

The  heart  is  enclosed  in  a  strong  fibrous  bag,  the 
Pericardium,  which  supports  it  and  prevents  over-distension. 
When,  in  disease,  fluid  accumulates  in  this  bag  the  auricles 
are  pressed  upon  and  the  flow  of  blood  into  them  is  impeded. 


C.  Physiology  of  the  Heart. 
I.  The  Cardiac  Cycle. 

Each  part  of  the  heart  undergoes  contractions  and 
relaxations  at  regular  rhythmical  intervals,  and  the  sequence 
of  events  from  the  occurrence  of  any  one  event  to  its  recur- 
rence constitutes  the  cardiac  cycle. 


A.  Frog. 

In  the  frog  (fig.  169)  a  contraction,  starting  from  the 
openings  of  the  veins,  suddenly  involves  the  sinus  venosus, 
causing  it  to  become  smaller  and  paler.  This  contraction  is 
rapid  and  of  short  duration,  and  is  followed  by  a  relaxation, 
the  cavity  again  regaining  its  former  size  and  colour.  As 
this    relaxation    begins,    the     two    auricles    are    suddenly 


394 


VETERINARY  PHYSIOLOGY 


contracted  and  pulled  downwards  towards  the  ventricle,  at 
the  same  time  becoming  paler,  while  the  ventricle  becomes 
more    distended    and    of   a    deeper    red.     The    rapid    brief 

auricular  contraction  now 
gives  place  to  relaxation, 
and,  just  as  this  begins, 
the  ventricle  is  seen  to 
become  smaller  and  paler, 
and,  if  held  in  the  fingers, 
is  felt  to  become  firmer. 
This  event  takes  place 
more  slowly  than  the 
contraction  of  either  sinus 
or  auricles.  The  chief 
change  in  the  ventricle 
is  a  diminution  in  its 
lateral  diameter,  though, 
it  is  also  decreased  in  the 
antero-posterior  and  verti- 
cal directions.  During 
ventricular  contraction  the  bulbus  is  seen  to  be  dis- 
tended and  to  become  of  a  darker  colour.  The  ventricular 
contraction  passes  off  suddenly,  the  ventricle  again  becom- 
ing larger  and  of  a  deep  red  colour.  At  this  moment 
the  bulbus  aortas  contracts  and  becomes  pale  and  then 
relaxes  before  the  next  ventricular  contraction  {Practical 
Physiology). 

Each  chamber  of  the  heart  thus  passes  through  two 
phases — a  contraction  phase,  a  systole,  of  short  duration  and 
a  longer  relaxation  phase,  the  diastole.  The  sequence  of 
events  in  the  frog's  heart  might  be  schematically  represented 
as  in  fig  169. 


SINUS. 


AUKicu:> 


VffiTmCLEL 


3ULBU5 


Fig.  169.— Scheme  of  the  Cardiac  Cycle 

in  the  Frog.  S.S. ,  sinus  systole  ;  A.S., 
auricular  systole;  V.S.,  ventricular 
systole;  B.S.,  bulbus  systole;  P., 
rest  of  all  chambers.  The  upstrokes 
represent  systole,  the  downstrokes 
diastole. 


B.  Mammal. 


1.  Rate  of  Recurrence. — The  rate  of  recurrence  of  the 
cardiac  cycle  varies  with  the  animal  examined.  In  man  it 
is,  in  adult  life,  about  72  per  minute.  In  the  adult  horse 
it  is  about  36  to  40  per  minute. 


HEART 

f  heart  per  minute  in  different  animals  :- 

Horse  . 

36  to  40 

Ox        . 

45  to  50 

Sheep  . 

70  to  80 

Dog      . 

90  to  100 

Rabbit . 

120  to  150 

395 


Many  factors  modify  the  rate  of  the  heart,  among  the 
most  important  of  which  are — 

(1)  The  Period  of  Life. — The  following  table  shows  the 
average  rate  of  the  heart  at  different  aofes  : — 


Horse 


New  born 
Under  1  year 
4  years 


92  to  132  per  minute 
50  to  68 
50  to  56 


(2)  The  Period  of  the  Bay. — The  rate  is  generally  lowest 
in  the  early  morning,  and  quickest  in  the  evening. 

(3)  Temperature  of  the  Body. — The  rate  varies  with  the 
body  temperature,  in  man  being  increased  about  ten  beats 
with  each  degree  Fahr.  of  elevation  of  temperature. 

(4)  The  condition  of  the  central  nervous  system  may 
modify  the  rate  of  the  heart,  any  disturbance  accompanied 
by  emotional  changes  either  accelerating  or  retarding  it. 

(5)  Muscular  exercise  markedly  accelerates  the  heart 
(Practical  Physiology). 

2.  Sequence  of  Events. — The  sequence  of  events  making 
up  the  cardiac  cycle  is  simpler  in  the  mammal  than  in  the 

frog- 

(1)  The  contraction  starts  in  the  neighbourhood  of  the 
sino-auricular  node.  This  is  indicated  by  the  fact  that  Lewis 
has  found  that  this  region  is  the  first  to  become  electro- 
positive, or  "  zincy,"  to  the  rest  of  the  auricles  (p.  212).  The 
contraction  spreads  out  rapidly  in  all  directions,  over  the 
auricles  and  up  the  mouths  of  the  great  veins,  as  the  circle 
of  waves  produced  by  throwing  in  a  stone  pass  over  the 
surface  of  the  pond.      It  next  seems   to  pass    to  the  strip 


396 


VETERINARY  PHYSIOLOGY 


of  primitive  tissue  along  the  back  of  the  auricular  septum, 
and  then  to  the  mouths  of  the  great  veins  partially  occluding 
them,  then  over  the  rest  of  the  auricles,  which  become 
smaller  in  all  directions  and  seem  to  be  pulled  down  towards 
the  ventricles.  The  contraction  of  the  auricles  in  mammals 
is  not  accompanied  by  so  marked  a  dilatation  of  the 
ventricles  as  in  the  frog. 

(2)  The  wave  of  contraction  in  the  auricles  is  propagated 
to  the  ventricles  through  the  auriculo-ventricular  band,  and 
when  this  is  diseased  the  passage  of  the  wave  of  contraction 
is  interfered  with. 

(3)  As  the  ventricles  contract,  the  auricles  relax. 

The  ventricular  contraction  develops  suddenly,  lasts  for 
some  time,  and  then  suddenly  passes  off.  The  wave  of 
contraction  is  chiefly  conducted  by  the  primitive  tissue  which 
runs  on  the  interior  aspect  of  the  ventricles.     Lewis  has  shown 


AVRICU6 


^/LtiTRICLE^ 


Fig.  170. — Scheme  of  the  Cardiac  Cycle  in  the  Human  Heart.     A.&.,  auri- 
cular sj'stole  ;    V.S.,  ventricular  systole  ;  P.,  pause. 

that,  if  this  layer  is  divided,  conduction  is  markedly  delayed, 
while,  if  the  outer  fibres  of  the  ventricles  are  cut,  no  marked 
delay  occurs. 

(4)  The  contraction  of  the  ventricles  is  followed  by 
a  period  during  which  both  auricles  and  ventricles  remain 
relaxed.      This  is  called  the  pause  of  the  cardiac  cycle. 

The  cardiac  cycle  in  mammals  may  be  represented  as  in 
fig.  170. 


iA 

r 

1     1  / 

|a51        \i.5. 

\ 

/ 

\      P 

3.  Duration  of  the  Phases. — Ventricular  systole  lasts 
three  times  as  long  as  auricular  systole. 

The  duration  of  these  two  phases  in  relationship  to  the 
pause  varies  very  greatly.     Whatever  may   be  the  rate   of 


HEART  397 

the  heart,  the  auricular  and  ventricular  systoles  do  not  vary, 
but  in  a  rapidly  acting  heart  the  pause  is  short,  in  a  slowly 
acting  heart  it  is  long.  Taking  the  ordinary  human  heart  rate 
of  72  per  minute,  the  auricular  systole  lasts  for  one-eighth 
of  the  whole  cardiac  cycle,  the  ventricular  for  three-eighths, 
and  the  pause  for  four-eighths. 

4.  Changes  in  the  Shape  of  the  Chambers. 

1.  Auricles.  —  These  simply  become  smaller  in  all 
directions  during  systole. 

2.  Ventricles. — The  changes  in  the  diameters  of  the 
ventricles  may  be  studied  by  iixing  them  in  the  various 
phases  of  contraction  and  measuring  the  alterations  in 
the  various  diameters. 

The  shape  in  diastole  when  the  muscular  fibres  are 
relaxed  is  determined  by  the  fibrous  pericardium  which 
surrounds  the  heart,  and  by  the  position  of  the  body, 
the  force  of  gravity  leading  to  the  expansion  of  the 
ventricles  at  their  dependent  part.  The  condition  at  the 
end  of  systole  may  be  studied  by  rapidly  excising  the  heart 
while  it  is  still  beating,  and  plunging  it  in  some  hot  solution 
to  fix  its  contraction. 

The  condition  iii  the  early  stage  of  systole,  before  the 
blood  has  left  the  ventricles,  may  be  studied  by  applying  a 
ligature  round  the  great  vessels,  and  then  plunging  the  heart 
in  a  hot  solution  to  cause  it  to  contract  round  the  contained 
blood  which  cannot  escape. 

Measurements  of  hearts  so  fixed  show  that,  at  the 
beginning  of  contraction,  the  antero-posterior  diameter  is 
increased,  while  the  lateral  diameter  is  diminished.  In 
contracting,  the  lateral  walls  appear  to  be  pulled  towards  the 
septum — the  increase  in  the  antero-posterior  diameter  being 
largely  due  to  the  blood  in  the  right  ventricle  pressing  on 
and  pushing  forward  the  thin  wall  of  the  infundibulum  below 
the  pulmonary  artery. 

As  the  ventricles  drive  out  their  blood,  both  antero- 
posterior and  lateral  diameters  are  diminished — but  the 
diminution  in  the  lateral  direction  is  the  more  marked,  and 
thus  the  section  of  the  heart  tends  to  become  more  circular. 


398  VETERINARY   PHYSIOLOGY 

There  is  no  great  shortening  in  the  long  axis  of  the 
ventricles ;  but  the  auriculo-ventricular  grooves  are  drawn 
somewhat  downwards  towards  the  apex,  which  does  not  alter 
its  position.  This  was  demonstrated  by  Leonardo  da  Yinci 
in  the  living  pig  by  inserting  long  pins  through  the  chest 
wall  into  the  wall  of  the  ventricle  and  observing  the 
movements. 

In  systole  the  ventricles  have  the  form  of  a  truncated 
cone. 

5.  The  Cardiac  Impulse. 

(1)  Cause. — During  contraction  the  heart  undergoes,  or 
attempts  to  undergo,  a  change  in  position.      In  the  relaxed 


Fig,  171. — Cardiograph  consisting  of  a  Receiving  Tambour,  with  a  button 
on  the  membrane  which  is  placed  upon  the  cardiac  impulse,  and  a 
Recording  Tambour  connected  with  a  lever. 


condition  it  hangs  downwards  from  its  plane  of  attachment, 
but  when  it  becomes  rigid  in  ventricular  contraction,  it  tends 
to  take  a  position  at  right  angles  to  its  base — Cor  sese  erigere, 
as  Harvey  describes  the  movement.  Since  the  apex  and 
front  wall  are  in  contact  with  the  chest,  the    result   of    this 


HEART  399 

movement  is  to  press  the  heart  more  forcibly  against  the 
chest  wall.  This  gives  rise  to  the  cardiac  impulse  with 
each  ventricular  systole  (fig.  168),  but  this  is  not  easily  felt 
in  the  horse  unless  the  action  of  the  heart  is  exaggerated. 

If  the  chest  is  opened  and  the  animal  placed  on  its  back 
this  elevation  of  the  apex  is  readily  seen. 

(2)  Position. — The  position  of  the  impulse  is  determined 
by  the  relationship  of  the  heart  to  the  anterior  chest  wall 
and  to  the  lungs. 

(3)  Character. — It  is  felt  as  a  forward  impulse  of  the 
tissues,  which  develops  suddenly,  persists  for  a  short  period, 
and  then  suddenly  disappears.  In  many  forms  of  heart 
disease  its  character  is  markedly  altered. 

The  cardiac  impulse  may  be  recorded  graphically  by  means 
of  any  of  the  various  forms  of  cardiograph  (fig,  173).  One  of 
the  simplest  consists  of  a  receiving  and  a  recording  tambour 
connected  by  means  of  a  tube  (fig.  171)  (Practical 
Physiology). 

6.  Changes  in  the  Intracardiac  Pressure. — These 
have  been  studied  in  the  horse  and  dog. 

(1)  Methods. — The  most  common  way  of  determining 
the  pressure  in  a  cavity  is  to  connect  it  to  a  vertical  tube 
and  to  see  to  what  height  the  fluid  in  the  tube  is  raised. 
If  such  a  method  be  applied  to  the  ventricles  of  the  heart, 
the  blood  in  the  tube  undergoes  such  sudden  and  enormous 
changes  m  level  that  it  is  impossible  to  get  accurate  results. 

The  same  objection  applies  to  the  method  of  connecting 
the  heart  with  a  manometer,  a  U  tube  filled  with  mercury. 
When  this  is  done,  the  changes  in  pressure  are  so  sudden 
and  so  extensive  that  the  mercury  cannot  respond  to  them 
on  account  of  its  inertia. 

Various  means  of  obviating  these  difficulties  have  been 
devised.  (1)  One  of  the  best  is  to  allow  the  changes  of  pressure 
to  act  upon  a  small  elastic  membrane  tested  against  known 
pressures.  A  tube  is  thrust  through  the  wall  of  the  heart 
and  connected  with  a  tambour  covered  by  a  membrane  to 
which  a  lever  is  attached.  (2)  Probably  the  most  delicate 
method  is  by  the  use  of  Piper's  stilette  manometer  (fig.  172), 


400  VETERINARY   PHYSIOLOGY 

which  consists  of  a  tube  or  cannula  with  a  sharp  pointed 
trocar,  which  can  be  thrust  out  of  the  end  of  the  tube  to 
perforate  the  chest  and  ventricular  wall,  and  then  retracted 
through  a  tap,  which  can  be  closed.  On  the  tube  is  a 
membrane  carrying  a  small  mirror,  from  which  a  beam  of 
light  may  be  reflected  on  to  a  sensitive  paper  covering  a 
moving  surface  so  that  the  variations  of  pressure  are 
photographed. 

(2)  Results. — A.  Pressure  in  the  Great  Veins  (small 
dotted  line  in  fig.  173). — The  pressure  in  these  is  so  low 
and  undergoes  such  small  variations  that  it  may  be 
investigated  by  a  water  manometer. 

When  the  auricles  contract,  the  flow  of  blood  from  the 
great  veins  into  these  chambers  is  arrested,  and,  as  a  result, 
the  pressure  in  the  veins  rises.      As   the  auricles  relax  the 


D 

Fig.  172. — Piper's  Stilette  Manometer.  A.,  trocar  for  puncturing  (with- 
drawn) ;  C,  tap  to  close  cannula;  E.,  rubber  membrane  with 
mirror,  F. 

pressure  falls,  but,  as  the  auricles  fill  up,  it  again  rises. 
When  the  ventricles  relax  blood  again  flows  in  from  the 
great  veins  and  the  pressure  falls,  again  to  rise,  as  the 
auricles  and  veins  are  both  filled  up,  towards  the  end  of  the 
pause. 

B.  Pressure  in  the  Auricles  (dash  line  in  fig.  173). — 
At  the  moment  of  auricular  contraction  there  is  a  marked 
rise  in  the  intra-auricular  pressure.  When  the  auricular 
systole  stops,  the  pressure  falls  rapidly,  but  the  fall  is 
interrupted  by  a  rise  due  to  the  upward  pressure  from  the 
closed  auriculo-ventricular  valves.  It  reaches  its  lowest 
level  early  in  ventricular  systole.  From  this  point  the 
pressure  in  the  auricles  rises  until  the  moment  when  the 
ventricles  relax,  when  another  fall  in  the  pressure  is 
observed.      The   pressure    again   rises   slightly    and   remains 


HEART 


401 


p.  A3 


ii 


mmmm 


TLOW    of  BLOOD 

from 


I    CrcM  Veins  to  Auricjes 


8.  Auricles  to  Ventricles 


a  Ventricles  to  Arteries 


CLOSURE    of 

I    Auriculo-Ventncular  Valve 


2  Semilunar  Valves 


SOUNDS    of  HEART 


CARDIAC     IMPULSE 


/ 


ELECTRO    CARDIOGRAM 


Fig.  173.  — Diagram  to  show  the  Relationship  of  the  Events  in  the  Cardiac 
Cycle    to  one  another.      A.S.,    auricular   systole;     V.S.,    ventricular 
systole  ;  P.,  pause. 
26 


402  VETERINARY  PHYSIOLOGY 

about   constant   from   this  point   until    the    next    auricular 
contraction. 

C.  Pressure  in  the  Ventricles  (continuous  line  in  fig. 
173). — The  intra-ventricular  pressure  rises  slightly  during 
auricular  systole.  It  rises  suddenly  at  the  moment  of 
ventricular  systole  to  reach  its  maximum,  but  on  the  trace 
there  is  sometimes  a  shoulder  due  to  the  opening  of  the  semi- 
lunar valves.  It  then  falls,  but  the  fall  is  gradual,  and  is 
interrupted  by  a  more  or  less  well-marked  period  during 
which  the  pressure  remains  constant.  When  the  ventricles 
relax,  the  pressure  suddenly  falls  to  zero,  then  rises  a 
little,  and  is  maintained  until  the  next  ventricular  systole. 
The  diastolic  expansion  of  the  ventricle  is  due  chiefly  to 
the  inflow  of  blood  from  the  auricles  and  veins,  possibly  in 
part  to  the  elasticity  of  the  muscular  wall,  and  to  the  filling 
of  the  coronary  arteries  which  takes  place  in  diastole. 

D.  Pressure  in  the  Arteries  (dot-dash  line  in  fig.  173). — 
This,  since  it  is  always  high  and  undergoes  no  great  and 
sudden  variations,  may  be  measured  by  means  of  a  mercury 
manometer.  The  aortic  pressure  is  high  throughout. 
There  is  a  sudden  rise  soon  after  the  beginning  of  ventricular 
systole,  as  the  blood  rushes  out  of  the  ventricles.  The 
pressure  then  falls,  but  the  fall  is  not  steady.  Often  it  is 
interrupted  by  a  more  or  less  marked  increase  corresponding 
to  the  later  part  of  the  ventricular  contraction.  At  the 
moment  of  ventricular  diastole  the  fall  is  very  sharp  and  is 
interrupted  by  a  well-marked  and  sharp  rise.  Following 
this,  the  fall  is  continuous  till  the  next  systohc  eleva- 
tion. 

These  changes  in  the  pressure  in  the  different  chambers 
are  due  to— 

\st  The  alternate  systole  and  diastole  of  the  chambers, 
the  first  raising,  the  second  lowering,,  the  pressure  in  the 
chambers. 

Ind.  The  action  of  the  valves. 

7.  Action  of  the  Valves  of  the  Heart. 

A.  Auriculo-ventricular  (tig.  174). — These  valves  have 
already    been   described   as  funnel-like   prolongations  of  the 


HEART 


403 


auricles  into  the  ventricles.  They  are  firmly  held  down  in 
the  ventricular  cavity  by  the  chordae  tendinete.  When  the 
ventricles  contract,  the  papillary  muscles  pull  the  cusps  of 
the  valves  together  and  thus  occlude  the  opening  between 
auricles  and  ventricles.  The  cusps  are  further  pressed  face 
to  face  by  the  increasing  pressure  in  the  ventricles,  and  they 
may  become  convex  towards  the  auricles.  They  thus  form 
a  central  core  around  and  upon  which  the  ventricles 
contract. 

On  the  left  side  of  the  heart,  the  strong  anterior  cusp  of 
the  mitral  valve  does  not  materially  shift  its  position.  It 
may  be  somewhat  pulled  backwards  and  to  the  left.  The 
posterior  cusp  is  pulled  forwards  against  the  anterior. 

On  the  right  side,  the  infundibular  cusp  of  the  tricuspid 


1.  2.  .3.  4.  5. 

Fk;.  174. — State  of  the  various  parts  of  the  Heart  throughout  the  Cardiac- 
Cycle.  1,  auricular  s3-stole  ;  2,  beginning  of  ventricular  systole  (latent 
period)  ;  3,  period  of  outflow  from  the  ventricle  ;  4,  period  of  residual 
contraction  ;  5,  beginning  of  ventricular  diastole. 

valve  is  stretched  between  the  superior  and  inferior  papillary 
muscles,  and  is  tluis  pulled  towards  the  bulging  septum, 
against  which  it  is  pressed  by  the  increasing  pressure  inside 
the  ventricles.  The  posterior  cusp  has  its  anterior  margin 
pulled  forward  and  its  posterior  margin  backwards,  and  is 
thus  also  pulled  toward  the  septum.  The  septal  cusp  remains 
against  the  septum.  The  greater  the  pressure  in  the 
ventricle,  the  more  firmly  are  the  two  outer  cusps  pressed 
against  the  septum,  and  the  more  completely  is  the  orifice 
between  the  auricle  and  the  ventricle  closed.  On  the  right 
side  of  the  heart  other  factors  play  an  important  part  in 
occluding  the  orifice  ;  the  muscular  fibres  which  surround 
the  auriculo-ventricular  opening  contract,  and  the  papillary 
muscles  pull  the  auriculo-ventricular  ring  downwards  and 
inwards  by  means  of  the  chordae  which  are  inserted  into  it. 


404  VETERINARY   PHYSIOLOGY 

Nevertheless,  the  occlusion  of  this  orifice  is  apt  to  be  in- 
complete when  the  right  side  of  the  heart  becomes  in  the 
least  over-distended,  and  this  gives  rise  to  what  may  be 
called  a  safety-valve  action  from  the  right  ventricle,  which 
prevents  over-distension. 

The  auriculo-ventricular  valves  are  open  duHng  the 
whole  of  the  cardiac  cycle,  except  during  the  ventricular 
systole  (fig.  173). 

B.  Semilunar  Valves. — Before  the  ventricles  contract 
these  valves  are  closed  and  the  various  segments  pressed 
together  by  the  high  pressure  of  blood  in  the  arteries. 

As  the  ventricles  contract  the  pressure  rises,  until  the 
intra- ventricular  pressure  becomes  greater  than  the  pressure 
in  the  arteries.  This  is  the  lyresphygmic  period.  Then  the 
cusps  of  the  valves  are  thrown  back  and  remain  open  until 
the  blood  is  expelled.  When  the  outflow  of  blood  is  com- 
pleted, the  cusps  are  again  approximated  by  the  pressure  of 
blood  in  the  arteries.  As  relaxation  of  the  ventricles  occurs, 
the  intra-ventricular  pressure  becomes  suddenly  very  low, 
and  the  high  pressure  of  the  blood  in  the  arteries  at  once 
falls  upon  the  upper  surfaces  of  the  cusps,  which  are  thus 
forced  together  and  downwards,  and  completely  prevent  any 
back-flow  of  blood. 

The  prejudicial  effect  of  too  great  pressure  upon  these 
cusps  is  obviated  by  the  lower  cusp  of  each  being  mounted 
on  the  top  of  the  muscular  septum  upon  which  the  pressure 
falls — the  other  cusps  shutting  down  upon  this  one  (fig. 
167). 

The  semilunar  valves  are  open  only  during  the  flow  of 
blood  from  the  ventricles  to  the  arteries  in  the  second  and 
third  periods  of  ventricular  systole  (fig.  173). 

8.  The  Flow  of  Blood  through  the  Heart. — The  circula- 
tion of  blood  through  the  heart  depends  upon  the  difterences  of 
pressure  in  the  difterent  chambers  and  upon  the  action  ot 
the  valves. 

A.  From  Great  Veins  into  Auricles. — This  occurs  when 
the  pressure  in  the  great  veins  is  greater  than  the  pressure 
in  the  auricles  (fig.  173). 


HEART  405 

The  pressure  in  the  auricles  is  lowest  at  the  moment  of 
their  diastole.  At  this  time  there  is  therefore  a  great  flow 
of  blood  into  them,  but  gradually  this  becomes  less  and  less, 
until,  when  the  ventricles  dilate,  another  fall  in  the  auricular 
pressure  takes  place  and  another  rush  of  blood  from  the 
great  veins  occurs.  Gradually  this  diminishes,  and,  by  the 
time  that  the  auricles  contract,  the  flow  from  the  great  veins 
has  stopped. 

The  contraction  of  the  mouths  of  the  great  veins  in 
auricular  systole  drives  blood  from  the  veins  into  the 
auricles,  and  prevents  any  back-flow  from  the  auricles. 

B.  From  Auricles  to  Ventricles. — As  the  ventricles  dilate, 
a  very  low  pressure  develops  in  them,  and  hence  a  great  rush 
of  blood  occurs  from  the  auricles.  During  the  later  stage 
of  ventricular  diastole,  the  intra-ventricular  pressure  becomes 
nearly  the  same  as  the  intra-auricular,  and  the  flow 
diminishes  or  may  stop.  When  the  auricles  contract,  a 
higher  pressure  is  developed  causing  a  fresh  flow  of 
blood  into  the  ventricles.  When  the  ventricles  contract 
the  auriculo- ventricular  valves  are  closed,  and  all  flow  of 
blood  from  the  auricles  is  stopped  (fig.  178). 

G.  From  Ventricles  to  Arteries. — When  the  ventricles 
begin  to  contract,  the  intra-ventricular  pressure  is  low,  while 
the  pressure  in  the  arteries  is  high,  which  keeps  the  semi- 
lunar valves  shut.  This  is  the  Latent  or  Presphygmic 
Period.  As  ventricular  systole  goes  on,  the  intra-ventricular 
pressure  rises,  until,  after  about  0"03  of  a  second,  it  becomes 
higher  than  the  arterial  pressure.  Immediately  the  semi- 
lunar valves  are  forced  open  and  a  rush  of  blood  occurs 
from  the  ventricles.  This  is  the  Period  of  Outfioiv,  which 
usually  lasts  less  than  0'2  second. 

(rt)  If  the  ventricles  are  contracting  actively,  and  if 
the  pressure  in  the  arteries  does  not  offer  a  great  resistance 
to  the  entrance  of  the  blood,  the  ventricles  rapidly  empty 
themselves  into  the  arteries,  and  the  intra-ventricular  pressure 
varies  as  shown  in  fig.  175,  b. 

(6)  If  the  heart,  however,  is  not  contracting  actively,  or  if 
the  arterial  pressure  offers  a  great  resistance  to  the  entrance  of 
blood,  then  the  outjlow  is  slow  and  more  continued,  and  in 


406 


VETERINARY   PHYSIOLOGY 


this  case,  the  trace  of  the  intra-ventricular  pressure  is  as  in 
fig.  175,  a,  with  a  well-marked  Period  of  Residual  Con- 
traction. It  is  not  the  absolute  force  of  the  cardiac 
contraction  or  the  absolute  intra-arterial  pressure  which 
governs  this,  but  the  relationship  of  the  one  to  the  other. 
The  heart  may  not  be  acting  very  forcibly,  but  still,  if  the 
pressure  in  the  arteries  is  low,  its  action  may  be  relatively 


Fig.    175. — Diagram  to  show  the  Relationship  of  the   Pulse    Wave   to   the 
Cardiac  Cycle  and  the  effect  of  altering  the  relationship  between  the 

activity  of  the  heart  and  the  arterial  blood  pressure. h  is 

the  curve  of  intra-ventricular  pressure,  and h^  is  a  pulse  curve 

with  an  active  heart  and  a  relatively  low  arterial  pressure.  — a  and  a^ 
are  the  same  with  a  sluggish  heart  and  a  relatively  high  arterial 
pressure.     The  period  of  outflow  under  each  condition  is  also  shown. 

The  Coronary  Arteries,  unlike  all  the  other  arteries,  are 
filled  during  ventricular  diastole.  During  systole  they  are 
compressed  by  the  contracting  muscle  of  the  heart,  and  it  is 
only  when  the  compression  is  removed  in  diastole  that  blood 
rushes  into  them.      This  helps  to  dilate  the  ventricles. 


The  interpretation  of  the  various  details  of  the  Cardiogram 
(fig.  173)  is  now  rendered  more  easy.  The  ventricles,  still 
full  of  blood,  are  suddenly  pressed  against  the  chest  wall  in 
systole.  As  the  blood  escapes  into  the  arteries  they  press 
with  less  force,  and  hence  the  sudden  slight  downstroke. 
But,  so  long  as  the  ventricles  are  contracted,  the  apex 
is  kept  tilted  forward,  and  hence  the  horizontal  plateau  is 


HEART  407 

maintained.      The  pressure  of  the   heart   disappears  as   the 
ventricles  relax. 

9.  Sounds  of  the  Heart. — On  listening  over  the  region 
of  the  heart,  a  pair  of  sounds  may  be  heard  with  each  cardiac 
cycle,  followed  by  a  somewhat  prolonged  silence.  These  are 
known  respectively  as  the  First  and  Second  Sounds  of  the 
Heart  (fig.  173)  {Practical  Physiology). 

By  placing  a  finger  on  the  cardiac  impulse,  while  listen- 
ing to  these  sounds,  it  is  easy  to  determine  that  the  first  sound 
occurs  synchronously  with  the  cardiac  impulse — I.e.  synchron- 
ously with  the  ventricular  contraction. 

It  develops  suddenly,  and  dies  away  more  slowly.  In 
character  it  is  dull  and  rumbling,  and  may  be  imitated  by 
pronouncing  the  syllable  lub.  In  pitch  it  is  lower  than  the 
second  sound. 

The  second  sound  is  heard  at  the  moment  of  ventricular 
diastole.  Its  exact  time  in  the  cardiac  cycle  has  been  deter- 
mined by  recording  it  on  a  cardiac  tracing  by  means  of  a 
microphone.  It  develops  suddenly  and  dies  away  suddenly. 
It  is  a  clearer,  sharper,  and  higher-pitched  sound  than  the 
first.      It  may  be  imitated  by  pronouncing  the  syllable  dupp. 

According  to  the  part  of  the  chest  upon  which  the  ear  is 
placed,  these  sounds  vary  in  intensity.  Over  the  apical 
region  the  first  sound  is  louder  and  more  accentuated  ;  over 
the  base  the  second  sound  is  more  distinctly  heard. 

A.  The  Cause  of  the  Second  Sound  is  simple.  At  the 
moment  of  ventricular  diastole,  when  this  sound  develops, 
the  only  occurrence  which  is  capable  of  producing  a  sound  is 
the  sudden  stretching  of  the  semilunar  valves  by  the  high 
arterial  pressure  above  them  and  the  low  intra-ventricular 
pressure  below  them.  The  high  arterial  pressure  comes  on 
them  suddenly  like  the  blow  of  a  drum-stick  on  a  drum-head, 
and,  by  setting  the  valves  in  vibration,  produces  the  sound. 

Aortic  and  Pulmonary  Areo.s. — The  second  sound  has 
thus  a  dual  origin — from  the  aortic  valve  and  from  the 
pulmonary  valve  ;  and  it  is  possible,  by  listening  in  suitable 
positions,  to  distinguish  the  character  of  each  of  these. 

B.  The  Cause  of  the  First  Sound  is  twofold.      When  it  is 


408  VETERINARY   PHYSIOLOGY 

heard,  two  changes  are  taking  phace  in  the  heart,  either  of 
which  would  produce  a  sound. 

1st  The  muscular  wall  of  the  ventricles  is  contracting. 

2nd.  The  auriculo-ventricular  valves  are  being  stretched. 

1  st.  That  the  first  factor  plays  a  part  in  the  production 
of  the  first  sound  is  proved  by  rapidly  cutting  out  the  heart 
of  an  animal  and  listening  to  the  organ  with  a  stethoscope 
while  it  is  still  beating — but  without  any  blood  passing 
through  it  to  stretch  the  valves.  With  each  beat  the  lub 
sound  is  distinctly  heard. 

Apparently  the  wave  of  contraction,  passing  along  the 
muscular  fibres  of  the  heart,  sets  up  vibrations,  and,  when 
these  are  conducted  to  the  ear,  the  external  meatus  picks  out 
the  vibration  corresponding  to  its  fundamental  note,  and  thus 
produces  the  characters  of  the  sound. 

2nd.  The  auriculo-ventricular  valves  are  being  subjected 
on  the  one  side  to  the  high  ventricular  pressure,  and  on  the 
other  to  the  low  auricular  pressure.  If  the  valves  are 
destroyed  or  diseased,  the  characters  of  the  first  sound  are 
materially  altered,  or  the  sound  may  be  entirely  masked  by 
a  continuous  musical  sound — a  murmur.  It  has  been  main- 
tained that  a  trained  ear  can  pick  out  in  the  first  sound  the 
note  corresponding  to  the  valvular  vibrations. 

The  idea  that  the  impulse  of  the  heart  against  the  chest 
wall  plays  a  part  in  the  production  of  this  sound  is  based 
upon  the  fallacious  idea  that  the  heart  "  hits  "  the  chest  wall. 
All  that  it  does  is  to  press  more  firmly  against  it. 

Mitral  and  Tricuspid  Areas. — On  account  of  the  part 
played  by  the  valves  in  the  production  of  the  first  sound,  it 
may  be  considered  to  be  double  in  nature — due  partly  to  the 
mitral  valve,  partly  to  the  tricuspid.  The  mitral  valve 
element  may  best  be  heard  not  over  the  area  of  the  mitral 
valve — which  lies  very  deep  in  the  thorax — but  over  the 
apex  of  the  heart,  as  at  this  situation  the  left  ventricle,  in 
which  the  valve  lies,  comes  nearest  to  the  thoracic  wall  and 
conducts  the  sound  thither. 

A  tJdrd  sound  has  been  described  by  Gibson.  It  is  heard 
during  the  diastole  of  the  ventricles,  and  it  has  been  ascribed 
to  the  rebound  of  the  semilunar  valves. 


HEART 


409 


By  means  of  the  microphone  apphed  over  the  heart,  two 
faint  sounds  have  been  recorded  during  auricular  systole,  but 
so  far  they  cannot  be  considered  as  of  clinical  importance. 

Cardiac  Murmurs- — When  the  valves  are  diseased  and  fail 
to  act  properly,  certain  continuous  sounds  called  cardiac 
murmurs  are  heard. 

These  owe  their  origin  to  the  fact  that,  while  a  current  of 


^Jim's 


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v.s. 

V.D. 

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Ill  lllllli 

hnm 

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rmi. 

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kmi-J 

EK'CE.l 


Fig.  176. — To  show  the  Periods  in  the  Cardiac  Cycle  at  which  the  various 
Murmurs  of  Stenosis  and  Incompetence  occur. 

fluid  passing  along  a  tube  of  fairly  uniform  calibre  is  not 
thrown  into  vibrations  and  therefore  produces  no  sound, 
when  any  marked  alterations  in  the  kimen  of  the  tube 
occurs — either  a  sudden  narrowing  or  a  sudden  expansion — 
the  flow  of  fluid  becomes  vibratory,  and,  setting  up  vibrations 
in  the  solid  tissues,  produces  a  musical  sound. 


410  VETERINARY   PHYSIOLOGY 

Such  changes  in  the  calibre  of  the  heart  are  produced  in 
two  ways  : — 

1st.  By  a  narrowing,  either  absolute  or  relative,  of  the 
orifices  between  the  cavities — stenosis. 

2nd.  By  a  non-closure  of  the  valves — incompetence. 

\st.  Stenosis. — If  one  of  the  auriculo-ventricular  orifices  is 
narrowed,  a  murmur  is  heard  during  the  period  at  which 
blood  normally  flows  through  this  opening  (p.  405).  A 
reference  to  fig.  176  at  once  shows  that  this  occurs  during 
the  whole  of  ventricular  diastole,  and  that  the  flow  is  most 
powerful  during  the  first  period  of  ventricular  diastole  and 
during  auricular  systole. 

If  the  aortic  or  the  pulmonary  valve  is  narrowed  the 
murmur  will  be  heard  (fig.  176)  during  ventricular  systole. 

The  narrowing  need  not  be  absolute.  A  dilatation  of  the 
artery  will  make  the  orifice  relatively  narrow,  and  will  pro- 
duce the  same  result. 

2nd.  Incompetence. — If  the  auriculo-ventricular  valves 
fail  to  close  properly,  then,  during  ventricular  systole,  blood 
will  be  driven  back  into  the  auricles,  and  a  murmur  will  be 
heard  during  this  period  (fig.  176). 

If  the  aortic  or  the  pulmonary  valve  fails  to  close,  the  blood 
will  regurgitate  into  the  ventricle  from  the  arteries  during 
ventricular  diastole,  and  a  murmur  will  be  heard  during  this 
period  (fig.  176). 

By  the  position  at  which  these  murmurs  are  best  heard 
the  pathological  condition  producing  them  may  be  deter- 
mined. 

II.  The  Work  of  the  Heart. 

The  enormous  variations  in  metabolism  which  the  muscles 
undergo  between  rest  and  activity  have  already  been 
considered  (p.  266).  The  oxygen  intake  and  output  of 
carbon  dioxide  may  increase  tenfold  during  muscular  work. 

To  supply  this  oxygen  the  flow  of  blood  through  the 
muscles  must  be  proportionately  increased,  and  this  increase 
is  secured  by  (1)  a  dilatation  of  the  blood-vessels  going  to  the 
muscles  with,  at  the  same  time  (2),  a  constriction  of  the 
blood-vessels  of  the  abdomen. 


HEART  411 

But  the  flow  of  blood  is  determined  by  the  difference  of 
pressure  between  the  arteries  and  veins,  and  hence  the 
arterial  pressure  must  be  maintained  when  the  dilatation  of 
the  small  arteries  is  allowing  the  increased  outflow  of  blood. 

This  must  be  met  by  an  increased  action  of  the  heart  to 
maintain  the  pressure.  The  heart  must  be  capable  of  great 
variations  in  its  action,  so  that  at  one  time  it  may  pump  out 
only  a  small  amount  of  blood,  at  another  an  enormously 
greater  quantity.  The  heart  must  be  able  to  perform  very 
varying  amounts  of  work. 

To  determine  the  variations  in  the  work  done,  it  is 
necessary  to  measure  the  amount  of  blood  expelled  and  the 
resistance  against  which  it  is  expelled.  A  certain  amount 
of  work  is  done  in  giving  velocity  to  the  blood,  but  this  is 
small  when  compared  with  the  work  of  overcoming  resist- 
ance. 

(1)  The  resistance  in  the  aorta  may  be  measured  by  a 
mercury  manometer  (p.  447). 

(2)  The  determination  of  the  amount  of  blood  expelled 
from  the  heart  has  proved  a  very  difficult  problem.  It  has 
been  attempted  both  by  direct  and  by  indirect  methods. 

A.  Direct  Methods. 
So  far  it  has  not  proved  possible  to  measure  the  output 
of  the    heart    by   a    direct   method   with   the    heart    acting 
normally  in  situ. 

(1)  Cardiometer  Method.— The  output  of  blood  at  each 
beat  of  the  heart  of  the  dog  may  be  measured  by  a  cardio- 
meter, a  rigid  walled  air-tight  case,  which  is  placed  round 
the  heart  and  connected  with  a  piston-recorder,  so  that  the 
decrease  in  the  volume  of  the  enclosed  heart,  due  to  the 
blood  leaving  it,  may  be  directly  recorded  by  means  of  a 
lever  attached  to  the  piston. 

(2)  The  Isolated  Heart-Lung  Preparation.— We  shall  presently 
consider  the  way  in  which  Starling  was  able  to  remove  the 
heart  and  lungs  from  a  dog  and  to  allow  blood  to  circulate 
through  them.  Fig.  178  shows  how,  by  means  of  the  side 
tube,  the  blood  driven  from  the  heart  may  be  collected  and 
its  amount  measured. 


412 


VETEEINARY   PHYSIOLOGY 


These  methods,  of  course,  give  no  measurement  of  the 
normal  output  of  the  heart. 

B.  Indirect  Methods. 
1.  The  Oxygen  Method. — (1)  By  finding  the  amount  of 
oxygen  which  the  blood  gains  per  unit  of  time  iu  passing 
through  the  lungs,  and  (2)  the  amount  of  oxygen  which  is 
taken  from  the  lungs  per  unit  of  time,  the  amount  of  blood 
passing  through  the  lungs,  i.e.  leaving  the  right  ventricle 
may  be  calculated.  Since  the  right  and  left  ventricles  must 
discharge  equal  amounts  of  blood,  the  output  of  the  left 
ventricle  is  thus  ascertained. 


CC.O2 


100     100     100     100    100    100    Cc.  Blood. 


Fig.  177. — To  illustrate  the  method  of  determining  the  amount  of  blood 
leaving  the  right  ventricle.  The  inverted  funnel  represents  the  lungs 
from  which  30  cc.  of  O2  have  been  taken  up  by  tlie  blood.  The 
blood  has  gained  5  cc.  of  Oo  per  100  cc  Therefore  600  cc.  of 
blood  must  have  passed  through  the  lungs,  i.e.  left  the  right  ventricle. 

(1)  The  amount  of  oxygen  in  the  blood  is  determined  as 
described  on  p.  497  (Practical  Physiology).  To  ascertain 
the  amount  of  oxygen  in  the  blood  going  to  the  lungs, 
blood  from  a  vein  is  taken  ;  for  the  amount  of  oxygen  in 
the  blood  leaving  the  heart  the  blood  from  an  artery  is  taken. 

If  lower  animals,  e.g.  goats,  are  used,  the  method  may  be 
made  even  more  accurate  by  taking  blood  by  means  of 
trocars  simultaneously  from  the  right  (venous  blood) 
and  left  ventricle  (arterial  blood). 

(2)  The  amount  of  oxygen  taken  from  the  lungs  is 
determined  by  means  of  the  Douglas  bag  (p.  261).  Suppose 
the  blood  gains  5   per  cent,  of  oxygen,  and  suppose  that  in 


HEART  413 

unit  of  time,  30  c.c.  of  oxygen  are  taken  up  by  the  lungs, 
then  this  30  c.c.  must  be  distributed  in  the  blood  to  the 
extent  of  5  c.c.  for  each  100  c.c.  of  blood  (fig.  177),  and 
hence  600  c.c.  of  blood  must  have  passed  through  the  lung 
in  unit  of  time. 

2.  The  Nitrous  Oxide  Method. — Instead  of  estimating  the 
increase  in  the  oxygen  of  the  blood,  a  measured  quantity 
of  nitrous  oxide,  N2O,  the  solubility  of  which  in  the  blood 
at  the  pressure  and  temperature  at  which  it  is  present  in  the 
lungs  is  known,  may  be  inhaled  into  the  lungs  and  estimate 
may  be  made  of — 

(1)  The  amount  of  NgO  in  the  arterial  blood.  (2)  The 
amount  of  N.3O  which  has  been  taken  from  the  lungs  by  the 
blood  per  unit  of  time. 

From  this  the  amount  of  blood  passing  through  the  lungs, 
i.e.  from  the  right  ventricle,  may  be  calculated  as  it  is  by  the 
oxygen  method. 

Krogh  has  shown  that  the  left  ventricle  in  man  pumps 
blood  into  the  arteries  at  the  following  rates  according  to 
the  condition  of  muscular  activity  : — 

1.  At  rest  about        3  litres  per  minute. 

2.  With  moderate  exercise      ,,  12        ,, 

3.  With  hard  „  „  21 

In  the  horse  the  quantities  are  probably  greater. 

The  Average  Work. — It  is  thus  impossible  to  attempt  to 
form  an  estimate  of  the  average  work  of  the  heart 
since  these  variations  are  so  great. 

The  Adaptation  of  the  Work. — A  more  interesting  question 
is — How  is  the  heart  able  to  adapt  itself  to  perform  the 
very  different  amounts  of  work  required  ?  This  has  been 
elucidated  by  Starling  by  the  use  of  the  isolated  heart-lung 
preparation  (fig.  178). 

(i.)  The  heart  and  lungs  are  carefully  removed  from  a 
dog,  all  the  vessels  being  clamped  or  ligatured,      (ii.)  The 


414 


VETERINARY   PHYSIOLOGY 


trachea  is  connected  with  a  pump  and  the  lungs  are  thus 
supplied  with  air  so  that  the  blood  is  oxygenated,  (iii.)  A  tube 
or  cannula  is  tied  into  a  carotid  artery,  CA.,  and  it  has  a  side 
attachment  to  a  mercury  manometer  to  measure  the  arterial 
pressure,  i¥;.  (iv.)  The  tube  passes  on  to  a  very  thin  piece  of 
tubing,  jK.,  enclosed  in  a  rigid  walled  glass  tube,  T.,  in  which  the 
pressure  can  be  raised  to  any  extent  which  may  be  desired, 
and  may  be  measured  by  a  manometer,  Mg.     (v.)  The  tube 


Yic.    178.— The  isolated  heart-lung  preparation  (the  lungs  are  not  shown).    For 
description  see  text.     (Starling.) 

passes  on  and  is  provided  with  a  by-pass,  X.,  from  which  the 
blood  may  be  allowed  to  escape  if  the  amount  passed  through 
the  heart  is  to  be  measured,  (vi.)  The  blood  passes  through 
an  arrangement  by  which  the  blood  is  kept  at  body 
temperature,  (vii.)  It  returns  to  the  superior  vena  cava,  SVC, 
and  right  auricle  by  a  tube  provided  with  a  clamp  so  that  the 
amount  entering  the  heart  may  be  controlled,  (viii.)  The 
blood  then  passes  to  the  right  ventricle,  and  so  through  the 
lungs   m    which    artificial   respiration   is  kept   up,    back    to 


HEART 


415 


the  left  side  of  the  heart,  (ix.)  Variations  in  the  size  of 
the  heart  may  be  recorded  by  enclosing  the  ventricles  in 
some  form  of  cardiometer  (p.  411). 

(1)  It  has  thus  been  shown  that  the  adaptation  is  largely 
independent  of  the  central  nervous  system,  although  this  too, 
as  will  laier  be  shown,  plays  an  important  part. 


Fia.  179. --To  show  the  effect  of  a  sudden  rise  in  arterial  pressure  upon  the 
cardiac  contractions  recorded  by  enclosing  the  heart  in  a  cardiometer. 
The  upstrokes  are  systolic.  Note  the  increased  diastole  with  the 
increase  in  the  systole,  i?./*.,  the  arterial  blood-pressure  ;  F.P.,  the 
venous  blood  pressure.     (Starling.) 


(2)  By  such  an  apparatus  it  is  found  that  the  output  of 
blood  is  not  dependent  on  the  arterial  pressure,  but  that 
within  wide  limits  of  pressure  the  heart  continues  to  pump 
out  the  same  amount  of  blood  at  each  systole,  thus  doing  a 
greater  and  greater  amount  of  work  as  tlie  arterial  pressure 
rises.      With  a  sudden  rise  of  pressure  it  may  fail  to  do  so 


416 


VETERINARY   PHYSIOLOGY 


for  the  first  few  beats,  and  the  ventricles  may  thus  become 
distended  (fig.  17  9),  but  with  this  distension  and  the  resulting 
elongation  of  the  muscular  fibres  of  the  heart  the  force  of 
contraction  is  increased  (p.  224),  and  the  heart  performs  the 
increased    work.       In    the    healthy  animal    this   dilatation 


Fig.  180. — To  show  the  effect  of  increasing  the  venous  filling  of  the  heart 
recorded  as  in  fig.  ITS.  The  upstrokes  are  systolic.  Note  the 
enormous  increase  of  the  diastolic  filling  with  the  very  marked 
increase  in  the  sj-stolic  contraction  leading  to  an  increased  output  of 
blood.     (Stakling.) 

is  temporary,  but,  in  conditions  of  debiUty,  over-distension 
beyond  the  physiological  limit  may  be  produced,  and 
permanent  distension  may  result.  Next  time  a  strain  is 
put  upon  the  heart  the  distension  may  be  still  further 
increased,  and  heart-failure  may  occur. 

The    muscle    of    the     heart    thus    acts     as     we     have 
seen    skeletal    muscle    to    act    within    physiological    limits, 


HEART  417 

the  force  of  contraction  varies  with  the  length  of  the 
fibres. 

For  increased  work  the  heart  muscle  requires  more 
oxygen,  and  the  increased  supply  is  secured  by  the  increased 
arterial  pressure  driving  more  blood  into  the  coronary 
arteries.     This  has  been  demonstrated  by  actual  experiment. 

The  output  of  the  heart  is  thus  not  controlled  by  the 
arterial  pressure. 

(3)  It  is  controlled  by  the  inflow  from  the  veins,  as  may 
be  shown  by  loosening  the  clamp  on  the  venous  tube,  and 
allowing  more  blood  to  enter  the  heart  (fig.  180).  The  in- 
creased inflow  leads  to  a  distension  of  the  ventricles,  and  so 
to  an  increased  force  of  contraction.  This  depends  upon  the 
lengthening  of  the  ventricular  fibres.  The  normal  heart 
drives  out  just  as  much  blood  as  it  receives  from  the  veins. 

The  way  in  which  the  heart  adjusts  itself  may  be  seen 
in  a  man  or  animal  starting  to  run. 

(1)  The  muscular  movements  pump  more  blood  into  the 
heart  from  the  veins. 

(2)  The  heart  is  distended,  and  pumps  more  blood  into 
the  arteries. 

(8)  The  pressure  in  these  is  further  raised,  in  spite  of 
the  dilatation  of  the  arterioles  to  the  muscles,  by  the  con- 
traction of  the  abdominal  vessels. 

In  discussing  the  rapid  adaptation  of  the  heart  to  the 
varied  requirements,  the  possibility  of  the  production  of  a 
chronic  dilatation  has  been  discussed. 

When,  as  the  result  of  some  obstruction  to  the  flow  of 
blood,  the  heart  is  called  upon  to  perform  continuously  an 
increased  amount  of  work,  it  is  found  that  the  muscle  of 
the  ventricular  walls  increases  or  hypertrophies.  In  this 
way  the  prejudicial  effects  of  grave  valvular  disease  of  the 
heart  may  be  compensated  for.  But  this  compensation  is  apt 
to  be  disturbed  and  heart-failure  to  be  induced  by  any 
interference  with  the  flow  of  blood  through  the  coronary 
arteries. 

The  self-regulation  of  the  heart  is  itself  insufiicient  to 
maintain   the  necessary  distribution  of  pressure  throughout 


418  VETERINARY   PHYSIOLOGY 

the  vessels.     Some  means  must  be  provided  of  preventing  a 
too  great  rise  of  arterial  pressure  and  of  preventing  an  over- 
distension of  the  heart  when  the  venous  return  is  too  great. 
This  is  effected   by   the   action   of  the   central   nervous 
system. 

III.  The  Influence  of  the  Central  Nervous  System 
on  the  Heart. 

In  the  frog,  a  branch  from  the  vagus  connects  the  central 
nervous  system  with  the  heart.  When  the  branch  is  cut  no 
effect  is  produced,  showing  that  it  is  not  constantly  in  action  ; 
but,  when  the  lower  end  is  stimulated,  the  heart  is  generally 
slowed  or  brought  to  a  standstill.  Sometimes  the  effect  is 
not  produced.  The  reason  for  this  is  that  the  cardiac 
branch  of  the  vagus  in  the  frog  is  really  a  double  nerve, 
derived  in  part  from  the  spinal  accessory,  and  in  part  from 
fibres  which  reach  the  vagus  from  the  superior  thoracic 
sympathetic  ganglion.  If  the  spinal  accessory  nerve,  or  the 
medulla  oblongata  from  which  it  springs  be  stimulated,  the 
heart  is  always  slowed  ;  and  if  the  sympathetic  fibres  are 
stimulated,  it  is  quickened.  Generally,  stimulation  of  the 
cardiac  branch  containing  these  two  sets  of  fibres  simply 
gives  the  result  of  stimulating  the  former,  but  sometimes 
the  stimulation  of  the  latter  masks  this  effect  {Practical 
Physiology). 

In  the  mammal  three  sets  of  nerve  fibres  pass  to  the 
heart : — 

Ist.  The  superior  cardiac  branch  of  the  vagus  starts 
from  near  the  origin  of  the  superior  laryngeal  nerve,  and 
passes  to  the  heart  to  end  in  the  endocardium  (fig.  ISl,  S.C). 

27id.  The  inferior  cardiac  branch  of  the  vagus  leaves  the 
main  nerve  near  the  recurrent  laryngeal,  and  passes  to  join 
the  superficial  cardiac  plexus  in  the  heart  (fig.  181,  I.G.). 

^rd.  The  symijathetic  nerve  fibres  come  from  the  superior 
thoracic  and  inferior  cervical  ganglia,  and  also  end  in  the 
superficial  cardiac  plexus  (fig.  181,  S.). 

\st.  The  Superior  Cardiac  Branch  of  the  Vagus  is  an  ingoing 
nerve.      Section  produces  no  effect  ;  stimulation  of  the  lower 


HEART 


419 


end  causes  no  effect ;   stimulation  of  the  upper  end  causes 
(1)    slowing  of   the   heart   and   (2)   a   marked    fall    in    the 


Fig.  181. — Connections  of  the  Heart  with  the  Central  Nervous  System. 
Au.,  auricle;  V.,  ventricle;  V.D.C.,  abdominal  vaso-dilator  centre; 
C./.C.,  cardiac  inhibitory  centre;  C.A.G.,  cardio-augmentor  centre; 
S.C,  superior  cardiac  branch  of  the  vagus;  I.C.,  inferior  cardiac 
branch  of  the  vagus  with  cell  station  in  the  heart;  S.,  cardio- 
sympathetic  fibres  with  cell  station  in  the  stellate  ganglion  ;  V.D.A6., 
vaso-dilator  fibres  to  abdominal  vessels.  The  continuous  lines  are 
outgoing,  the  broken  lines  are  ingoing  nerves. 

pressure  of  blood  in  the  arteries,  and  it   may  cause    pain. 
The    slowing^   of  the   heart   is   a   reflex   effect    through    the 


420  VETERINARY   PHYSIOLOGY 

inferior  cardiac  branch  ;  and  the  fall  of  blood  pressure, 
which  is  the  most  manifest  effect,  is  due  to  a  reflex  dilata- 
tion of  the  vessels  of  the  abdomen,  causing  the  blood  to 
accumulate  there,  and  thus  to  lessen  the  pressure  in  the 
arteries  generally.  On  account  of  its  effect  on  the  blood 
pressure,  this  nerve  is  called  the  depressoo'  nerve. 

^nd.  Inferior  Cardiac  Branch  of  Vagus. — Section  of  the  vagus 
or  of  this  branch  causes  acceleration  of  the  action  of  the 
heart.  The  nerve  is  therefore  constantly  in  action.  Stimu- 
lation of  its  central  end  has  no  effect ;  stimulation  of  its 
peripheral  end  causes  a  slowing  or  stoppage  of  the  heart. 
Less  blood  is  pumped  into  the  arteries,  and  the  pressure  in 
them  falls  (fig.  188).  It  is  therefore  the  checking  or  in- 
hibitory nerve  of  the  heart.  The  right  vagus  is  chiefly 
connected  with  the  sino-auricular  node,  and  its  stimulation 
slows  the  rate  of  the  heart.  The  left  vagus  is  specially 
connected  with  the  auriculo-ventricular  node,  and  its  stimu- 
lation tends  to  slow  or  prevent  conduction  of  contraction 
from  the  auricles  to  the  ventricles.  But  these  two  actions 
are  not  always  clearly  differentiated. 

1.  Course  of  the  Fibres. — These  fibres  leave  the  central 
nervous  system  by  the  spinal  accessory,  and  pass  to  the 
heart  to  form  connections  with  the  cells  of  the  cardiac 
plexuses. 

2.  Centre. — The  fibres  arise  from  cells  in  the  medulla 
oblongata,  which  can  be  stimulated  to  increased  activity 
either  directly  or  reflexly. 

(1)  Direct  stimulation  is  brought  about  by  (a)  sudden 
anaemia  of  the  brain,  as  when  the  arteries  to  the  head  are 
clamped  or  occluded  ;  (6)  increased  venosity  of  the  blood, 
as  when  respiration  is  interfered  with  ;  (c)  the  concurrent 
action  of  the  respiratory  centre  (see  p.  585). 

(2)  Reflex  stimulation  is  produced  through  many  nerves, 
e.g.  those  of  the  abdomen — a  point  of  great  importance  in 
abdominal  surgery.  The  superior  cardiac  branch  of  the  vagus 
from  the  ventricles  and  wall  of  the  aorta  is  stimulated  Avhen  the 
arterial  pressures  rises  and  leads  to  a  reflex  slowing  of  the 
heart,  which,  along  with  the  dilatation  of  the  abdominal 
vessels,  reduces  the  pressure. 


HEART  421 

The  reflex  stimulation  of  the  centre  may  be  used  to 
determine  its  position.  It  can  be  induced  after  removal  of 
the  brain  above  the  medulla,  but  destruction  of  the  medulla 
entirely  prevents  it, 

(3)  The  action  of  this  centre  may  be  checked  or  inhibited. 
This  happens  when  the  diastolic  filHng  is  markedly  increased 
as  a  result  of  increased  venous  inflow,  and  since  it  does 
not  occur  after  section  of  the  vagi  it  is  a  central  effect. 

3.  Mode  of  Action. — These  inhibitory  fibres  appear  to  act 
by  stimulating  the  intra-cardiac  nervous  mechanism.  When 
these  peripheral  neurons  have  been  poisoned  by  atropine,  the 
vagus  cannot  act  {Practical  Physiology).  Nicotine,  which 
poisons  the  synapses  between  the  vagus  nerve  and  the  terminal 
neurons,  also  prevents  stimulation  of  the  vagus  from  slowing 
the  heart,  but,  as  is  shown  by  experiments  on  the  heart  of 
the  frog,  direct  stimulation  of  the  terminal  neurons  does 
act. 

Gaskell  found  that  stimulating  the  inhibitory  fibres  causes 
a  positive  variation  of  the  current  of  injury,  indicating  that 
the  difference  between  the  living  part  of  the  heart  and  the 
injured  part  is  increased  (p.  213),  and  he  concluded  that 
they  excite  anabolic  changes  in  the  heart. 

4.  Result  of  Action. 

(a)  The  rate  of  botli  auricles  and  ventricles  is  slowed, 
but  the  effect  on  the  auricles  is  more  marked  than  upon  the 
ventricles.  The  rigljt  vagus  has  usually  the  most  marked 
effect  upon  the  rate  of  the  heart  (fig.  182,  A.). 

(6)  The /o?-ce  of  contraction  of  the  auricles  is  decreased. 
In  the  ventricles  the  systole  becomes  less  complete  and  the 
cavities  become  more  and  more  distended,  partly  as 
a  result  of  decrease  in  the  force  of  contraction,  partly 
as  a  mechanical  result  of  over-distension  due  to  the 
decreased  output. 

(c)  Conduction,  especially  from  auricles  to  ventricles,  is 
decreased  so  that  the  ventricles  may  not  contract  with  each 
auricular  contraction,  and  a  condition  of  heart-block  may  be 
established.  This  is  usually  best  marked  upon  stimulation 
of  the  left  vagus. 

C  Sympathetic  Fibres.— The  outgoing  fibres  are  the  aug- 


422 


VETERINARY   PHYSIOLOGY 


mentors  and  accelerators  of  the  heart's  action.  ^Vhen  they 
are  cut  the  heart  beats  slower,  therefore  they  are  constantly 
in  action.  When  the  peripheral  end  is  stimulated,  the  rate 
and  force  of  the  heart  are  increased. 

1.    Course   of  the  Fibres.- — These  are   small   medullated 
fibres,  which  leave  the  spinal  cord  by  the  anterior  roots  of 


Fig.  182. — Simultaneous  Tracing  from  Auricles  and  Ventricles.  A.,  during 
stimulation  of  the  vagus  ;  B.,  during  stimulation  of  the  sympathetic. 
Each  downstroke  marks  a  systole,  each  upstroke  a  diastole.  (From 
Roy  and  Adami.  ) 


the  2nd,  3rd,  and  4th  dorsal  nerves,  and  in  most  animals  pass 
to  the  stellate  ganglion  where  they  have  their  cell  stations 
(fig.  181).  From  the  cells  in  this  ganglion,  non-medullated 
fibres  run  on  in  the  annulus  of  Vieussens,  and  from  this  and 
from  the  inferior  cervical  ganglion,  they  pass  out  apparently 
directly  to  the  muscular  fibres  of  the  heart. 


HEART  423 

2.  The  Centre  is  in  the  medulla,  and  it  may  be  reflexly 
stimulated  through  various  ingoing  nerves,  such  as  the 
sciatic  ;  or  it  may  be  set  in  action  from  the  higher  nerve 
centres  in  various  emotional  conditions.  It  is  also  called 
into  play,  along  with  inhibition  of  the  action  of  the  vagus,  when 
increased  venous  inflow  in  diastole  leads  to  over-distension. 

3.  Mode  of  Action. — The  fibres  seem  to  act  (a)  upon  the 
muscular  fibres,  by  increasing  their  excitability  and  conduc- 
tivity ;  (6)  upon  the  inhibitory  mechanism,  by  throwing  it 
out  of  action. 

4.  Results  of  Action  (fig.  182,  B). 

(a)  The  rate  of  the  rhythmic  movements  of  auricles  and 
ventricles  is  increased. 

(6)  The  force  of  contraction  of  auricles  and  ventricles  is 
increased. 

Thus,  the  output  of  blood  from  the  heart  is  increased, 
and  the  pressure  of  blood  in  the  arteries  is  raised. 

It  is  probable  that  the  cardiac  sympathetic  also  carries 
ingoing  fibres  which  enter  the  cord  in  the  lower  cervical 
region.  The  pain  experienced  down  the  inside  of  the  arm 
in  heart  disease  in  man  is  generally  thought  to  be  due  to  the 
implication  of  these  fibres  leading  to  sensations  which  are 
referred  to  the  corresponding  somatic  nerves. 

The  vagus  is  thus  the  protecting  nerve  of  the  heart, 
reducing  its  work  and  diminishing  the  pressure  in  the 
arteries,  and  it  is  called  into  action  when  the  systolic 
pressure  rises  too  high,  while  it  is  inhibited  when  the  venous 
inflow  is  too  much  increased. 

The  sympathetic  is  the  whip  which  forces  the  heart  to 
increased  action  in  order  to  keep  up  the  pressure  m  the 
arteries,  and  it  is  brought  into  action  by  increase  in  the 
venous  inflow,  so  that  the  intrinsic  response  is  supplemented 
by  this  extrinsic  eflect. 


424  VETERINARY   PHYSIOLOGY 


IV.  The  Maintenance  and  Control  of  the  Cardiac 
Rhythm. 

That  there  is  an  intrinsic  mechanism  in  the  heart  for 
the  maintenance  and  control  of  its  action  is  shown  by  the 
fact  that  the  excised  heart  continues  to  beat  in  cold-blooded 
animals  for  a  considerable  time  without  any  supply  of  blood 
and  in  warm-blooded  animals  if  oxygenated  blood  is  supplied 
at  a  suitable  temperature. 

In  considering  the  nature  of  this  mechanism,  it  must  be 
borne  in  mind  :  first,  that  two  distinct  questions  have  to  be 
investigated  : — 

\st.   How   the   rhythmic  contractions  are   initiated    and 
maintained  ; 

Ind.   How  they  are  propagated  over  the  heart  ; 
and  second,  that  nerve  structures  as  well  as  muscular  fibres  exist 
in  the  heart,  so    that  either  one  or  other  or  both  of  these 
may  be  involved  in  the  starting  and    conduction    of  con- 
tractions. 

1 .  The  Initiation  and  Maintenance  of  Rhythmic  Contraction. — 
(1)  In  the  embryo,  the  heart  begins  to  beat  before  any 
nervous  structures  can  be  shown  to  have  migrated  into  it. 

(2)  Hooker  finds  that,  after  removal  at  a  very  early  stage 
of  development  of  the  anterior  part  of  the  neural  canal  from 
which  the  neurons  to  the  heart  come,  the  formation  of  the 
heart  still  goes  on  while  it  beats  in  a  normal  manner. 

(3)  A  little  piece  of  the  heart  of  an  embryo  kept  under 
aseptic  precautions  in  the  animal's  blood  plasma  will  grow, 
and  will  manifest  typical  rhythmic  contractions. 

(4)  Even  in  the  adult  amphibian  it  is  possible  to  start 
rhythmic  contractions  in  the  apex  of  the  ventricle — a  part 
in  which  nerve  cells  have  not  been  observed — either  by 
repeated  rhythmic  stimulation  or  by  distending  it  with 
Ringer's  solution  perfused  through  a  tube.  The  conclusion 
thus  seems  inevitable  that  rhythmic  contraction  is  primarily 
a  function  of  the  muscle. 

But  there  is  evidence  that,  when  nerve  structures  have 


HEART  425 

grown   out   to  reach  the  heart,  they  play  a  not  unimportant 
part  in  initiating  contraction. 

(1)  There  is  the  negative  evidence  that,  if  during  Hfe  the 
apex  of  the  frog's  ventricle  has  been  separated  from  the  rest 
by  crushing,  it  remains  passive. 

(2)  Chloral,  apparently  by  poisoning  the  nervous  struc- 
tures, may  stop  the  rhythmic  action  of  the  heart,  but  leave 
the  muscle  capable  of  responding  to  stimulation 

(3)  Carlson  has  shown  that  the  heart  of  the  king  crab 
stops  if  the  nervous  structures  are  dissected  off  it.  But  the 
muscle  of  this  heart  is  of  the  type  of  skeletal  muscle,  and  it 
is  perhaps  unsafe  to  apply  these  results  to  ordinary  heart 
muscle. 

(4)  The  contractions  normally  start  in  the  sinus  region, 
a  part  of  the  heart  richly  supplied  with  nerve  cells  and 
fibres  (p.  388),  The  importance  of  this  part  of  the  heart  in 
originating  the  movements  of  the  rest  of  the  organ  is  shown 
by  experiments  on  the  heart  of  the  frog  (Practical  Physiology). 

If  a  ligature  be  applied  between  the  sinus  and  auricles, 
the  sinus  goes  on  beating  while  the  rest  of  the  heart  stops 
(Stannius'  Exjyeriment).  This  shows  the  dominant  influence 
of  the  sinus.  But,  if  now  a  ligature  be  applied  between  the 
auricles  and  ventricles,  these  latter  generally  begin  to  beat 
with  a  slower  rhythm  than  the  sinus  (Practical  Physiology). 

The  second  part  of  this  experiment  seems  to  indicate 
that  each  part  of  the  heart  has  the  property  of  rhythmic 
contractility.  It  has  also  been  shown  that  if  the  ventricle 
be  made  to  beat  faster  than  the  sinus,  the  contraction  wave 
may  travel  in  the  reverse  direction,  from  ventricle  to  sinus. 
It,  and  not  the  sinus,  becomes  the  "  pace-maker." 

The  whole  question  of  the  relative  parts  played  by  nerve 
and  muscle  in  starting  contraction  is  still  unsettled.  The 
evidence  seems  to  indicate  that  the  rhythmicity  is  a  function 
of  the  primitive  cardiac  tissue  which  is  at  first  purely 
muscular,  but  which  later  contains  nervous  elements,  and  in 
connection  with  which,  in  the  mammal,  the  chief  masses  of 
nerve  cells  occur. 

The  maintenance  of  this  rhythmic  contraction  and  relaxa- 


426  VETERINARY  PHYSIOLOGY 

tion  deiDends  upon  the  presence  of  certain  electrolytes  in  the 
circulating  blood.  A  due  admixture  of  salts  of  sodium, 
potassium,  and  calcium  is  essential.  For  the  frog's  heart 
Ringer  found  that  the  proportions  which  give  the  best  results 
are — 

NaCl        ....         0-70    per  cent. 

KCl  ....         0-08 

CaCl        ....         0025 

The  mammalian  heart  may  be  kept  contracting  for  a  long 
time  after  removal  from  the  body  by  perfusing  the  coronary 
arteries  through  a  tube  fixed  in  the  aorta  with  a  suitable 
saline  solution  well  oxygenated  and  kept  at  the  temperature 
of  the  animal. 

Sodium  salts  when  supplied  alone  to  the  heart  in  con- 
siderable amounts  cause  relaxation.  Potassium  salts  in  much 
smaller  amounts  have  the  same  effect.  Calcium  salts  when 
in  excess  cause  a  sustained  contraction. 

The  amount  of  carbon  dioxide  in  the  blood  circulating  in 
the  coronary  system  affects  the  action  of  the  heart.  A 
decrease  in  the  amount  accelerates  the  heart,  an  increase 
leads  to  increased  diastolic  relaxation  ;  but  at  first  the  con- 
tractions are  also  so  increased  that  the  output  of  blood 
remains  unaltered.  Later  the  contractions  decrease  in  force 
and  the  heart  may  stop  in  diastole. 

The  intra-cardiac  nervous  mechanism  seems  to  exercise 
a  controlling  influence  on  cardiac  contraction.  (1 )  If  the  region 
between  the  sinus  and  auricles  in  the  frog's  heart  is  stimu- 
lated by  the  interrupted  current  from  the  induction  coil,  the 
heart  is  slowed  or  stopped,  even  after  the  synapses  with  the 
nerves  coming  from  the  central  nervous  system  have  been 
poisoned  by  nicotine.  If  atropine,  which  poisons  the  terminal 
plexus,  be  first  applied,  electric  stimulation  is  without  result 
(Practical  Physiology). 

(2)  Further,  if  the  intra- ventricular  pressure  in  the  heart  of 
the  frog  is  raised  by  clamping  the  aorta,  a  slowing  of  the 
rhythm  occurs  even  after  section  of  the  vagi,  but  not  after 
the  intra-cardiac  neurons  have  been  poisoned  by  atropine. 


HEART  427 

2.  Conduction  of  Contraction. — There  is  little  evidence 
that  nerve  structures  play  a  part  in  the  conduction  of  the 
impulse  when  once  started.  The  syncytial  structure  of  heart 
muscle  is  specially  well  fitted  to  secure  the  propagation  of 
the  contraction,  and  poisoning  the  nerve  structures  with 
chloral  does  not  abolish  this. 

While  conduction  is  a  function  of  the  muscular  tissue  of 
the  heart,  it  is  undoubtedly  modified  through  the  action  of 
nerves  (p.  418). 

The  propagation  of  the  wave  of  contraction  over  the 
auricles  and  ventricles  and  the  part  played  by  the  primitive 
tissue  have  been  already  considered  (p.  395). 


V.  The  Nature  of  Cardiac  Contraction. 

The  contraction  of  the  ventricle  lasts  for  a  considerable 
fraction  of  a  second.  Is  it  of  the  nature  of  a  single  contrac- 
tion, or  of  a  tetanus  ? 

(i.)  A  single  stimulus  applied  to  heart  muscle  produces  a 
single  prolonged  contraction  (Practical  Physiology). 

(ii.)  It  is  impossible  to  tetanise  the  heart  by  rapidly 
repeated  induction  shocks.  This  is  due  to  the  long  refractory 
period  after  contraction  has  occurred.  The  resistance  to 
further  stimulation  gradually  wanes,  till,  just  before  the  onset 
of  the  next  contraction,  a  very  small  stimulus  is  effective. 
By  a  slower  sequence  of  stimuli  it  is  therefore  possible  to 
produce  an  incotnplete  fusion  of  contractions. 

(iii.)  The  steady  passage  of  the  contraction  wave  along 
the  heart  is  against  the  idea  that  the  normal  action  of  the 
heart  is  a  tetanus. 

(iv.)  That  it  is  really  a  single  contraction  is  demonstrated 
by  taking  advantage  of  the  fact  that  the  contracting  part  of 
a  muscle  is  electro-positive  ("  zincy  ")  to  the  rest.  By  the 
use  of  the  string  galvanometer,  it  is  possible  to  show  that 
the  region  of  the  sino-auricular  node  first  becomes  "  zincy," 
and  that  this  variation  then  travels  over  the  auricle  and 
onward  to  the  ventricle. 

In  man,  by  leading  off  from  the  right  hand  and  left  foot 


428  VETERINARY   PHYSIOLOGY 

to  the  galvanometer,  it  is  possible  to  get  an  electrocardiogram, 
the  left  foot  being  the  pole  connected  with  the  apex,  the 
right  hand  that  connected  with  the  base.  Such  tracings 
not  only  throw  important  light  upon  the  nature  and  course 
of  contraction,  but  have  proved  of  considerable  im- 
portance in  the  diagnosis  of  abnormal  conditions  of  the 
heart.  The  string  galvanometer  is  generally  used  for  this 
purpose  (p.  214). 

The  base  of  the  heart  shows  first  (a)  an  electro-positive 
phase  (fig.  173)  due  to  auricular  contraction.  This  may 
be  followed  by  (6)  an  electro-negative  variation  just  at  the 
beginning  of  ventricular  contraction,  which  has  been  ascribed 
to  the  early  contraction  of  the  papillary  muscles  at  the  apex, 
making  that  part  of  the  heart  electro-positive  to  the  base, 
(c)  This  is  immediately  followed  by  the  most  marked  electro- 
positive variation,  due  to  the  contraction  of  the  ventricle 
starting  at  the  base,  (d)  At  the  very  end  of  ventricular 
contraction  another  small  electro-positive  variation  occurs, 
and  this  has  been  ascribed  to  the  contraction  ending  in  the 
infundibulum  of  the  right  ventricle. 

Heart  muscle  resembles  visceral  muscle  in  that  the  minimum 
stimulus  is  also  a  maximum  stimulus — i.e.  the  smallest 
stimulus  which  will  make  the  muscle  contract  makes  it 
contract  to  the  utmost.  This  seems  to  be  due  to  the  fact 
that,  while  in  skeletal  muscle  a  small  stimulus  calls  into  play 
a  few  fibres  a  more  powerful  stimulus  calls  into  play  a  greater 
number,  whereas  in  heart  muscle  all  are  stimulated  at  once 
by  the  minimum  effective  stimulus,  because  of  the  continuity 
of  the  fibres  in  the  syncytial  network.  The  general  law  of 
the  "  all  or  nothing"  in  contraction  applies  to  heart  muscle. 
But  while  this  is  the  case,  the  strength  of  stimulus  necessary 
to  call  forth  a  contraction  varies  at  different  periods  of  the 
cardiac  cycle  as  indicated  above. 

In  cardiac  muscle,  perhaps  more  than  in  any  other,  a 
staircase  increase  in  the  extent  of  contraction  with  a  series  of 
stimuli  is  manifested. 

Tone  of  Heart  Muscle. — As   already  indicated,   tonus   is  a 


HEART  429 

marked  feature  of  visceral  muscle  (p.  215),  and  it  is  also 
manifest  in  skeletal  muscle  (p.  211),  It  would,  therefore, 
be  curious  if  it  were  absent  in  cardiac  muscle.  The  rapid 
rhythmic  contraction  makes  it  more  difficult  to  investigate, 
and  some  physiologists  actually  deny  its  existence  and 
maintain  that  tone  of  the  heart  muscle,  which  should 
prevent  over-distension,  is  rendered  unnecessary  by  the  action 
of  the  fibrous  pericardium. 

But  there  is  considerable  evidence  that  it  does  exist,  and 
that  it  plays  a  not  unimportant  part.  Gaskell  found  in  the 
heart  of  cold-blooded  animals  that  perfusing  a  fluid  con- 
taining a  weak  alkali  gradually  decreased  the  diastolic  filling 
and  finally  stopped  the  heart  in  systole,  while  perfusing  a 
weak  solution  of  lactic  acid  increased  the  diastole  and 
reduced  the  systole,  and,  finally,  brought  the  heart  to  a 
standstill  in  full  diastole.      Strophanthus  acts  like  an  alkali. 

Clinically  a  condition  of  over-distension  of  the  heart  is 
frequently  observed  and  the  administration  of  strophanthus 
is  found  to  decrease  the  distension.  Physicians  generally 
regard  this  as  due  to  loss  of  tone.  Some  investigators 
maintain  that  it  is  simply  due  to  too  great  diastolic  filling, 
with  too  great  lengthening  of  the  muscle  fibres. 

Pathological  Disturbances  of  Contraction  and  Conduction. 

(1)  Heart-block. — Since,  in  the  mammalian  heart, 
muscular  continuity  between  auricles  and  ventricles  through 
the  band  of  His  is  of  small  extent,  the  wave  of 
contraction  is  delayed  at  this  point,  and  in  the 
dying  heart  and  in  various  pathological  conditions,  the 
contraction  frequently  fails  altogether  to  pass  this  block,  and 
the  ventricles  do  not  contract  after  each  auricular  systole, 
and  may  either  stop,  or  contract  only  after  two  or  three 
auricular  contractions  have  occurred.  In  such  cases  the 
pulse  rate  is  reduced  to  a  half  or  even  less  of  its  normal 
rate.  A  condition  of  bradycardia  through  "heart-block"  is 
produced.  This  condition  is  revealed  by  a  study  of  the 
pulse  in  the  veins  of  the  neck  or  by  the  electrocardiogram. 

(2)  Auricular    Flutter- — Another  condition  in  which  the 


430  VETERINARY   PHYSIOLOGY 

contraction  wave  is  not  regularly  propagated  from  auricles 
to  ventricles  is  seen  in  the  condition  of  auricular  flutter 
when,  as  the  result  of  increased  excitability  of  the  sino- 
auricular  node,  the  normal  "  pacemaker,"  the  rate  of 
contraction  of  the  auricles  is  enormously  increased,  some- 
times to  two  or  three  hundred  per  minute.  In 
such  cases  the  contraction  reaches  the  auriculo-ventricular 
band  while  the  ventricular  fibres  have  not  completed 
their  contraction  while  they  are  still  in  the  refractory 
phase.  Hence,  only  one  ventricular  contraction  for  every 
two  or  three  auricular  contractions  may  occur.  In  these 
cases  there  is  practically  a  heart-block,  but  the  rate  of  the 
ventricles  is  not  decreased  as  in  true  heart-block.  The 
condition  is  fairly  common,  and  it  may  be  detected  by  the 
study  of  the  venous  pulse,  or  still  better,  by  the  electro- 
cardiogram. 

(3)  Fibrillation. — While  under  normal  conditions  the 
contractions  are  conducted  in  an  orderly  manner  over  auricles 
and  ventricles,  interference  with  the  coronary  circulation  with 
the  consequent  decreased  supply  of  oxygen  to  the  wall  of  the 
heart  is  apt  to  lead  to  marked  inco-ordination  of  the  contractions, 
so  that  some  bundles  of  fibres  are  contracting  while  others  are 
relaxing.  Thus  a  peculiar  fluttering  and  ineffective  fibrillar 
contraction  ov  fibrillation  is  seen  in  the  myocardium.  This 
may  affect  either  the  auricles  or  the  ventricles.  Since  the 
auricles  are  practically  the  cisterns  of  the  heart,  the  con- 
dition does  not  so  seriously  interfere  with  the  circulation 
when  it  affects  them  as  when  it  affects  the  ventricles  which 
are  the  pumps.  Ventricular  fibrillation  prevents  the  proper 
expulsion  of  blood  and  soon  leads  to  a  fatal  result.  It  may 
be  produced  by  powerful  electrical  stimulation  of  the 
ventricles  and  is  one  factor  in  causing  death  in  electrocution. 


BLOOD   VESSELS  431 


III.  CIRCULATION  IN  THE  BLOOD  AND  LYMPH 
VESSELS. 

The  general  distribution  of  the  various  vessels — arteries, 
capillaries,  veins,  and  lymphatics — has  been  already  con- 
sidered (fig.  162,  p.  383). 

1.  STRUCTURE. 

{The  structure  of  the  walls  of  each  kind  of  Vessel 
must  he  studied  practically.) 

The  capillaries  are  minute  tubes  of  about  1 2  micromilli- 
metres  in  diameter,  forming  an  anastomosing  network 
throughout  the  tissues.  Their  walls  appear  to  be  composed 
of  a  single  layer  of  endothelium. 

On  passing  from  the  capillaries  to  arteries  on  the  one 
side,  and  to  veins  and  lymphatics  on  the  other,  non-striped 
muscle  fibres  make  their  appearance  encircling  the  tube. 
Between  these  fibres  and  the  endothelium,  a  fine  elastic 
membrane  next  appears,  while,  outside  the  muscles,  a  sheath 
of  fibrous  tissue  develops.  Thus  the  three  essential  coats  of 
these  vessels  are  produced  : — 

Tunica  intima,  consisting  of  endothelium  set  on  the 
internal  elastic  membrane. 

Tunica  media,  consisting  chiefly  of  circularly  arrano-ed 
visceral  muscular  fibres. 

Tunica  adventitia,  consisting  of  loose  fibrous  tissue. 

A.  Arteries. — The  coats  of  the  arteries  are  thick.  In 
the  large  arteries,  the  muscular  fibres  of  the  media  are 
largely  replaced  by  elastic  fibres,  so  that  the  vessels  may 
better  stand  the  strain  of  the  charge  of  blood  which  is  shot 
from  the  heart  at  each  contraction. 

The  great  characteristic  of  the  walls  of  the  laiye  arteries 
is  the  toughness  and  elasticity  given  by  the  abundance  of 
elastic  fibrous  tissue,  and  of  the  small  arteries,  the  con- 
tractility due  to  the  preponderance  of  muscular  fibres. 


432  VETERINARY   PHYSIOLOGY 

B.  Veins. — In  the  veins,  double  flaps  of  the  tunica 
intima  form  valves  which  prevent  any  back -flow  of  blood. 
The  walls  of  the  veins  are  thin. 


2.  PHYSIOLOGY. 

The  circulation  of  blood  in  the  vessels  is  that  of  a  fluid 
in  a  closed  system  of  elastic-walled  tubes,  at  one  end  of 
which  (the  great  arteries)  a  high  pressure,  and  at  the  other 
(the  great  veins)  a  low  pressure,  is  kept  up.  As  a  result  of 
this  distribution  of  pressure,  there  is  a  constant  flow  of  blood 
from  arteries  to  veins. 

Many  points  in  connection  with  the  circulation  may  be 
conveniently  studied  on  a  model,  or  schema,  made  of 
india-rubber  tubes  and  a  Higginson's  syringe  (Practical 
Physiology). 

A.  Blood    Pressure. 

The  distribution  of  pressure  is  the  cause  of  the  flow  of 
blood,  and  must  first  be  considered. 


1.  General  Distribution  of  Pressure. 

That  the  pressure  throughout  the  greater  part  of  the 
blood-vessels  is  positive — greater  than  the  pressure  of  the 
atmosphere — is  indicated  by  the  fact  that  if  a  vessel  is 
opened,  the  blood  flows  out  of  it.  The  force  with  which 
blood  escapes  is  a  measure  of  the  pressure  in  that  i^articular 
vessel.  If  an  artery  be  cut,  the  blood  escapes  with  great 
force  ;  if  a  vein  be  cut,  with  much  less  force. 

1.  Arteries. — If  the  pressure  in  the  aorta,  in  the  radial, 
in  the  dorsalis  pedis,  and  in  one  of  the  smallest  arteries  is 
measured,  it  is  found  that  there  is  no  marked  change  till  the 
very  smallest  arteries  are  reached,  when  the  pressure  rapidly 
falls.  In  the  aorta  the  pressure  may  be  over  150  mm.  Hg, 
while  in  the  capillaries  it  may  be  only  about  20  mm.  Hg. 
This  distribution  of  arterial  pressure  may  be  plotted  as  in 
fig.  183,  Ar. 

2.  Veins, — If  the  pressure  in  any  of  the  small  veins,  in 


BLOOD  VESSELS 


433 


a  medium  vein,  and  in  a  large  vein  near  the  heart  be 
measured,  it  will  be  found — 

1st.  That  the  venous  pressure  is  less  than  the  lowest 
arterial  pressure. 

271(1.  That  it  is  highest  in  the  small  veins,  and  becomes 
lower  in  the  larger  veins.  In  the  great  veins  entering  the 
heart  during  inspiration  it  is  lower  than  the  atmospheric 
pressure. 

3.   Capillaries. — The    pressure    in    the    capillaries     must 

C. 


Fig.  183. — Diagram  of  the  Distribution  of  Mean  Blood  Pressure  throughout 
the  Blood  Vessels.     Ar.,  the  arteries  ;  C,  the  capillaries  ;    V.,  the  veins. 

obviously  be  intermediate  between  that  in  the  arteries  and 
in  the  veins. 


The  pressure  in  any  part  of  a  system  of  tubes  depends 
upon  two  factors  : — 

1st.  The  force  propelling  fluid  into  that  part  of  the 
system. 

2nd.  The  resistance  to  the  outflow  of  fluid  from  that 
part  of  the  system. 

The  pressure  in  the  arteries  is  high,  (1)  because  with  each 
beat  of  the  heart  the  contents  of  the  ventricle  are  thrown 
with  the  whole  contractile  force  of  the  heart  into  the  corre- 
sponding artery  ;  and  (2)  because  the  resistance  offered  to 
the  outflow  of  blood  from  the  arteries  into  the  capillaries 
and  veins  is  enormous,  since  the  blood,  as  it  passes  into 
28 


434  VETERINARY  PHYSIOLOGY 

innumerable  small  vessels,  is  subjected  to  greater  and  greater 
friction — just  as  a  river,  in  flowing  from  a  deep  narrow 
channel  on  to  a  broad  shallow  bed,  is  subjected  to  greater 
friction. 

Thus,  in  the  arteries  the  powerful  propulsive  force  of  the 
heart  and  the  great  resistance  to  outflow  keep  the  pressure 
high. 

When  the  capillaries  are  reached,  much  of  the  force  of 
the  heart  has  been  lost  in  dilating  the  elastic  coats  of 
the  arteries,  and  thus  the  inflow  into  the  capillaries  is  much 
weaker  than  the  inflow  into  the  arteries.  At  the  same  time, 
the  resistance  to  outflow  is  small,  for,  in  passing  from 
capillaries  to  veins,  the  channel  of  the  blood  is  becoming 
less  broken  up  and  thus  offers  less  friction  to  the  flow  of 
the  blood. 

When  the  veins  are  reached,  the  propelling  force  of  the 
heart  is  still  further  weakened,  and  hence  the  force  of  inflow 
is  very  small.  But  there  is  no  resistance  to  outflow  from 
the  veins  into  the  heart  during  diastole.  Further,  the  great 
veins,  before  they  reach  the  heart,  pass  into  the  thorax,  an 
air-tight  box  in  which,  during  each  inspiration,  a  low 
pressure  is  developed. 

What  has  been  said  of  the  pressure  in  the  veins  applies 
equally  to  that  in  the  lymphatics. 

2.  Rhythmic  Variations  in  Blood  Pressure. 

Before  considering  the  methods  of  investigating  the 
pressure  in  these  different  vessels,  and  the  changes  which 
they  undergo,  certain  rhythmic  variations  in  pressure 
may  first  be  considered. 

A.  Changes  in  Pressure  Synchronous  with  the 
Heart  Beats. 

1.  The  Arterial  Pulse. 
With    each    ventricular    systole,    the    contents    of   each 
ventricle  are  thrown  into  the  already  full  arteries,  and  the 
pressure  in  these  vessels  is  suddenly  raised. 


BLOOD   VESSELS  435 

If  the  finger  be  pressed  upon  an  artery,  a  distinct 
expansion,  due  to  this  rise  of  pressure,  will  be  felt  following 
each  systole.  This  is  the  arterial  pulse.  It  is  simply  a 
rise  of  pressure,  and  it  has  nothing  to  do  with  the  flow  of 
blood. 

The  pulse  wave  may  be  compared  to  a  wave  at  sea,  which 
is  also  a  Avave  of  increased  pressure,  the  only  difference 
being  that,  while  the  wave  at  sea  travels  freely  over  the 
surface,  the  pulse  wave  is  confined  in  the  column  of  blood, 
and  manifests  itself  by  expanding  the  walls  of  the  arteries. 

If  a  vein  be  investigated  in  the  same  way,  it  will  be 
found  that  no  such  pulse  can  be  detected.  In  the  capillaries, 
also,  this  pulse  does  not  exist. 

It  is  best  marked  in  the  great  arteries,  and  becomes  less 
and  less  distinct  as  the  small  terminal  arteries  are  reached. 

1.   Causes  of  the  Pulse. — The  arterial  pulse  is  due  to — 

1st.  The  intermittent  inflow  of  blood.  The  arteries 
expand  from  the  sudden  increase  of  pressure  due  to  each 
sudden  rush  of  blood  from  the  heart  into  the  arterial 
system. 

'Unci.  The  resistance  to  outflow  from  the  arteries  into  the 
capillaries. 

If  blood  could  flow  freely  from  the  arteries  into  the 
capillaries,  then  the  inrush  of  blood  from  the  heart  would 
simply  displace  the  same  amount  of  blood  into  the 
capillaries  and  the  arteries  would  not  be  expanded.  As 
already  indicated,  the  friction  between  the  walls  of  the 
innumerable  small  arterioles  and  the  blood  is  so  great  that 
the  flow  out  of  the  arteries  is  not  sufficiently  free  to  allow 
the  blood  to  pass  into  the  capillaries  as  rapidly  as  it  is  shot 
into  the  arteries.  Hence,  with  each  beat  of  the  heart,  an 
excess  of  blood  must  accumulate  in  the  arteries  to  be  passed 
on  into  the  capillaries  and  veins  between  the  beats. 

3rcZ.  The  elasticity  of  the  walls.  To  allow  of  their 
expanding  to  accommodate  this  excess  of  blood  the  walls 
of  the  arteries  must  be  elastic. 

It  is  upon  these  three  factors  that  the  arterial  pulse 
depends.  Do  away  with  any  of  them,  and  the  pulse  at 
once  disappears. 


436  VETERINARY  PHYSIOLOGY 

2.  Why  is  there  no  Pulse  in  the  Veins? — Their  walls  have 
a  certain  amount  of  elasticity,  but,  instead  of  there  being  a 
resistance  to  the  outflow  of  blood  from  the  veins  into  the 
heart,  this  is  favoured  by  the  suction  action  of  the  thorax 
in  inspiration.  Hence,  even  if  an  intermittent  inflow  were 
well  marked,  the  absence  of  resistance  to  outflow  would  in 
itself  prevent  the  development  of  a  venous  pulse.  But  the 
inflow  is  not  intermittent.  The  arteries  are  so  overfilled 
that  just  as  much  blood  passes  into  the  veins  between  the 
beats  as  during  the  beats  of  the  heart.  Hence  the  most 
important  factor  in  causing  a  pulse,  an  intermittent  inflow, 
is  absent. 

With  no  intermittent  inflow,  and  with  no  resistance  to 
outflow,  the  development  of  a  pulse  is  impossible. 

In  certain  abnormal  conditions,  where,  from  the  extreme 
dilatation  of  the  arterioles,  the  inflow  into  the  veins  is  very 
free,  and  where  the  outflow  from  the  part  of  the  body  is  not 
so  free,  a  local  venous  pulse  may  develop. 

A  special  pulse  in  the  great  veins  near  the  heart  is 
considered  on  p.  444. 

3.  Characters  of  the  Pulse  Wave. — If  a  finger  be  placed  on 
the  carotid  artery  and  another  upon  the  radial  artery,  it  will 
be  felt  that  the  artery  near  the  heart  expands  (pulses)  before 
that  further  from  the  heart  {Practical  Physiology).  The 
pulse  develops  first  in  the  arteries  near  the  heart  and  passes 
outwards  towards  the  periphery.  The  reason  for  this  is 
obvious.  The  arteries  are  always  overfilled  with  blood. 
The  ventricle  drives  its  contents  into  the  overfilled  aorta, 
and,  to  accommodate  this,  the  aortic  wall  expands.  But, 
since  the  aorta  communicates  with  the  other  arteries,  this 
increased  pressure  passes  outwards  along  them,  expanding 
their  wall  as  it  goes. 

It  greatly  simplifies  the  study  of  the  pulse  to  regard  it 
in  this  light,  and  to  study  it  just  as  we  should  study  a  wave 
at  sea. 

1.  Velocity. — To  determine  how  fast  a  wave  is  travelling, 
the  time  may  be  ascertained  which  it  takes  to  pass  from 
one  point  to  another  at  a  known  distance  from  the  first. 
So  with  the  pulse  wave :   two  points  on  an  artery  at  a  known 


BL0{3D   VESSELS  437 

distance  from  one  another  may  be  taken,  and  the  time  which 
the  wave  takes  to  pass  between  them  may  be  measured. 

In  this  way  it  is  found  that  the  pulse  Avave  travels  at 
about  9  or  10  metres  per  second — about  thirty  times  as 
fast  as  the  blood  flows  in  the  arteries  (p.  465). 

2.  Length  of  the  Wave. — To  determine  this  in  a  wave  at 
sea  is  easy,  if  we  know  its  velocity  and  know  how  long  it 
takes  to  pass  any  one  point.  The  same  method  may  be 
appHed  to  the  pulse  wave.  We  know  its  velocity,  and,  by 
placing  the  finger  on  an  artery,  we  may  determine  that  one 
wave  follows  another  in  rapid  succession,  so  that  there  is  no 


A.  B. 

Fig.  184.— Two  Pulse  Tracings — A.  with  a  relatively  sluggish  heart  and 
relatively  high  arterial  pressure  ;  B.  with  a  relatively  active  heart 
and  relatively  low  arterial  pressure.  Both  show  the  primary  crest 
exaggerated  by  the  inertia  of  the  sphygmograph. 

pause  between  them.  Each  wave  lasts  the  length  of  a 
cardiac  cycle.  There  are  about  40  cycles  per  minute — i.e. 
per  60  seconds;  hence,  each  must  last  1'5  second.  The 
pulse  wave  takes  I'o  second  to  pass  any  place,  and  it 
travels  at  10  metres  per  second  ;  its  length  then  is  15 
metres,  or  about  five  times  the  length  of  the  body.  It  is 
then  an  enormously  long  wave,  and  it  has  disappeared  at 
the  periphery  long  before  it  has  hnished  leaving  the  aorta. 

3.  The  Height  of  the  Wave. — The  height  of  the  pulse 
wave,  as  of  a  wave  at  sea,  depends  primarily  on  the  pressure 
causing  it.  It  is  really  the  difference  between  the  maximum 
systolic  pressure  (fig.  190)  and  the  minimum  diastolic 
pressure.      It  may  be  most  accurately  measured  by  determin- 


438 


VETERINARY  PHYSIOLOGY 


ing  these  by  means  of  the  Riva  Rocci  apparatus  (p.  448). 
The  character  of  the  arterial  wall  modifies  it  very  largely 
and  the  true  height  of  the  pulse  wave  in  the  great  arteries 
near  the  heart  is  masked  by  the  thickness  of  the  arterial 
wall. 

The  pulse  wave  is  highest  near  the  heart,  and  becomes 
lower  and  lower  as  it  passes  out  to  the  periphery,  where  it 
finally  disappears  altogether  (fig.  190).  This  disappearance 
is  due  to  its  force  becoming  expended  in  expanding  the 
arterial  wall. 

4.   The  Form  of  the  Wave. — Waves  at  sea  vary  greatly  in 


Fig. 


185. — Diagram  of  Dudgeon's  Sphygniograph.  CI.,  clockwork  driving 
the  smoked  paper,  Tr.,  under  the  writing  point  of  the  liver,  L.,  8p., 
is  a  steel  spring,  with  a  button,  B.,  which  is  applied  over  the  radial 
artery.  With  each  expansion  of  the  artery  the  button  is  moved 
upwards,  and  causes  a  movement  of  the  system  of  levers  indicated  by 
the  arrows. 


form,  and  the  form  of  the  wave  might  be  graphically  recorded 
on  some  moving  surface,  such  as  the  side  of  a  ship,  by  some 
floating  body.  If  the  ship  were  stationary,  a  simple  vertical 
line  would  be  produced  as  the  wave  passed,  but,  if  she  were 
moving,  a  curve  would  be  recorded,  more  or  less  abrupt 
according  to  her  speed.  From  this  curve  the  shape  of  the 
wave  might  be  deduced,  if  the  speed  of  the  vessel  were 
known. 

The  same  method  may  be  applied   to  the  arterial  pulse. 
By  recording  the  changes  produced  by  the  pulse  wave  as  it 


BLOOD  VESSELS 


439 


passes  any  point  in  an  artery  the  shape  of  the  wave  may  be 
deduced  from  the  tracing. 

This  may  be  done  by  any  of  the  various  forms  of 
sphygmograph  (fig.  185),  {Practical  Physiology). 

Such  a  tracing  is  not  a  true  picture  of  the  wave,  but 
simply  of  the  eifect  of  the  wave  on  one  point  of  the  arterial 
wall.  Its  apparent  length  depends  upon  the  rate  at  which 
the  recording  surface  is  travelling,  and  not  upon  the  length 
of  the  wave. 

Its  apparent  height  depends  (i.)  upon  the  length  of  the 


i^WVKf4x]^^ 


fJ^NsKKNKKN^vKKKf^ 


Fig.  186. — Three  Sphygmographic  Tracings  made  from  the  Radial  Artery  of 
a  healthy  man  in  the  course  of  one  hour  without  removing  the 
sphygmograph.  1  was  made  immediately  after  muscular  exercise  ;  2 
was  made  after  sitting  still  for  half  an  hour  ;  and  3,  after  an  hour. 

recording  lever,  (ii.)  upon  the  resistance  offered  by  the  instru- 
ment, (iii.)  upon  the  degree  of  pressure  with  which  the  instru- 
ment is  applied  to  the  artery,  and  (iv.)  on  the  thickness  of 
the  arterial  wall. 

Such  a  trace  (figs.  184  and  186)  shows — 

1st.  That  the  pulse  waves  generally  follow  one  another 
without  any  interval. 

Ind.  That  the  rise  of  the  wave  is  much  more  abrupt 
than  the  fall. 

3rd  That  there  are  one  or  more  secondary  waves  upon 
the  descent  of  the  primary  wave. 


440  VETERINARY   PHYSIOLOGY 

One  of  these  is  constant  and  is  very  often  well  marked. 
It  forms  a  second  crest,  and  is  hence  called  the  dicrotic 
wave  (fig.  184,  c). 

Between  the  chief  crest  and  this  secondary  crest,  a 
smaller  crest  is  often  manifest  (fig.  184,  A.,  b).  From  its 
position,  it  may  be  called  the  predicrotic  wave.  If 
the  wave  has  only  one  crest  the  pulse  is  called  a  one- 
crested  or  monocrotic  wave.  If  the  dicrotic  crest  is  well 
marked  it  is  called  dicrotic. 

That  the  wave  actually  has  the  characters  disclosed  by  a 
sphygmographic  tracing  may  be  demonstrated  by  letting  the 
blood  from  a  cut  artery  play  upon  a  moving  surface  when  a 
Hcemautograph  showing  the  waves  is  produced. 

To  understand  the  various  parts  of  the  pulse  wave,  it  is 
necessary  to  compare  it  with  the  changes  in  the  intra- 
ventricular pressure  throughout  the  cardiac  cycle.  This 
may  be  done  by  taking  synchronously  tracings  of  the  intra- 
ventricular pressure  and  of  the  aortic  pressure  (fig.  173, 
p.  401). 

Such  a  tracing  shows  that,  at  the  moment  of  ventricular 
systole,  the  pressure  in  the  aorta  is  higher  than  that  in  the 
left  ventricle. 

As  ventricular  systole  advances,  the  intra-ventricular 
pressure  rises  and  becomes  higher  than  the  aortic.  At  that 
moment,  the  aortic  valves  are  thrown  open  and  a  rush  of 
blood  takes  place  into  the  aorta,  raising  the  pressure  and 
expanding  the  artery,  and  causing  the  upstroke  and  crest  of 
the  pulse  curve.  In  a  sphygmographic  tracing  this  crest  is 
exaggerated  by  the  inertia  of  the  instrument  (fig.  184). 
After  the  ventricle  has  emptied  itself,  the  intra-ventricular 
pressure  tends  somewhat  to  fall,  and,  at  the  same  time,  a 
fall  in  the  intra-aortic  pressure  begins,  and  goes  on  till 
ventricular  diastole,  while  the  elastic  wall  of  the  artery 
recovers  and  reduces  the  size  of  the  vessel.  With  diastole, 
the  intra-ventricular  pressure  suddenly  becomes  less  than  the 
intra-aortic,  and  the  semilunar  valves  are  forced  downwards 
towards  the  ventricles,  and  thus  the  capacity  of  the  aorta  is 
slightly  increased  and  the  pressure  falls  sharply.      This  fall 


BLOOD   VESSELS  441 

in  pressure  is  indicated  by  the  dicrotic  notch.  But  the 
elasticity  of  the  semilunar  valves  at  once  makes  them  spring 
up,  thus  increasing  the  pressure  in  the  aorta  and  causing  the 
second  crest,  the  dicrotic  wave  (fig,  184,  c).  After  this  the 
pressure  in  the  arteries  steadily  diminishes  till  the  minimum 
is  reached,  to  be  again  increased  by  the  next  ventricular 
systole. 

If  all  the  blood  does  not  leave  the  ventricle  in  the  first 
gush,  e.g.  when  the  intra-aortic  pressure  is  high  as  compared 
with  the  force  of  the  heart  (fig.  175,  continuous  line),  there 
is  a  residual  outflow  which,  by  catching  the  lever  of  the 
sphygmograph  on  its  back-spring  from  the  initial  crest,  may 
again  raise  it,  causing  the  predicrotic  wave. 

It  is  thus  manifest  that  the  form  of  the  pulse  ivave  varies 
according  to  the  relationship  between  the  arterial  pressure 
and  the  activity  of  the  heart.  It  is  not  the  actual  activity 
of  the  heart  or  the  actual  arterial  pressure,  but  their  relation- 
ship to  one  another  which  is  of  importance.  Thus,  a  heart 
actually  weak  may,  with  a  low  arterial  pressure,  be  relatively 
active. 

(A)  If  the  heart  is  active  and  strong  m  relation  to  the 
arterial  pressure,  the  main  mass  of  the  blood  is  expelled  in 
the  first  sudden  outflow,  and  the  residual  flow  is  absent  or 
slight  (fig.  175,  dotted  line).  In  this  case  there  is  a  sudden 
and  marked  rise  of  the  arterial  pressure,  followed  by  a  steady 
fall  till  the  moment  of  ventricular  diastole.  The  rebound  of 
the  semilunar  valves  is  marked  and  causes  a  very  prominent 
dicrotic  wave,  while  the  predicrotic  wave  is  small  or  absent 
(fig.  184,  B.).  Such  a  condition  is  well  seen  after  violent 
nmscular  exertion,  and  in  certain  fevers.  In  these  con- 
ditions the  dicrotic  wave  may  be  so  well  marked  that  it  can 
be  felt  with  the  finger. 

(B)  On  the  other  hand,  if  the  ventricles  are  acting  slowly 
or  feebly  in  relationship  to  the  arterial  pressure,  the  initial 
outflow  of  blood  does  not  take  place  so  rapidly  and  com- 
pletely (fig.  175,  continuous  line),  and  the  initial  rise  in  the 
pulse  is  thus  not  so  rapid.  The  residual  outflow  of  blood  is 
more  marked  and  causes  the  well-marked  secondary  rise  in 
the  pulse  curve — the   predicrotic   wave.      In    certain   cases, 


442  VETERINARY   PHYSIOLOGY 

this  may  be  higher  than  the  primary  crest,  producing  the 
condition  known  as  the  anacrotic  pulse.  The  relatively 
high  intra-arteriai  pressure  in  such  a  case  prevents  the 
development  of  a  well-marked  dicrotic  wave. 

In  extreme  cases  of  this  kind,  when  the  arterial  walls  are 
very  tense,  they  may  recover  in  an  irregular  jerky  manner, 
and  may  give  rise  to  a  series  of  katacrotic  crests  producing  a 
polycrotic  pulse  (fig.  186,  s). 

From  what  has  been  said,  it  will  be  seen  that  a  study  of 
the  pulse  wave  gives  most  valuable  information  as  regards 
the  state  of  the  circulation,  and  the  physician  constantly 
makes  use  of  the  pulse  in  diagnosis. 

Palpation  of  the  Pulse. — On  placing  the  finger  on  the 
artery  the  points  to  determine  are — 

1st.  The  rate  of  the  pulse — i.e.  the  rate  of  the  heart's 
action. 

Ind.  The  rhythm  of  the  pulse — i.e.  of  the  heart's  action, 
as  regards — (1)  Strength  of  the  various  heats. — Normally 
the  beats  differ  little  from  one  another  in  force — since  the 
various  heart-beats  have  much  the  same  strength.  Respira- 
tion has  a  slight  effect  which  will  afterwards  be  considered 
(see  p.  535).  In  pathological  conditions  great  differences 
in  the  force  of  succeeding  pulse  waves  may  occur.  (2) 
Time  relationship  of  the  beats. — Normally  the  beats  follow 
one  another  at  regular  intervals — somewhat  shorter  during 
inspiration — somewhat  longer  during  expiration.  In  patho- 
logical conditions  great  irregularities  in  this  respect  may 
occur. 

Srd.  The  volume  of  the  pulse  wave.  Sometimes  the 
wave  is  high  and  greatly  expands  the  artery — sometimes  it 
is  less  high  and  expands  the  artery  less.  This  is  a  measure 
of  the  difference  between  systolic  and  diastolic  pressure. 
The  former  condition  is  called  a  full  pulse  {pulsus  pleniis), 
the  latter  a  small  pulse  (pulsus  parvus).  The  fulness  of 
the  pulse  depends  upon  two  factors : — 1st.  The  average 
tension  in  the  arteries  between  the  pulse-beats — the  diastolic 
pressure.  If  this  is  high,  the  walls  of  the  artery  are  already 
somewhat  stretched,  and  therefore  the  pulse  wave  may  expand 


BLOOD  VESSELS  443 

them  further  only  slightly.  On  the  other  hand,  if  the  diastolic 
pressure  is  low,  the  arterial  wall  is  lax,  and  is  readily  stretched 
to  a  greater  extent.  '2.nd.  The  force  of  the  heart  which  de- 
termines the  systolic  pressure.  To  stretch  the  arterial  wall  to 
a  large  extent  requires  an  actively  contracting  heart  throwing 
a  large  wave  of  blood  into  the  arterial  system  at  each  systole. 
The  full  pulse  is  well  seen  after  violent  exertion,  when  the 
heart  is  active  and  the  peripheral  vessels  moderately  dilated. 
It  is  also  seen  in  a  slow  pulse,  on  account  of  the  greater 
diastolic  filling  of  the  ventricles, 

Mh.  Tension  of  the  pulse.  This  is  really  a  measure  of 
the  maximum  systolic  blood  pressure,  which  may  be  more 
accurately  measured  by  the  Riva  Rocci  apparatus  (p.  448). 
To  test  it,  two  fingers  must  be  placed  upon  the  artery,  and 
the  one  nearer  the  heart  pressed  more  and  more  firmly  on 
the  vessel  until  the  pulse  wave  is  no  longer  felt  under  the 
other  finger. 

The  tension  of  the  pulse  varies  directly  with  the  force  of 
the  heart  and  with  the  peripheral  resistance.  The  first  state- 
ment is  so  obvious  as  to  require  no  amplification.  It  is  also 
clear  that,  if  the  peripheral  resistance  is  low,  so  that  blood 
can  easily  be  forced  out  of  the  arteries  into  the  capillaries, 
the  arterial  wall  will  not  be  so  forcibly  expanded  as  when 
the  resistance  to  outflow  is  great.  Hence  a  high-tension 
pulse  is  indicative  of  a  strongly  acting  heart  with  constric- 
tion of  the  peripheral  vessels.  It  is  well  seen  during  the 
shivering  fit  which  so  frequently  precedes  a  febrile  attack, 
since  at  that  time  the  peripheral  vessels  are  constricted  and 
the  heart's  action  excited. 

oth.  The  form  of  the  pulse  wave  may  be  investigated  by 
means  of  the  finger  alone,  or  by  means  of  the  sphygmo- 
graph.      The  points  to  be  observed  are  : — 

(1)  Does  the  wave  come  up  suddenly  under  the  finger  ? 
In  the  pulsus  celer  (or  active  pulse)  it  does  so ;  in  the 
pulsus  tardus,  on  the  other  hand,  it  comes  up  slowly.  The 
former  condition  is  indicative  of  an  actively  contracting  heart 
with  no  great  peripheral  resistance — the  latter  indicates  that 
the  heart's  action  is  weak  in  relationship  to  the  arterial 
blood  pressure. 


444  VETERINARY   PHYSIOLOGY 

(2)  Does  the  wave  fall  slowly  or  rapidly  ?  Normally  the 
fall  should  not  be  so  sudden  as  the  ascent.  When  the 
aortic  valves  are  incompetent  the  descent  becomes  very 
rapid  (p.  410)  in  the  so-called  "  water-hammer  "  pulse. 

(3)  Are  there  any  secondary  waves  to  be  observed  ? 
The  only  one  of  these  which  can  be  detected  by  the  finger  is 
the  dicrotic  wave,  and  this  only  when  it  is  well  marked. 
When  it  can  be  felt,  the  pulse  is  said  to  be  dicrotic,  and,  as 
before  stated,  this  indicates  an  actively  contracting  heart 
with  an  arterial  pressure  low  relatively  to  the  strength  of  the 
ventricles  (p.  441). 

2.  The  Capillary  Pulse. 

For  the  reasons  already  given,  there  is  normally  no  pulse 
in  the  capillaries  (p.  434).  If,  however,  the  arterioles  to  a 
district  are  freely  dilated,  so  that  little  resistance  is  offered 
to  the  escape  of  blood  from  the  arteries,  and  if,  at  the  same 
time,  the  outflow  from  the  capillaries  is  not  proportionately 
increased,  intermittent  inflow  and  resistance  to  outflow  are 
developed,  and  a  pulse  is  produced.  Such  a  condition  is 
seen  in  certain  glands  during  activity. 

3.  The  Venous  Pulse. 

1.  The  absence  of  a  general  venous  pulse  has  been 
already  explained.  But  just  as  in  the  capillaries,  so  in  the 
veins,  a  local  pulse  may  develop. 

2.  In  the  veins  entering  the  auricles  and  in  the  veins  at 
the  root  of  the  neck  a  pulse  occurs.  This  may  be  recorded 
by  means  of  M'Kenzie's  polygraph  which  consists  of  a  small 
metal  cup  connected  with  a  recording  tambour.  The  cup 
is  applied  closely  to  the  skin  over  the  vein.  This  pulse 
has  no  resemblance  to  the  arterial  pulse,  although  depending 
on  the  same  three  factors. 

Its  form  is  indicated  in  fig.  187. 
Its  features  are  to  be  explained  as  follows  : — 
(a)  Normal. — Blood  is  constantly  flowing  into  the  great 
veins,  pressed  on  from  behind,      (i.)  When  the  auricles  con- 
tract, the  outflow  from  these  veins  into  the  heart  is  suddenly 


BLOOD  VESSELS 


445 


checked,  and  consequently  the  veins  distend,  causing  a  crest 
(^.*S^.)-  -^t  the  moment  of  auricular  diastole  the  outflow  is 
again  free,  a  rush  of  blood  takes  place  into  the  distending 
auricles,  and  thus  the  pressure  in  the  veins  falls,  (ii.)  But,  as 
this  is  occurring,  two  things  happen — (a)  blood  is  shot  from 
the  ventricles  into  the  arteries,  and  the  carotid,  lying  behind 
the  jugular  vein,  transmits  its  pulse  through  the  vein  ;  (b)  the 
auriculo-ventricular  valves  are  closed  and  pressed  upon  from  the 
ventricular  side,  and  thus  a  wave  of  pressure  is  sent  back 
through  the  auricles.  These  two  together  cause  a  second 
wave  early  in  ventricular  systole,     (iii.)  While  the  ventricle  is 


Fig.  187.— Tracings  of  the  Pulse  in  the  Great  Veins  in  Relationship  to  the 

Cardiac  Cycle.     normal  venous  pulse,   on  which  is  shown  the 

fourth  crest  which  is  often  absent.     a  and  b,  venous  pulse  in 

tricuspid  incompetence. 


contracted,  blood  cannot  pass  on  from  the  auricles,  and  hence 
it  accumulates  in  the  great  veins  and  makes  a  third  crest  at 
the  end  of  ventricular  systole.  At  the  moment  when  the 
ventricles  dilate,  a  sudden  rush  of  blood  takes  place  from  the 
veins  and  auricles  into  the  ventricles,  and  thus  a  sudden  fall 
in  the  pressure  is  produced,  (iv.)  This  may  be  interrupted  by 
the  rebound  of  the  auriculo-ventricular  ring,  which  was  pulled 
downwards  during  ventricular  systole,  and  this  may  cause  a 
fourth  crest  on  the  pulse.  Gradually,  as  the  ventricles  fill, 
the  pressure  in  the  auricles  and  veins  increases  till  the  next 
auricular  systole. 


446  VETERINARY   PHYSIOLOGY 

(6)  Pathological. — If  the  auriculo-ventricular  valves  are 
incompetent  (p.  410)  blood  is  forced  back  into  the  auricles 
and  veins  when  the  ventricles  contract,  and  the  crest  during 
ventricular  S3^stole  becomes  more  and  more  marked  (fig,  187,6). 
The  height  of  this  crest  is  a  good  index  of  the  amount  of 
regurgitation. 


B.    Changes    in    Pressure    Synchronous    with   the 
Respiration. 

1.   Arterial. — Not     only    do    rhythmic     changes    in    the 
arterial  pressure    occur  with    each    beat  of  the  heart,  but 


Fig.  188. — Tracing  of  the  Arterial  Blood  Pressure  to  show  large  respiratory- 
variations,  and  small  variations  due  to  heart  beats  upon  these,  and 
the  sudden  fall  in  the  pressure  produced  by  stimulating  the  inferior 
cardiac  branch  of  the  vagus  nerve. 

larger  changes  are  caused  by  the  respirations — the  rise  in 
pressure  in  great  measure  corresponding  to  the  phase  of 
inspiration,  the  fall  in  pressure  to  the  phase  of  expiration. 
This  statement  is  not  quite  accurate,  as  will  be  seen  when 
considering  the  influence  of  respiration  on  circulation  (see  p. 
535).  These  variations  are  easily  seen  in  a  tracing  of  the 
arterial  pressure  taken  with  the  mercurial  manometer  (fig. 
188). 


BLOOD   VESSELS  447 

2.  Venous. — A  pulse,  synchronous  with  the  respirations, 
may  also  be  observed  in  the  great  veins  at  the  root  of  the 
neck  and  in  the  venous  sinuses  of  the  cranium  when  it  is 
opened.  With  each  inspiration  they  tend  to  collapse  ;  wath 
each  expiration  they  again  expand.  The  reason  for  this  is 
that  during  inspiration  the  pressure  inside  the  thorax  becomes 
low,  and  hence  blood  is  sucked  from  the  veins  into  the  heart. 
Hence  the  danger  that  in  operating  on  the  neck  a  vein  may 
be  opened  and  air  sucked  into  the  circulation  to  block  the 
vessels  in  the  lungs.  During  expiration,  the  intra- thoracic 
pressure  becomes  higher  and  thus  the  entrance  of  blood 
into  the  heart  is  opposed. 


3.  Mean  Blood  Pressure. 

L  Pressure  in  the  Arteries. 

(1)   Methods. 

A.  In  Lower  Animals. —  1.  The  first  investigation  of  the 
pressure  in  the  blood-vessels  was  made  by  the  Rev.  Stephen 
Hales  in  1733.  He  fixed  a  long  glass  tube  in  the  femoral 
artery  of  a  horse  laid  on  its  back,  and  found  that  the  pressure 
supported  a  column  of  blood  of  8  feet  3  inches,  while,  when 
the  tube  Avas  placed  in  a  vein,  only  1  foot  was  supported. 
The  capillary  pressure  is,  of  course,  intermediate  between 
these  two. 

2.  At  the  present  time,  instead  of  letting  the  blood 
pressure  act  directly  against  the  force  of  gravity,  it  is  found 
more  convenient,  in  studying  the  pressure  in  an  artery,  to 
let  it  act  through  a  column  of  mercury  placed  in  a  U  tube 
(fig.  189,  A.).  (1)  To  record  the  changes  in  pressure  a 
float  is  placed  upon  the  mercury  in  the  distal  limb  of  the 
tube,  and  this  carries  a  writing  style  which  records  the 
changes  upon  a  moving  surface.  (2)  The  tube  and  the 
proximal  end  of  the  manometer  are  filled  with  a  strong 
solution  of  sodium  sulphate  to  prevent  clotting  and  to 
transmit  the  pressure  to  the  mercury,  (3)  Before  the 
artery  is  undamped,  the  pressure  is  raised  in  the  proximal 


448 


VETERINARY  PHYSIOLOGY 


end  of  the  manometer  so  that  it  nearly  equals  that  in  the 
artery,  and  thus  prevents  the  animal  from  bleeding  into  the 
tube. 

With  such  an  apparatus  a  record  such  as  is  shown  in 
fig.  188  is  given.  The  actual  pressure  is  measured  by 
taking  the  difference  between  the  level  of  the  mercury  in 
the  two  limbs  of  the  tube.  To  make  the  measurement,  it  is 
customary  to  describe  an  abscissa  when  the  mercury  is  at 
the  same  level  in  the  two  sides  of  the  tube.      The  heigrht  of 


¥Z> 


Pig.  189. — A.,  The  Mercurial  Manometer  with  Recording  Float,  used  in 
taking  records  of  the  arterial  blood  pressure  of  lower  animals.  The 
clamped  tube  is  to  allow  of  the  pressure  being  raised.  The  long  tube 
is  connected  with  the  cannula  placed  in  the  artery.  B.,  The  Riva 
Rocci  Sphygmometer,  for  measuring  the  arterial  pressure  in  man. 
M.,  manometer  ;  P.,  pump  ;    V.,  valve. 


the  style  above  the  abscissa  must  be  multiplied  by  two  to 
give  the  pressure,  on  account  of  the  depression  in  the 
proximal  limb  which  accompanies  the  rise  in  the  distal 
limb. 

On  the  record  made  with  such  an  instrument,  the 
rhythmic  variations  in  the  arterial  blood  pressure  already 
considered  on  p.  434  et  seq.  are  clearly  visible  (Practical 
Physiology). 


BLOOD   VESSELS 


449 


B.  In  the  intact  Animal. — 1.  To  measure  the  systolic 
pressure  it  is  necessary  to  find  the  pressure  which  must  be 
appHed  to  an  artery  in  order  to  prevent  the  pulse  from  passing. 

This  may  be  done  with  Riva  Rocci's  apparatus  (fig.  189, 
{B.)  by  applying  a  bag  round  the  limb  so  that  it  rests  upon 
the  brachial  artery.  The  bag  is  firmly  strapped  on  by 
means  of  a  broad  supporting  belt,  and  it  is  connected  with  a 
pump,  by  which  the  pressure  within  it  may  be  raised,  and 
with  a  mercurial  manometer  by  which  the  pressure  applied 
may  be  measured  in  mm.  of  mercury.  The  pressure  is  then 
raised  either  (a)  till  the  pulse  beyond  the  band  is  no  longer 
felt,  or  (6)  till  no  sound  is  heard  with  each  pulse  wave  through 
a  stethoscope  applied  to  the  artery  beyond  the  band.  The 
pressure  is  then  gradually  relaxed  till  (a)  the  pulse  is  again 


Fig.  190. — To  show  the  Difference  between  Systolic,  Diastolic,  and  Mean 
Blood  Pressure  throughout  the  Arterial  System.  S.,  systolic  pressure  ; 
D.,  diastolic  pressure  ;  J/.,  mean  pressure. 


felt  or  (6)  the  sound  with  each  pulse  wave  heard  through  the 
stethoscope  reappears.  The  column  of  mercury,  at  this 
moment,  indicates  the  systolic  pressure  in  the  artery 
{Practical  Physiology). 

2.  The  diastolic  pressure  may  be  measured  by  further 
relaxing  the  pressure  and  noting  the  point  at  which  the 
pulse  sound  again  disappears. 


(2)   Normal  Arterial  Pressure. 

By  these  methods  it  has    been   found    that    the    systolic 
pressure  in  the  brachial  artery  of  man  is  about  120   mm.  of 
29 


450  VETERINARY   PHYSIOLOGY 

mercury,  while  the  diastohc  pressure  is  only  about  70  mm. 
The  difference  between  these,  of  course,  gives  the  pulse 
pressure. 

(3)  Factors  controlling  Arterial  Pressure. 

(1)  The/o7^ce  of  the  heart  and  (2)  the  degree  of  'peripheral 
resistance  both  modify  the  arterial  pressure,  and  normally 
these  so  act  together  that  any  disturbance  of  one  is  com- 
pensated for  by  changes  in  the  other.  Thus,  if  the  heart's 
action  becomes  increased  and  tends  to  raise  the  arterial 
pressure,  the  peripheral  resistance  falls  and  prevents  any 
marked  rise.  Similarly,  if  the  peripheral  resistance  is 
increased,  the  heart's  action  is  diminished,  and  the  rise  in 
the  pressure  is  checked.  Under  certain  conditions,  however, 
this  compensatory  action  is  not  complete,  and  changes  in 
the  arterial  pressure  are  thus  brought  about. 

(3)  The  volume  of  blood  has  a  comparatively  small 
influence  on  the  arterial  pressure  (1)  because  by  changes  in 
the  degree  of  contraction  of  the  peripheral  vessels,  the 
volume  of  the  vascular  system  may  be  adapted  to  the 
volume  of  blood  contained,  and  (2)  because  there  is  a  very 
free  exchange  of  water  between  the  blood  and  the  tissues 
through  the  walls  of  the  capillaries.  Hence,  after  a 
hemorrhage,  the  volume  of  the  blood  is  rapidly  restored, 
and  hence,  after  transfusion  of  salt  solution,  the  fluid  rapidly 
passes  out  of  the  vessels.  But  if  an  excessive  loss  of  blood 
occurs,  or  if  a  large  quantity  of  blood  stagnates  in  any 
region  and  is  thus  put  out  of  effective  circulation,  the  vessels 
may  not  be  able  to  adapt  themselves,  and  the  arterial 
pressure  may  fall.  From  observations  made  during  the 
Great  War  it  appears  that  in  man  a  loss  of  40  per  cent,  of 
the  blood  is  generally  accompanied  by  a  fall  of  the  systolic 
pressure  to  about  80  mm.  Hg. 

When  the  pressure  falls  in  this  way  it  may  be  restored 
by  injecting  into  a  vein  a  sufficient  amount  of  some  fluid 
which  has  no  prejudicial  action  on  the  blood  and  which 
does  not  too  readily  transude  out  of  the  capillaries.  In 
man,  gum  arable  in  6  per  cent,  solution  has  been  used. 


BLOOD  VESSELS  451 

L  Heart's  Action.— The  influence  of  this  may  be  readily 
demonstrated  by  stimulating  the  vagus  nerve  while  taking  a 
tracing  of  the  arterial  pressure.  The  heart  is  inhibited,  less 
blood  is  forced  into  the  arteries,  and  the  pressure  falls 
(tig.  188). 

If,  on  the  other  hand,  the  augnientor  nerve  is  stimulated, 
the  increased  heart's  action  drives  more  blood  into  the 
arteries,  and  the  pressure  rises. 

II.  Peripheral  Resistance.— The  resistance  to  outflow  from 
the  arteries  to  the  capillaries  and  veins  depends  upon  (1)  the 
resistance  offered  in  the  small  arteries,  the  walls  of  which 
are  surrounded  by  visceral  muscle  fibres.  When  these  fibres 
are  contracted,  the  vessels  are  small  and  the  resistance  is 
great.  When  they  are  relaxed,  the  vessels  dilate,  and  the 
resistance  to  outflow  is  diminished.  This  muscular  tissue  of 
the  arterioles  acts  as  a  stop-cock  to  the  flow  of  blood  from 
the  arteries  to  the  capillaries.  It  is  of  great  importance 
—  1st,  in  maintaining  the  uniform  pressure  in  the 
arteries  ;  2nd,  in  regulating  the  flow  of  blood  into  the 
capillaries. 

(2)  The  condition  of  the  capillaries. — Krogh  has  shown 
that  in  resting  muscle  most  of  the  capillaries  are  closed, 
while  in  contracting  muscle  they  are  dilated  and  filled 
with  blood  even  when  the  arterial  pressure  has  not 
been  allowed  to  rise.  The  capillaries  thus  seem  able  to 
contract  and  expand.  Recently  Dale  has  shown  that 
the  administration  of  histamine  to  dogs  and  monkeys 
causes  such  a  dilatation  with,  at  the  same  time,  a 
contraction  of  the  arterioles.  Krogh  also  finds  that  a 
dilatation  of  capillaries  may  be  produced  by  stimulating 
directly. 

He  considers  that  the  state  of  the  arterioles  regulates 
the  pressure  of  blood  in  the  arteries,  while  the  state 
of  the  capillaries  regulates  the  rate  of  jioiv.  A  slow 
current  through  dilated  capillaries  means  arteriole  con- 
striction. 

Dilatation  of  the  arterioles  and  of  the  capillaries  is  gener- 
ally local,  and  its  effects  upon  the  general  arterial  pressure  is 


452  VETERINARY   PHYSIOLOGY 

generally  compensated  for  by  contraction  in  other  parts  of 
the  body  (p.  460). 

(3)  The  viscosity  of  the  blood. —  The  friction  between 
the  walls  of  the  blood-vessels  and  the  blood  is  modified 
by  the  viscosity  of  the  latter  (p.  475).  After  severe 
haemorrhage  this  is  markedly  decreased,  and  the  resistance 
to  the  flow  in  the  small  vessels  is  correspondingly  decreased, 
and  hence  the  arterial  pressure  tends  to  fall. 

Q)  Methods  of  Studying  the  Condition  of  the  Arterioles  and 
Capillaries. 

1st.  By  direct  observation. — 1.  With  the  naked  eye.  A 
red  engorged  appearance  of  any  part  of  the  body  may  be  due 
to  dilatation  of  the  arterioles  leading  to  it.  But  the 
capillaries  may  be  more  particularly  dilated,  when,  as  a 
result  of  partial  stagnation  and  the  more  complete  removal  of 
oxygen  from  the  blood,  the  part  may  have  a  bluish  colour. 
The  engorgement  may,  however,  be  due  to  some  obstruction 
to  the  outjioiv  of  blood  from  the  part.  2.  With  the  micro- 
scope. In  certain  transparent  structures,  such  as  the  web  of 
the  frog's  foot,  or  the  wing  of  the  bat,  or  the  mesentery,  it  is 
possible  to  measure  the  diameter  of  the  arterioles  and  the 
capillaries  by  means  of  an  eye-piece  micrometer,  and  to 
study  their  dilatation  and  contraction. 

2nd.  Engorgement  of  the  capillaries  brought  about  either 
by  dilatation  of  the  arterioles,  of  the  capillaries,  or  of  both, 
or  simply  by  increased  force  of  the  heart  raising  the  arterial 
pressure,  manifests  itself  also  in  an  increased  size  of  the  jxirt. 
Every  one  knows  how,  on  a  hot  day,  when  the  arterioles  of 
the  skin  are  dilated,  it  is  difficult  to  pull  on  a  glove  which, 
on  a  cold  day,  when  the  cutaneous  vessels  are  contracted, 
feels  loose.  By  enclosing  a  part  of  the  body  in  a  case  with 
rigid  walls,  filled  with  fluid  or  with  air,  and  is  connected 
with  some  form  of  recording  tambour,  an  increase  or 
decrease  in  the  size  of  the  part,  due  to  the  state  of  its 
vessels,  may  be  registered.  Such  an  instrument  is  called  a 
bulk-measurer  (plethysmograph  or  oncograph). 

SrcZ.  When  the  arterioles  in  a  part  are  dilated  and 
the  blood  is  flowing  freely  into  the  capillaries,  the  part 
becomes  warmer,  and,  by  fixing  a  thermometer  to  the  surface, 


BLOOD   VESSELS  453 

conclusions  as  to  the  condition  of  the  arterioles  may  be 
drawn.  The  temperature  of  the  surface  of  the  body  is  also 
modified  by  the  activity  of  the  heart ;  if  the  heart  begins 
to  fail  the  temperature  tends  to  fall. 

Uh.  By  streaming  blood  through  the  vessels,  gener- 
ally of  a  frog,  and  observing  the  rate  at  which  it  escapes, 
the  changes  in  the  state  of  the  small  vessels  may  be  made 
out.  This  perfusion  method  is  much  used  in  studying  the 
action  of  drugs  (Practical  Physiology). 

5th.  Since  the  state  of  the  arterioles  influences  the  arterial 
pressure  (p.  450),  if  the  heart's  action  is  kept  uniform, 
changes  in  the  arterial  blood  pressure  indicate  changes  in  the 
arterioles — a  fall  of  pressure  indicating  dilatation,  a  rise  of 
pressure,  constriction. 

(•2j  Normal  Condition  of  the  Arterioles. — Normally  the 
arterioles  are  in  a  state  of  semi-contraction  ;  but  if  the 
arterioles  in  some  transparent  tissue  be  examined,  they  will 
be  found  to  undergo  periodic  slow  changes  in  calibre.  The 
ear  of  a  white  rabbit  shows  such  slow  changes  ;  it  seems  at  one 
time  pale  and  bloodless,  at  another  time  red  and  engorged. 
During  this  latter  phase  numerous  vessels  appear  which  in 
the  former  condition  were  invisible.  These  slow  changes  are 
independent  of  the  heart's  action  and  of  the  rate  of  respira- 
tion. They  appear  to  be  due  to  the  periodic  rhythmic 
contraction  of  the  walls  of  the  vessels. 

During  the  functional  activity  of  a  part,  a  free  supply  of 
blood  to  its  capillaries  is  required.  This  is  brought  about 
by  a  relaxation  of  the  muscular  coats  of  the  arterioles  leading 
to  the  part,  and  probably  by  an  active  as  well  as  a  passive 
dilatation  of  the  capillaries.  When  the  part  returns  to  rest, 
the  free  flow  of  blood  is  checked  by  the  contraction  of  the 
muscular  walls  of  the  arterioles,  and  probably  of  the  walls 
of  the  capillaries. 

The  action  of  the  arterioles  is  well  seen  under  the 
influence  of  certain  drugs  (vaso- dilators  and  vaso- 
constrictors). If  nitrite  of  aniyl  is  inhaled  by  the  animal, 
it  will  be  seen  that  the  skin  and  mucous  membranes 
become  red  and  engorged  with  blood,  while  at  the  same 
time  the  arterial  pressure  falls.      Nitrites  cause  the  muscular 


454  VETERINARY   PHYSIOLOGY 

coat  of  the  arterioles  to  relax,  and  thus,  by  diminishing 
peripheral  resistance,  permit  blood  to  flow  freely  from  the 
arteries  into  the  capillaries. 

Salts  of  barium  have  precisely  the  opposite  effect,  causing 
the  skin  to  become  pale  from  imperfect  filling  of  the 
capillaries,  and  producing  a  marked  rise  in  the  arterial 
pressure.  Contraction  of  the  muscles  of  the  arterioles  is 
produced,  and  the  flow  of  blood  from  arteries  to  capillaries 
is  retarded. 

Histamine  seems  to  have  the  peculiar  action  of  constrict- 
ing the  arterioles,  and  in  some  animals  at  least  of  dilating 
the  capillaries. 

(3)  Nervous  Mechanism  Controlling  the  Arterioles- — If  the 
sciatic  nerve  is  cut,  the  small  vessels  in  the  foot  at  once 
dilate.  If  the  nerve  is  stimulated,  they  contract.  The  same 
results  follow  if  the  anterior  roots  of  the  lower  spinal  nerves, 
from  which  the  sciatic  takes  origin,  are  first  cut  and  then 
stimulated. 

TJie  Ci^ntral  nervous  system,  therefore,  exerts  a  constant 
tonic  influence  ui^on  the  arterioles,  keeping  them  in  a  state 
of  semi-contraction,  and  this  action  may  be  increased,  and 
thus  a  constriction  of  the  arterioles  produced.  In  this  way, 
if  the  effect  is  a  general  one,  the  flow  of  blood  from  arteries 
to  capillaries  is  obstructed  and  the  arterial  pressure  may  be 
raised.  This  influence  may  also  be  diminished,  so  that  the 
arterioles  dilate  and  allow  an  increased  flow  into  the 
capillaries  from  the  arteries.  Thus  the  arterial  pressure 
may  be  lowered  if  the  action  is  not  too  local,  and  is  not 
compensated  for  by  constriction  elsewhere. 

These  mobile  arterioles,  under  the  control  of  the  central 
nervous  system,  constitute  a  vaso-motor  mechanism,  which 
plays  a  part  in  nearly  every  vital  process  in  the  body.  By 
it  the  pressure  in  the  arteries  is  governed,  the  supply  of 
blood  to  the  capillaries  and  tissues  is  controlled,  and  the  loss 
of  heat  from  the  skin  is  largely  regulated  (p.  269). 

This  vaso-motor  mechanism  consists  of  two  parts  : — 

Is^.  Peripheral. — This  consists  of  the  muscular  fibres  in 
the  walls  of  the  arterioles  with  the  nerve  structures  among 
them. 


BLOOD   VESSELS  455 

2i}d.  Central. — The  portions  of  the  central  nervous 
system  presiding  over  these  and  the  nerves  which  pass 
from  them. 

1st.  Peripheral  Mechanism. — The  muscular  fibres  are 
maintained  in  a  state  of  tonic  semi-contraction  by  nerves 
passing  to  them,  and  when  these  nerves  are  divided,  the 
muscular  fibres  relax.  But  if,  after  these  nerves  have  been 
cut,  the  animal  be  allowed  to  live,  in  a  few  daj's  the 
arterioles  ar/ain  pass  into  a  state  of  tonic  semi-contraction, 
although  no  union  of  the  divided  nerve  has  taken  place. 

Certain  drugs,  e.g.  digitalis  and  the  salts  of  barium,  act 
as  direct  stimulants  to  these  nuiscle  fibres,  while  nitrites 
inhibit  their  activity. 

The  precise  part  played  by  the  nerve  plexus  in  the  walls 
of  the  arterioles  has  not  been  definitely  established,  but 
certain  drugs  appear  to  act  specially  upon  it.  Thus, 
apocodeine,  while  it  does  not  prevent  barium  salts  from 
constricting  the  vessels,  prevents  the  constricting  action  of 
adrenalin,  even  when  the  nerves  are  cut.  Hence,  it  must 
be  concluded  that  apocodeine  paralyses  a  nervous  mechanism 
in  the  arteriole  wall  which  is  stimulated  by  adrenalin.  On 
the  other  hand,  nicotine  seems  to  block  the  action  of  barium, 
but  not  that  of  adrenalin.  Deductions  from  the  antagonistic 
action  of  drugs  are  by  no  means  satisfactory,  as  their  action 
varies  so  much  with  dosage  and  with  the  functional 
condition  of  the  tissues  at  the  time  when  they  are 
administered. 

'2nd.  Central  Mechanism. — When  a  nerve,  going  to  any 
part  of  the  body,  is  cut,  the  arterioles  of  the  part 
generally  dilate  ;  when  it  is  stimulated,  the  arterioles 
usually  contract ;  sometimes,  however,  they  dilate.  In  no 
case  does  section  of  a  nerve  cause  constriction  of  the 
arterioles. 

These  facts  prove  that  the  central  vaso-motor  nervous 
mechanism  may  be  divided  into  two  parts  : — 

A.  Vaso-constrictor  mechanism. 

B.  Vaso-dilator  mechanism. 

A.  Vaso-constrictor  Mechanism- — The  fact  that  section  of 
most    nerves   at   once   causes  a   dilatation   of  the  arterioles 


456 


VETERINARY   PHYSIOLOGY 


proves  that  they  are  constantly  transmitting  vaso- constrictor 
impulses  from  centres  in  the  nervous  system. 

(1)  Course  of  the  Nerves. — The  course  of  these  fibres  has 
been  investigated  by  section 
and  by  stimulation  (lig. 
191). 

They  leave  the  spinal 
cord,  chiefly  in  the  dorsal 
region,  by  the  anterior  roots 
of  the  spinal  nerves,  pass 
into  the  sympathetic  ganglia, 
where  they  have  their  cell 
stations,  and  then,  as  non- 
medullated  fibres,  pass,  either 
(«)  along  the  various  sym- 
pathetic nerves  to  the  vis- 
cera, or  (h)  back  through 
the  grey  ramus  (see  fig.  46, 
p.  106)  into  the  spinal 
nerve,  in  which  they  run 
to  their  terminations. 

(2)  Mode  of  Action  of 
the  Mechanism.  —  This 
mechanism  is  constantly  in 
action,  maintaining  the  tonic 
contraction  of  the  arterioles. 
(a)  Reflex  Stimulation. — 
If  any  afferent  nerve  be 
stimulated  the  effect  is  to 
cause  a  general  constriction 
of  arterioles,  and  thus  to 
raise  the  general  arterial 
pressure.  A  central  mech- 
anism therefore  exists  cap- 
able of  reflex  excitation.  In  ordinary  conditions,  so  many 
afferent  nerves  are  constantly  being  stimulated,  that  it  is  not 
easy  to  say  how  far  the  tonic  action  of  this  centre  is  reflex 
and  dependent  on  the  stream  of  afferent  impulses. 

(6)   Direct  Stimulation.  —  The    centre    may    undoubtedly 


Fig.  191. — Diagram  of  the  Distribu- 
tion of  Vaso-niotor  Xerves.  The 
continuous  line  shows  the  vaso- 
constrictors, the  dotted  line  the 
vaso-dilators.  C.iV., cranial  nerves; 
Vag.,  vagus;  T.S.,  thoracic  sym- 
pathetic; A.S.,  abdominal  sym- 
pathetic ;  N.L.,  nerves  to  the  leg. 


BLOOD    VESSELS  4-57 

be  directly  acted  upon  by  the  condition  of  the  blood  and 
lymph  circulating  through  it.  When  the  blood  becomes 
charged  with  carbon  dioxide,  as  in  asphyxia,  this  centre  is 
stimulated  and  a  general  constriction  of  arterioles  with  high 
blood  pressure  results  (p.  548).  The  same  thing  happens  as 
a  result  of  a  marked  decrease  in  the  amount  of  oxygen  in 
the  blood.  This  leads  to  an  imperfect  oxidation  of  such 
products  as  lactic  acid,  and  to  their  accumulation  in  the 
blood,  and  this  concentration  of  H  ions  stimulates  the  vaso- 
motor mechanism. 

(3)  Position  of  the  Centres. — (a)  Primary  Centre. — In 
investigating  the  position  of  the  centre  advantage  may  be 
taken  of — 

1st.  Its  constant  tonic  influence.  Removal  of  the  centre 
at  once  causes  dilatation  of  the  arterioles. 

2nd.  The  fact  that  it  may  be  reflexly  stimulated.  If  the 
vaso-constrictor  centre  be  removed,  stimulation  of  an  afferent 
nerve  no  longer  causes  constriction  of  the  arterioles. 

Removal  of  the  whole  brain  above  the  pons  Varolii  leaves 
the  action  of  the  centre  intact. 

Separation  of  the  pons  Varolii  and  medulla  oblongata  from 
the  spinal  cord  at  once  causes  a  dilatation  of  the  arterioles  of 
the  body  with  a  marked  fall  in  arterial  pressure,  and  prevents 
the  production  of  reflex  constriction  by  stimulation  of  an 
afferent  nerve. 

The  main  part,  at  least,  of  the  vaso-constrictor  mechanism 
therefore  is  situated  in  the  pons  Varolii  and  medulla  oblongata. 
The  extent  of  this  centre  has  been  determined  by  slicing 
away  this  part  of  the  brain  from  above  downwards,  and 
studving  the  influence  of  reflex  stimulation  after  the  removal 
of  each  slice. 

It  is  found  that,  at  a  short  distance  below  the  tectum,  the 
removal  of  each  succeeding  part  is  followed  by  a  diminution 
in  the  reflex  constriction,  until,  at  a  point  close  to  and  just 
above  the  calamus  scriptorius,  all  reflex  response  to  stimula- 
tion stops. 

The  centre  is  therefore  one  of  very  considerable  longi- 
tudinal extent. 

(6)    Secondary  Centres. — It  has   been   found  that  if,  after 


458  VETERINARY   PHYSIOLOGY 

section  of  the  spinal  cord  high  up,  the  animal  be  kept  alive 
for  some  days,  the  dilated  arterioles  again  contract.  If  the 
spinal  cord  below  the  point  of  section  be  now  destroyed, 
another  marked  fall  of  blood  pressure  occurs.  This  shows 
that  secondary  vaso-constrictor  centres  exist  all  down  the  grey 
matter  of  the  spinal  cord.  Normally  these  are  under  the 
control  of  the  dominant  centre,  but  when  this  is  out  of  action 
they  then  come  into  play. 

B.  Vaso-dilator  Mechanism. — A  good  example  of  a  vaso- 
dilator nerve  is  to  be  found  in  the  chorda  tympani  branch  of 
the  facial  nerve,  which  sends  fibres  to  the  submaxillary  and 
sublingual  salivary  glands.  If  this  nerve  be  cut,  no  change 
takes  place  in  the  vessels  of  the  glands,  but,  when  it  is 
stimulated,  the  arterioles  dilate  and  allow  an  increased  flow 
of  blood  through  the  capillaries.  These  fibres,  therefore, 
instead  of  increasing  the  activity  of  muscular  contraction, 
inhibit  it.  The  gastric  branches  of  the  vagus  carrying  vaso- 
dilator fibres  to  the  mucous  membrane  of  the  stomach,  and 
the  nervi  erigentes  carrying  vaso-dilator  fibres  to  the  external 
genitals,  are  further  examples  of  vaso-dilator  nerves. 

1.  Course  of  the  Fibres. — The  vaso-dilator  nerves  of  most 
parts  of  the  body  run  side  by  side  with  the  vaso-constrictor 
nerves,  and  hence  curious  results  are  often  obtained.  If  the 
sciatic  nerve  of  a  dog  be  cut,  the  arterioles  of  the  foot  dilate. 
If  the  peripheral  end  of  the  cut  nerve  be  stimulated,  the 
vessels  contract.  But  after  a  few  days,  if  the  nerve  be  pre- 
vented from  uniting,  the  arterioles  of  the  foot  recover  their 
tonic  contraction.  If  the  sciatic  nerve  be  now  stimulated,  a 
dilatation,  and  not  a  constriction,  is  brought  about.  The 
vaso-constrictor  fibres  seem  to  die  more  rapidly  than  the 
vaso-dilator  fibres  which  run  alongside  of  them.  Under 
certain  conditions,  the  activity  of  the  vaso-dilator  fibres 
seems  to  be  increased.  Thus,  if  the  sciatic  nerve  be  stimu- 
lated when  the  limb  is  warm,  dilatation  rather  than  con- 
striction may  occur.  Again,  while  rapidly  repeated  and 
strong  induction  shocks  are  apt  to  cause  constriction,  slower 
and  weaker  stimuli  tend  to  produce  dilatation. 

The  vaso-dilator  nerves  pass  out  chiefly  by  the  anterior 


BLOOD   VESSELS 


459 


roots  of  the  various  spinal  nerves,  and  do  not  pass  through 
the  sympathetic  gangha,  but  run  as  medullated  fibres  to 
their  terminal  ganglia  (fig.  191).  Bayliss  has  shown  that  the 
vaso-dilator  fibres  for  the  hind  limb  of  the  dog  leave  the  cord 
by  the  posterior  roots.  Evidence  has  been  adduced  that 
dilatation  can  be  brought  about  by  irritation  of  the  skin  even 
after  the  posterior  root  is  cut  above  the  ganglion,  but  not 
when  it  is  cut  below  it  (p.  98).  The  possible  action  of  this 
secondary  vaso-dilator  mechanism  in  the  production  of  inflam- 
mation and  of  such  trophic 
disturbances  as  shingles  or 
herpes  zoster  is  worthy  of 
attention. 

The  probable  existence  of 
&, 'peripheral  vaso-dilator  rtiech- 
anism  indicated  by  the  action 
of  nitrites  and  of  weak  acids 
may  be  of  importance  in  ex- 
plaining local  dilatation  of 
vessels  during  the  functional 
activity  of  a  part  (p.  455). 

2.  Mode  of  Action. — 
{a)  Reflex  Stimulation. — This 
mechanism  is  not  constantly 
in  action,  since  section  of  a 
vaso-dilator  nerve  does  not 
cause  constriction.      It  may,  however,  be  excited  reflexly. 

Stir>iulatio7i  of  an  afferent  nerve  causes  a  dilatation  of 
the  arterioles  in  the  part  from  which  it  comes,  and  a  con- 
striction of  the  arterioles  throughout  the  rest  of  the  body.  If 
a  sapid  substance  such  as  pepper  be  put  in  the  mouth,  the 
buccal  mucous  membrane  and  the  salivary  glands  become 
engorged,  while  there  is  a  constriction  of  the  arterioles 
throughout  the  body.  The  vaso-dilator  mechanism  is  not 
general  in  its  action  like  the  vaso-constrictor,  but  is  specially 
related  to  the  difterent  parts  of  the  body. 

Again,  it  has  been  shown  that  stimulation  of  the  central 
end  of  the  depressor  nerve  (superior  cardiac  branch  of  the 
vagus)   causes   a  dilatation  of  the  arterioles,  chiefly  in  the 


Fig.  192. — To  show  Local  Vaso- 
dilatation with  General  Vaso- 
constriction. S.,  skin;  V.D., 
local  vaso-dilator  centre;  V.C., 
vaso-constrictor  centre. 


460  VETERINARY    PHYSIOLOGY 

abdominal  cavity,  but  also  throughout  the  body  generally. 
This  is  the  most  generalised  vaso-dilator  reflex  known  (see 
p.  418). 

(6)  Stimulation  from  the  Cerebrum. — Not  onlydoes  peripheral 
stimulation  thus  act  reflexly,  but  various  states  of  the  brain, 
accompanied  by  emotions,  may  stimulate  part  of  the  vaso- 
dilator mechanism,  as  in  the  act  of  blushing. 

The  vaso-constrictors  and  vaso-dilators  have  a  reciprocal 
action,  and  this  is  of  the  greatest  importance  in  ph3'si- 
ology  and  pathology.  It  explains  the  increased  vascularity 
of  a  part  when  active  growth  is  going  on.  The  changes 
in  the  part,  or  the  products  of  these,  stimulate  the  afferent 
nerve.  This  reflexh'  stimulates  the  vaso-dilator  mechanism 
of  the  part,  and  thus  causes  a  free  flow  of  blood  into  the 
capillaries,  and,  at  the  same  time,  maintains  or  actually 
raises  the  arterial  pressure  by  causing  a  general  constriction 
of  the  arterioles,  and  thus  forces  more  blood  to  the  situation 
in  which  it  is  required.  It  also  explains  the  vascular  changes 
in  inflammation  (fig.  192). 

This  reciprocal  action  may  be  disturbed  just  as  the 
reciprocal  action  of  motor  and  inhibitory  nerves  in  reflex 
action  may  be  disturbed.  The  administration  of  strychnine, 
which  in  reflex  action  converts  inhibitory  into  motor  responses, 
also  converts  vaso-dilator  into  vaso-constrictor  responses,  while 
chloroform  tends  to  convert  vaso-constrictor  into  vaso-dilator 
actions. 

(3)  Position  of  the  Centres.— While  the  dominant  vaso- 
constrictor centre  is  in  the  medulla,  the  vaso-dilator  centres 
seem  to  be  distributed  in  the  medulla  and  spinal  cord. 
The  vagus  is  the  great  outgoing  vaso-dilator  nerve  from  the 
centres  in  the  medulla,  and  the  nervi  erigentes,  or  pelvic 
nerves,  from  the  sacral  part  of  the  cord. 

II.  Pressure  in  the  Capillaries. 

This  may  be  determined  (a)  by  finding  the  pressure 
required  to  blanch  the  skin  or  to  occlude  the  capillaries  of 
some  transparent  membrane,  or  (6)  by  inserting  a  hypodermic 


BLOOD  VESSELS 


461 


needle  connected  with  a  reservoir  of  water  and  a  manometer, 
and  estimating  the  capillary  pressure  by  the  pressure 
required  to  drive  the  water  into  the  subcutaneous  tissue. 
The  assumption  is  made  that  the  pressure  in  the  tissues  is 
about  equal  to  that  in  the  capillaries.  The  movement  of 
the  water  may  be  determined  by  watching  the  movement  of 
a  bubble  of  air  in  the  tube  (fig.  193). 

At  the  level  of  the  heart  the  capillaries  may  be  com- 
pressed by  a  pressure  of  some  10  to  20  mm.  Hg,  but  in  the 
leg  about  90  mm.  is  required. 

It  has  already  been  shown  that  the  pressure  is  less  than 
in  the  arteries  and  greater  than  in  the  veins. 


Fig.  193.— Method  of  Estimating  Capillary  Blood  Pressure  (see  text). 


Like  the  pressure  in  the  arteries,  the  pressure  in  the 
capillaries  depends  upon  the  two  factors — 

1st.   Force  of  inflow. 

2nd.   Resistance  to  outflow. 

It  must  be  remembered  that  all  the  blood  does  not  flow 
through  capillaries,  but  that  in  many  situations  arterioles 
open  directly  into  venules.  Thus,  in  obstruction  of  the 
capillaries,  the  blood  may  find  its  way  through  to  the  veins. 

1st.  Variations  in  the  Force  of  Inflow. — The  capillary 
pressure  may  undergo  marked  local  cnanges  through  the 
vaso-motor  inechanism.  Wherever  the  function  of  a  part  is 
active,  dilatation  of  the  arterioles  and  an  increased  capillary 
pressure  exist,  and,  as  has  been  already  seen,  the  condition 
of  the  capillaries  plays  a  part. 

But  the  capillary  pressure  may  also  be  modified  by  the 


462 


VETERINARY   PHYSIOLOGY 


A--.  c  V 

\  '; 

I  ? 

Il 

!„;_. ^' ., c 


heart's  action,  inasmuch  as  the  arterial  pressure,  by  which 
blood  is  driven  into  the  capillaries,  depends  upon  this.  In 
cardiac  inhibition  not  only  is  arterial  pressure  lowered,  but 
capillary  pressure  may  also  fall.  In  augmented  heart  action 
both  arterial  and  capillary  pressure  are  raised  (fig.  194,  B.). 

2nd.  Variations  in  Resistance  to  Outflow. — Normally  the 
flow  from  capillaries  to  veins  is  free  and   unobstructed  ;  but, 

if  the  veins  get  blocked, 
or  if  the  flow  in  them  is 
retarded  by  gravity,  the 
capillaries  get  engorged 
with  blood.  This  in- 
creased pressure  in  the 
capillaries  is  very  differ- 
ent from  that  caused  by 
increased  inflow.  The 
flow  through  the  vessels 
is  slowed  or  may  be 
stopped  instead  of  being 
accelerated,  and  the  blooil 
gets  deprived  of  its  oxygen 
and  of  its  nourishing 
constituents,  loaded  with 
wast6  products,  and  tends 
to  exude  into  the  lymph  spaces,  causing  dropsy  (fig. 
194,  C).  A  very  simihir  condition  results  if  the  arterioles 
are  contracted  and  the  capillaries  dilated  ;  the  same  stagna- 
tion of  blood  may  occur. 

It  is  therefore  most  important  to  distinguish  between 
high  capillary  pressure  from  dilated  arterioles  or  an  active 
heart,  and  high  pressure  due  to  venous  obstruction. 

A  condition  very  similar  to  that  described,  but  producing 
a  capillary  pressure  high  relatively  to  the  pressure  in  the 
arteries — though  not  absolutely  high — is  seen  in  cases  of 
failure  of  the  heart,  when  that  organ  is  not  acting  sufficiently 
strongly  to  pass  the  blood  on  from  the  venous  into  the 
arterial  system.  Here  the  arterial  pressure  becomes  lower  and 
lower,  the  venous  pressure  higher  and  higher,  and,  along  with 
this,  the  capillary  pressure  becomes  high  in  relationship  to 


Fig.  194. —The  Changes  in  Blood 
Pressure  in  the  Capillaries  produced 
by    increasing    the    arterial    pressure 

,  and  by  obstructing  the  venous 

flow .       A.,    arteries  ;     G. , 

capillaries  ;    V. ,  veins. 


BLOOD   VESSELS  463 

the  arterial  pressure.  The  blood  is  not  -driven  through 
these  channels,  and  congestion  of  the  capillaries  and  dropsy 
may  result. 

3rd  The  influence  of  gravity  plays  a  very  important  part  on 
the  capillary  pressure,  since  it  has  so  marked  an  influence  on 
the  flow  of  blood  in  the  veins.  At  the  level  of  the  heart 
the  pressure  is  about  20  mm.  Hg.  In  the  feet  it  is  much 
higher.  When,  through  heart  failure  or  want  of  exercise, 
the  blood  is  not  properly  returned  from  the  legs,  this 
increased  pressure  becomes  very  marked,  stagnation  of  blood 
occurs,  and  swelling  of  the  legs  is  apt  to  occur. 

■ith.  Volume  of  the  Blood. — The  pressure  in  the  capillaries 
may  also,  to  a  certain  extent,  be  varied  by  the  ivithdrawal 
of  ivater  from  the  hotly,  as  in  purgation  or  in  diuresis,  or  by 
the  addition  of  large  quantities  of  fluid  to  the  blood.  In  both 
cases  there  is  a  rapid  readjustment  by  the  passage  of  water 
from  the  tissues  to  the  blood,  or  vice  versa.  The  venous 
system  is  so  capacious  that  very  great  changes  in  the  amount 
of  blood  in  the  vessels  may  take  place  without  materially 
modifying  the  arterial  or  capillary  pressure. 


III.   Pressure  in  the  Veins. 

The  pressure  in  the  veins  is  so  low  that  it  may  best 
be  determined  in  the  lower  animals  by  a  water  manometer. 

In  man  it  may  be  estimated  in  a  prominent  superficial  vein 
by  stroking  the  vein  downwards  from  the  peripheral  side  of 
a  valve,  applying  the  band  of  a  Riva  Rocci  apparatus  (p.  448), 
and  then  relaxing  the  pressure  and  allowing  the  blood  to 
flow  up  into  the  emptied  vein,  and  reading  the  pressure  at 
which  this  occurs.  It  may  also  be  estimated  in  the  veins  of 
the  hand  by  finding  at  what  level  above  the  heart  they 
collapse. 

In  the  veins  the  force  of  inflow  is  small ;  the  resistance 
to  outflow  is  nil.  Hence  the  pressure  is  low,  and  steadily 
diminishes  from  the  small  veins  to  the  large  veins  entering 
the  heart  (fig.  183). 

The  venous  pressure  may  be  modified  by  variations  in  these 
two  factors.      Constriction  of  the  arterioles  tends  to  lower  the 


464  VETERINARY   PHYSIOLOGY 

venous  pressure,  dilatation  to  raise  it.  In  the  legs  the  veins 
are  abundantly  supplied  with  valves  which  support  the  long 
column  of  blood.  When  the  veins  of  the  legs  become  over- 
distended  and  the  valves  incompetent,  the  veins  become  large 
and  tortuous  and  are  known  as  varicose  veins.  The  condi- 
tion is  temporarily  relieved  by  elevating  the  legs. 

Gorripression  of  the  thorax  retards  the  flow  of  blood  from 
the  great  veins  into  the  heart,  and  thus  tends  to  raise  the 
venous  and  to  lower  the  arterial  pressure. 

Venous  pressure  may  be  temporarily  modified  by  the 
loss  or  gain  of  water,  but  the  venous  system  is  so  capacious 
that  it  can  accommodate  a  considerably  increased  volume  of 
fluid  without  any  marked  rise  of  pressure.  Further,  there 
is  a  very  rapid  adjustment  between  the  fluid  in  the  vessels 
and  in  the  tissues. 

IV.  Pressure  in  the  Lymphatics. 

No  exact  determination  of  the  lymph  pressure  in  the 
tissue  spaces  has  been  made,  but,  since  there  is  a  constant 
flow  from  these  spaces  through  the  lymphatic  vessels  and 
through  the  thoracic  duct  into  the  veins  at  the  root  of  the 
neck,  the  pressure  in  the  tissue  spaces  must  be  higher  than 
the  pressure  in  the  great  veins. 

This  pressure  is  kept  up  by  the  formation  of  lymph  from 
the  blood,  and  from  the  cells  of  the  tissues  (see  p.  508). 


B.  FLOW    OF    BLOOD. 

The  flow  of  blood,  as  already  indicated,  depends  upon  the 
distribution  of  pressure,  a  fluid  always  tending  to  flow  from 
the  point  of  higher  pressure  to  the  point  of  lower  pressure. 
Since  a  high  pressure  is  maintained  in  the  aorta  and  a  low 
pressure  in  the  veins  entering  the  heart  and  in  the  cavities 
of  the  heart  during  its  diastole,  the  blood  must  flow  through 
the  vessels  from  arteries  to  veins  {Practical  Physiology).  If 
for   any  reason   the  difference  of  pressure  is  decreased,   the 


BLOOD   VESSELS  465 

rate  of  flow  must  be  decreased.  This  occurs  in  various 
forms  of  heart  failure,  and  when  the  heart  has  an  insufficient 
supply  of  blood  to  contract  upon. 

1.  Velocity. 

The  velocity  of  the  flow  of  a  fluid  depends  upon  the 
width  of  the  channel.  Since  in  unit  of  time  unit  of  volume 
must  pass  each  point  in  a  stream,  if  the  fluid  is  not  to 
accumulate  at  one  point,  the  velocity  must  vary  with  the 
sectional  area  of  the  channel.  In  other  words,  the  velocity 
(V)  of  the  stream  is  equal  to  the  amount  of  blood  passing 
any  point  per  second  (v)  divided  by  the  sectional  area  of 
the  stream  (S) — 

V 

Avhere  S  is  the  radius  squared  multiplied  by  the  constant 
3-14. 

In  the  vascular  system  the  sectional  area  of  the  aorta  is 
small  when  compared  with  the  sectional  area  of  the  smaller 
arteries  ;  while  the  sectional  area  of  the  capillary  system  may 
be  no  less  than  700  times  greater  than  that  of  the  aorta.  In 
the  venous  system  the  sectional  area  steadily  diminishes, 
although  it  never  becomes  so  small  as  in  the  corresponding 
arteries,  and,  where  the  great  veins  enter  the  heart,  it  is 
about  twice  the  sectional  area  of  the  aorta  (fig.  195). 

This  arrangement  of  the  sectional  area  of  the  vascular 
system  gives  rise  to  a  rapid  flow  in  the  arteries,  a  somewhat 
slower  flow  in  the  veins,  and  a  very  slow  flow  in  the 
capillaries. 

The  suddenness  of  the  change  of  pressure  has  a  certain 
influence  on  the  rapidity  of  flow,  as  is  well  seen  in  a  river. 
If  from  any  cause  the  pressure  is  raised  at  one  point, 
the  flow  will  tend  to  be  more  rapid  from  that  point 
onwards  till  the  normal  distribution  of  pressure  is  re- 
established. When  the  difference  between  the  pressure 
at  the  arterial  end  and  at  the  venous  end  of  a  set  of 
capillaries  is  increased,  a  more  rapid  flow  of  blood  takes 
place  through  the  tissues. 
30 


466 


VETERINARY  PHYSIOLOGY 


Friction  has  also  a  certain  etfect.  A  river  runs  much 
faster  in  mid -stream  than  along  the  margins,  because  near 
the  banks  the  flow  is  delayed  by  friction,  and,  the  more 
broken  up  and  subdivided  is  the  channel,  the  greater  is  the 
friction  and  the  more  is  the  stream  slowed. 

In  the  capillary  system  considerable  resistance  is  offered 
by  friction  in  the  innumerable  small  channels,  and  this  is 
markedly  influenced  by  the  viscosity  of  the  blood  (p.  475). 


Measurement  of  Velocity. — The    velocity    of    flow    in    the 
arteries  and  veins  may  be  measured  by  various  methods,  of 


A  R 

/ 

--:-r"' 

\ 

\ 

\ 

c 

V 

Fig. 


195. — Diagram  of  the  Sectional  Area  of  the  Vascular  System,  upon 
which  the  velocity  of  the  flow  depends.  A.R.,  arteries;  C, 
capillaries  ;    V. ,  veins. 


which  one  of  the  best  is  that  by  means  of  the  strovuchr,  an 
instrument  by  which  the  volume  of  blood  passing  a  given 
point  in  an  artery  or  vein  in  a  given  time  may  be  deter- 
mined (Practical  Physiology). 

The  velocity  of  the  flow  in  the  capillaries  may  be 
measured  in  transparent  structures  by  means  of  a  microscope 
with  an  eye-piece  micrometer. 

The  velocity  of  the  blood  varies  greatly  but  is  roughly  as 
follows  : — 

Carotid  of  the  dog  about  .         .     300  mm.  per  sec. 
Capillaries  about      .         .         .     O'S  to  1  m.     ,, 
Veiu  (jugular)  about         .         .150  mm. 


BLOOD  VESSELS  467 

Definite  figures  for  the  velocity  of  the  lymph  stream 
cannot  be  given. 

Disturbance  of  any  of  the  factors  which  govern  the  rate 
of  fiow  will  bring  about  alterations  in  the  velocity  of  the 
blood  in  arteries,  capillaries,  and  veins. 


2.  Special  Characters  of  Blood  Flow. 

(a)  Arteries. — This  may  be  investigated  by  a  hcemodromo- 
graph,  which  consists  of  a  paddle  suspended  in  a  box.  By 
means  of  a  tube  at  each  end  the  box  is  inserted  into  the 
course  of  an  artery,  and  with  each  acceleration  of  the  flow 
the  paddle  is  pressed  forward.  The  movements  of  the  other 
end  of  the  paddle  are  recorded  through  a  tambour  on  a 
cylinder. 

The  flow  of  blood  in  an  artery  is  rhythmically  accelerated 
with  each  ventricular  systole.  This  is  due  to  the  pulse  wave. 
As  the  wave  of  high  pressure  passes  along  the  vessels,  the 
blood  tends  to  flow  first  forwards  and  then  backwards  from 
it — so  that  in  front  of  the  wave  there  is  an  acceleration  of 
the  stream  and  behind  it  a  retardation,  just  as  occurs  in  a 
wave  at  sea. 

(b)  Capillaries. — In  the  capillaries  the  flow  is  uniform, 
unless  when  in  excessive  dilatation  of  the  arterioles  the  pulse 
wave  is  propagated  to  them. 

(c)  Veins. — In  most  veins  the  flow  is  uniform,  but  in  the 
great  veins  near  the  heart  it  undergoes  acceleration — 

l.?;",  with  each  diastole  of  auricle  and  of  ventricle  (p. 
444)  ; 

2nd,  with  each  inspiration  (p.  447). 

In  all  vessels,  the  blood  in  the  centre  of  the  stream  moves 
more  rapidly  than  that  at  the  periphery  on  account  of  the 
friction  between  the  blood  and  the  vessels.  This  rapid 
"  axial  "  and  slow  "  peripheral "  stream  is  well  seen  in  a 
small  vessel  placed  under  the  microscope.  The  erythrocytes 
are  chiefly  carried  in  the  axial  stream,  while  the  leucocytes 
are  more  confined  to  the  peripheral  stream,  where  they  may 


468  VETERINARY    PHYSIOLOGY 

be  observed  to  roll  along  the  vessel  wall  with  a  tendency  to 
adhere  to  it. 

When,  from  any  cause,  the  flow  through  the  capillaries 
is  brought  to  a  standstill,  the  leucocytes  creep  out  through 
the  vessel  walls  and  invade  the  tissue  spaces.  This  is  the 
process  of  diapedesis,  which  plays  an  important  part  in 
inflammation. 


C.    SPECIAL     CHARACTERS    OF    THE     CIRCULATION 
IN  CERTAIN    SITUATIONS. 

1.  Circulation  Inside  the  Cranium  (tig.  196). — Here  the 
blood  circulates  in  a  closed  cavity  with  rigid  walls,  and  there- 
fore its  amount  can  vary  only  at  the  expense  of  the  cerebro- 
spinal fluid  (p.  511).  This  is  small  in  amount,  some  150 
c.cm.,  and  permits  of  only  small  variations  in  the  volume  of 
blood. 

Increased  arterial  pressure  in  the  body  does  not  therefore 
markedly  increase  the  amount  of  blood  in  the  brain,  but 
simply  drives  the  blood  more  rapidly  through  it. 

There  seems  to  be  no  regulating  nervous  mechanism 
connected  with  the  arterioles  of  the  brain,  and  the  cerebral 
pressure  simply  follows  the  changes  in  the  general  arterial 
pressure.  The  splanchnic  area  is  the  great  regulator  of  the 
supply  of  blood  to  the  brain. 

Since  the  cerebral  arteries  are  supported  and  prevented 
from  distending  by  the  solid  wall  of  the  skull,  the 
arterial  pulse  tends  to  be  propagated  into  the  veins.  In  these 
veins  the  respiratory  pulse  also  is  very  well  marked. 

The  condition  of  the  intra-cranial  circulation  is  indicated 
by  the  circulation  in  the  fundus  of  the  eye  which 
communicates  with  it,  and  this  may  be  observed  by  means 
of  an  ophthalmoscope  (p.  143). 

2.  Circulation  in  the  Lungs — The  action  of  the  vaso-con- 
strictor  nerves  is  feeble,  and  adrenalin  fails  to  cause  a 
constriction  of  the  arterioles.  The  amount  of  blood  in  the 
lungs  is  regulated  by  the  blood  pressure  in  the  systemic 
vessels,  and  hence  the  intravenous  administration  of  adrenalin, 


BLOOD   VESSELS 


469 


by   increasing    the    arterial   pressure,    drives  more    blood  to 
them. 

The  circulation  through  the  lungs  is  impeded  in  many 
cases    of   heart    disease,    and    especially    in    mitral    stenosis 


INTRACRANIAL    CIRCULATION 


PULMONARV      CIRCULATION 


ABDOMINAL     CIRCULATION 


Fig.  196. — Scheme  of  the  Circulation,  modified  from  Hill,  to  illustrate  the 
influence  of  the  various  extra-cardiac  factors  which  maintain  the  flow 
of  Vjlood. 

(p.  410),  and  a  condition   of  'passive  congestion   is  set  up, 
which  may  lead  to  h;eraorrhage  from  the  lungs. 

3.  Circulation  in  the  Heart  Wall. — The  sympathetic  fibres 
are  vaso- dilator  not  vaso-constrictor  to  the  arterioles  of  the 
coronary  vessels  and  the  administration  of  adrenalin  dilates 
these  arterioles. 

4.  Circulation  in  the  Spleen. — Here  the  blood  has  to  flow 
through    a    labyrinth    of   large    sinusoid    capillaries    in    the 


470  VETERINARY   PHYSIOLOGY 

pulp,  and  it  is  driven  on  by  the  alternate  contraction  and 
relaxation  of  the  non-striped  muscle  in  the  capsule  and 
trabeculse  (see  p.  215). 

5.  Bone-Marrow. —  The  tissue  is  surrounded  by  rigid  bony 
walls,  and  the  amount  of  blood  can  vary  only  slightly.  The 
circulation  is  through  sinusoid  capillaries. 

D.  EXTRA-CARDIAC    FACTORS    MAINTAINING 
CIRCULATION. 

The  central  pump,  the  heart,  is  not  the  only  factor  main- 
taining the  flow  of  blood  through  the  vessels  (flg.  196). 

1.  Movements  of  Respiration.  —  (i.)  The  thorax,  in  the 
movements  of  respiration,  is  a  suction  pump  of  considerable 
power,  which  draws  blood  into  the  heart  during  inspiration. 
The  auricles  may  be  regarded  as  the  cisterns  of  the  heart, 
the  abdominal  ;blood-vessels  as  the  great  blood  reservoir, 
and  the  diaphragm,  contracting  in  inspiration,  presses  the 
blood  from  this  reservoir  up  into  the  thorax  and  heart. 

(ii.)  Expiration  also  helps,  for  the  blood,  which  has  filled 
the  vessels  of  the  lungs  in  inspiration,  is  driven  on  into  the 
left  side  of  the  heart  in  expiration.  The  blood  is  thus  forced 
on  into  the  arteries.  The  respiratory  movements  apparently 
play  a  great  j)art  in  maintaining  the  circulation  when  the 
heart  has  undergone  extensive  calcareous  degeneration. 

2,  Intermittent  Muscular  Exercise.  —  This  acts  in  three 
ways:  (1)  by  increasing  the  respiratory  movements;  (2) 
by  augmenting  the  action  of  the  heart ;  (3)  by  the  contract- 
ing and  relaxing  muscles  pressing  on  the  blood-vessels,  and 
so  forcing  the  blood  onwards  into  the  veins  and  to  the  heart, 
back-flow  in  the  veins  being  prevented  by  the  valves  ; 
(4)  by  the  increased  venous  filling  of  the  heart  leading  to 
stronger  contractions  fp.  417),  and  reflexly  to  acceleration 
(p.  421). 

The  arterial  blood  pressure  is  thus  raised  and  the  intra- 
cranial circulation  accelerated,  so  that  more  blood  is  sent  to 
the  brain.  Too  marked  a  rise  of  pressure  during  such 
exercise  is  prevented  by  dilatation  of  the  arterioles  throughout 
the  body. 


BLOOD  VESSELS  471 

In  sustained  muscular  strain  the  thorax  is  fixed,  and 
hence,  (a)  at  first  (1)  the  pressure  on  the  heart  and  thoracic 
organs  is  raised,  and  the  increased  pressure  in  the  thorax 
helps  to  support  the  heart  and  to  prevent  over-distension. 
(2)  The  rigid  thorax  prevents  the  blood  being  sucked  into 
the  heart  by  the  respiratory  movements.  (3)  The  abdominal 
vessels  are  pressed  upon  by  the  contraction  of  the  abdominal 
muscles,  and  the  blood  is  pressed  on  to  the  heart,  while  the 
sustained  contraction  of  the  limb  muscles  tends  to  prevent 
the  free  flow  of  blood  through  the  capillaries. 

Arterial  pressure  is  thus  raised,  and  the  blood  is  forced 
to  the  central  nervous  system  in  which  the  pressure  rises, 
and,  if  a  weak  spot  in  the  vessels  is  present,  rupture  is 
apt  to  occur. 

(b)  Later,  if  the  strain  is  still  further  sustained,  the  high 
intra-thoracic  pressure  tends  to  prevent  proper  diastolic 
filling  of  the  heart,  and  the  pumping  action  of  respiration  is 
in  abeyance.  The  abdominal  vessels  being  pressed  upon 
prevents  the  free  flow  of  blood  through  them  to  the  heart, 
and  the  venous  inflow  fails  and  the  force  of  contraction  of 
the  heart  decreases.  Thus,  less  blood  is  sent  to  the  arteries 
and  the  arterial  pressure  falls,  less  blood  goes  to  the  brain, 
and  fainting  may  result  (see  below). 

E.   INFLUENCE   OF  POSTURE   ON   CIRCULATION. 

In  the  "head  down"  position,  as  in  the  horse  in  drinking, 
the  accumulation  of  blood  in  the  head  is  prevented  by  the 
vessels  being  packed  inside  the  skull,  and  in  the  right  side 
of  the  heart  by  the  supporting  pericardium. 

In  man,  in  the  erect  posture,  the  position  of  the  abdominal 
reservoir  of  blood  at  a  lower  level  than  the  heart  increases 
the  work  of  that  organ.  Especially  is  this  the  case  with 
animals,  in  which  the  abdominal  wall  is  lax,  so  that  the  blood 
can  accumulate  in  the  abdominal  vessels,  e.g.  rabbits  bred  in 
confinement.  In  these,  failure  of  the  heart  or  fainting  may 
occur  when  they  are  placed  in  the  "  head  up  "  position.  In 
the  normal  position  of  quadrupeds  the  work  is  much  easier, 
for  the  reservoir  is  on  the  same  level  as  the  pump. 


472  VETERINARY  PHYSIOLOGY 


F.    FAINTING. 

This  is  a  sudden  loss  of  consciousness  produced  by  failure 
in  the  supply  of  blood  to  the  brain.  It  is  accompanied  by 
loss  of  control  over  the  muscles.  It  may  be  induced  by 
any  sudden  lowering  of  the  arterial  blood  pressure,  whether 
due  to  decreased  inflow  of  blood  or  to  decreased  peripheral 
resistance. 

1.  Decreased  inflow  may  be  caused  by  —  (1)  Cardiac 
inhibition  brought  about  reflexly  (a)  by  strong  stimulation  of 
ingoing  nerves,  and  more  especially  of  the  nerves  of  the 
abdomen  ;  (b)  by  strong  stimulation  of  the  upper  brain 
neurons  accompanied  by  changes  in  the  consciousness  of  the 
nature  of  emotions — (2)  Failure  of  the  heart  to  pump  blood 
from  veins  to  arteries  against  the  force  of  gravity,  as  when  a 
hutch  rabbit  is  held  in  the  "  head  up  "  position  for  some  time. 

2.  Decreased  resistance  to  outflow  through  sudden 
dilatation  of  arterioles  may  result  from  changes  in  the 
upper  brain  neurons,  sometimes  as  a  result  of  digestive  dis- 
turbances. 

However  induced,  the  anaemic  state  of  the  brain  leads  to 
a  stimulation  of  the  cardio-inhibitory  centre  and  the 
condition  is  thus  accentuated.  In  man  the  cerebral  anaemia 
is  accompanied  by  pallor  of  the  face. 

The  treatment  consists  in  depressing  the  head  to  allow 
the  force  of  gravity  to  act  in  filling  the  cerebral  vessels  and 
in  giving  diffusible  stimulants  to  increase  the  action  of 
the  heart. 

G.   THE    TIME  TAKEN  BY  THE  CIRCULATION. 

This  has  been  determined  by  injecting  ferrocyanide  of 
potassium  into  the  proximal  end  of  a  cut  vein,  and  finding 
how  long  it  took  to  appear  in  the  blood  flowing  from  the 
distal  end.  From  observation  in  the  horse,  dog,  and  rabbit, 
it  appears  that  the  time  corresponds  to  about  twenty-seven 
beats  of  the  heart,  so  that  in  man  it  should  amount  to  about 
twenty-three  seconds. 


BLOOD   VESSELS  473 


H.  FLOW  OF  BLOOD  THROUGH  DIFFERENT  ORGANS. 

This  may  be  studied — 

(-4)  In  lower  animals  in  the  following  ways  : — (1)  By 
use  of  the  Stromuhr  (p.  466)  ;  (2)  by  the  plethysmo- 
graph  method.  This  consists  in  enclosing  the  organ 
in  a  plethysmograph,  and,  while  the  blood  is  flowing, 
clamping  the  vein  for  a  very  brief  period.  The  organ 
expands  according  to  the  amount  of  blood  which  flows  in, 
and  the  increased  volume  gives  a  measure  of  the  blood  flow. 
The  vein  is  again  undamped,  and  the  observation  may  be 
repeated. 

In  lower  animals  it  has  been  found  that  the  flow 
of  blood  through  different  organs  when  measured  per  100 
grm.  of  organ  per  minute  is  very  different,  in  the 
stomach  only  about  21  c.c,  in  the  kidney,  150  c.c,  and  in 
the  thyreoid  no  less  than  5  60  c.c. 

(8)  The  time  taken  by  the  blood  to  pass  through  an 
organ  may  be  determined  by  injecting  some  electrolyte, 
e.g.  NaCl  solution,  into  the  artery,  and  measurmg  the 
electrical  conductivity  of  the  blood  in  the  vein  by  means  of  a 
Wheatstone's  bridge.  When  the  salt  solution  reaches  the 
vein  this  is  increased. 


SECTION   V. 

The  Fluids  carrying  Nourishment  to  the  Tissues. 

BLOOD   AND   LYMPH. 

The  blood  carries  the  necessary  nourishment  to  the  tissues, 
and  receives  their  waste  products.  But  it  is  enclosed  in  a 
closed  system  of  vessels,  and  does  not  come  into  direct 
relationship  with  the  cells.  Outside  the  blood-vessels,  and 
bathing  the  cells,  is  the  lymph  which  plays  the  part  of 
middleman  between  the  blood  and  the  tissues,  receiving 
nourishment  from  the  former  for  the  latter,  and  passing  the 
waste  from  the  latter  into  the  former. 

A.  BLOOD. 

TJie  physical,  chemical  and  histological  characters  of 
blood  must  he  investigated  practically. 

I.  General  Characters. 

Colour.  — Blood,  when  it  has  stood  for  some  time,  is  dark 
purple,  but  when  shaken  with  air  it  assumes  a  bright 
cinnabar  red  colour.  Elements  of  Blood.  —  Microscopic 
examination  shows  that  blood  is  composed  of  a  clear  fluid 
(Liquor  Sanguinis  or  Plasma)  in  which  float  myriads  of 
small  disc-like  yellowish-red  cells  (Erythrocytes),  a  smaller 
number  of  greyish  cells  (Leucocytes),  and  certain  very  minute 
grey  particles  (Blood  Platelets).  The  Opacity  of  Blood  is  due 
to  the  erythrocytes,  and,  when  the  pigment  is  dissolved  out 
of  them  by  water  and  they  are  rendered  transparent,  the 
blood  as  a  whole  becomes  transparent  and  is  said  to  be 
"laked"  (p.  485).      The  Specific  Gravity  is  about  1055.      It 


BLOOD  475 

may  be  estimated  by  finding  the  specific  gravity  of  a  solution 
of  sodium  sulphate  or  of  chloroform  and  benzene  in  which  a 
drop  of  blood  remains  where  it  is  placed,  neither  sinking 
nor  floating.  The  lowering  of  the  freezing-point  of  blood  or 
A  is  0-o6  C.  This  is  equivalent  to  the  osmotic  pressure 
of  a  solution  of  about  0  9  per  cent,  of  NaCl  and  is  the  same 
as  that  of  the  cells  of  the  tissues.  Viscosity— The  viscosity 
of  blood,  or  the  intermolecular  and  intermolar  friction,  may 
be  measured  by  the  time  taken  to  pass  through  a  given 
length  of  capillary  tube  compared  by  the  time  taken  by 
water.  It  depends  partly  on  the  viscosity  of  the  plasma, 
which,  being  of  the  nature  of  an  emulsoid  colloid,  manifests 
viscosity,  but  chiefly  upon  the  blood  cells.  Hence  when  these 
are  diminished  in  number  the  viscosity  of  the  blood  is 
decreased.  The  Taste  and  Smell  are  characteristic,  and 
must  be  experienced.  Reaction. — Blood,  so  far  as  the 
balance  of  H  and  OH  ions  is  concerned,  is  slightly  alkaline, 
its  hydrogen  ion  concentration,  Cjj,  being  lower  than  that  of 
pure  water  (see  Appendix  ni.). 

The  cells  of  the  blood  constitute  about  33  per  cent.,  one- 
third  of  its  weight,  and  the  total  sohds  of  the  blood  are 
about  20  per  cent. 

II.  Clotting  or  Coagulation. 

Blood,  when  shed,  becomes  a  firm  jelly  in  the  course  of 
three  or  four  minutes.  The  primary  object  of  the  process 
is  to  seal  wounds  in  the  blood-vessels,  and  so  to  prevent 
haemorrhage.  When  the  blood  is  collected  in  a  beaker  or 
other  dish,  the  process  starts  from  the  sides,  and  spreads 
throughout  the  blood  until,  when  clotting  is  complete,  the 
dish  may  be  inverted  without  the  blood  falling  out.  In  a 
short  time,  drops  of  clear  fluid  appear  upon  the  surface  of 
the  clot,  and,  in  a  few  hours,  these  have  accumulated  and 
run  together,  while  the  clot  has  contracted  and  drawn  away 
from  the  sides  of  the  vessel,  until  it  finally  floats  in  the  clear 
fluid — the  Serum.  If  clotting  occurs  slowly,  e.g.  when  the 
shed  blood  is  cooled,  the  erythrocytes  subside,  leaving  a 
layer  of  clear  plasma  above,  which,  when  coagulation  takes 
place,  forms  a  "  bufty  coat "  in  the  upper  part  of  the  clot. 


476  VETERINARY   PHYSIOLOGY 

Clotting  is  due  to  changes  in  the  plas'ina,  since  this  fluid 
will  coagulate  in  the  absence  of  corpuscles. 
The  change  may  be  represented  thus  : — 

Blood 


I 

Plasma 

I 


1  I 

Serum  Clot      Corpuscles 


The  change  consists  in  the  formation  of  a  series  of  fine 
elastic  threads  of  fibrin  throughout  the  plasma,  and,  if  red 
corpuscles  are  present,  they  are  entangled  in  the  meshes  of 
the  network  and  give  the  clot  its  red  colour. 

These  threads  may  be  readily  collected  in  mass  upon  a 
stick  with  which  the  blood  is  whipped  as  it  is  shed.  The 
red  fluid  blood  which  is  left,  consisting  of  blood  cells  and 
serum,  is  said  to  be  dejibrinated. 

Blood 


Plasma 
I 


.1.  I 

Fibrin  Serum    Corpuscles 

Defibrinated  Blood 

A  study  of  clotting  blood  by  means  of  the  ultra- 
microscope  shows  that  the  fibrin  first  separates  as  small 
acicular  particles  which  run  together  to  form  threads. 

Fibrin  is  a  protein  substance.  It  is  slowly  dissolved  in 
solutions  of  neutral  salts.  It  is  coagulated  by  heat,  and  is 
precipitated  when  an  excess  of  a  neutral  salt  is  added.  It 
therefore  belongs  to  the  group  of  globulins. 

The  plasma,  before  clotting,  and  the  serum,  squeezed  out 
from  the  clot,  both  contain  in  the  same  proportions  an 
albumin  (serum  albumin)  and  a  globulin,  or  series  of  globulins, 
which  may  be  classed  together  as  serum  globulin.      But  the 


BLOOD  477 

plasma  contains  a  small  quantity — about  0'4  per  cent. — of 
another  globulin  (fibrinogen)  which  coagulates  at  a  low- 
temperature,  and  which  is  absent  from  serum.  It  is  this 
which  undergoes  the  change  from  the  soluble  form  to  the 
insoluble  form  in  coagulation.  If,  by  taking  advantage  of 
the  fact  that  it  is  more  easily  precipitated  by  sodium  chloride 
than  the  other  proteins,  it  is  separated  from  them,  it  may 
still  be  made  to  clot.  The  source  of  this  substance  seems  to 
be  the  intestine  and  liver,  and  when  these  are  removed  it  is 
not  formed. 

The  essential  points  in  coagulation  were  discovered  by 
Andrew  Buchanan  in  184.5.  He  showed  that  something 
which  he  called  "soluble  fibrin"  exists  in  the  plasma  and  that 
this  changes  to  insoluble  fibrin.  He  further  showed  that 
the  addition  of  the  white  cells  of  the  blood  brings  about  the 
change. 

The  process  of  clotting  is  due  to  the  action  of  a 
substance,  thrombin,  Avhich  does'  not  exist  as  such  in  the 
blood,  but  which  is  formed  by  the  union  of  a  precursor  with 
calcium  ions.  This  is  proved  by  the  fact  that  if  blood  is 
directly  collected  in  alcohol,  it  is  found  to  yield  no  thrombin, 
although  when  treated  with  alcohol  after  clotting  it  is  rich  in 
this  substance.      Its  precursor  may  be  called  prothrombin. 

If  calcium  salts  are  precipitated  by  the  addition  of 
oxalates  to  the  blood,  clotting  does  not  take  place.  The 
mere  conversion  of  the  calcium  from  an  ionised  state  to  a 
non-ionised  state,  such  as  that  in  which  it  exists  in  the 
citrate,  prevents  clotting.  Hence,  when  unclotted  blood  is 
wanted,  it  may  be  collected  in  a  vessel  containing  some 
potassium  oxalate  or  a  solution  of  sodium  citrate  {Chemical 
Physiology). 

Although  prothrombin  and  calcium  ions  exist  together 
in  the  blood,  they  do  not  form  the  thrombin  necessary  to 
produce  clotting,  apparently  because  an  anti-prothrombin  is  also 
present.  This  may  be  separated  from  fibrinogen  and  from 
prothrombin  by  heating  the  plasma  to  60°  C,  which  pre- 
cipitates the  fibrinogen  and  destroys  the  prothrombin,  but 
leaves  the  antithrombin  unaltered. 

Before  clotting  can  occur,  antithrombin  must  be  thrown 


478  VETERINARY    PHYSIOLOGY 

out  of  action  by  the  development  of  some  substance  which 
neutralises  it.  Such  a  substance  is  yielded  by  the  breaking 
down  of  tissue  cells  or  the  cells  of  the  blood,  especially  the 
platelets.  It  is  perhaps  best  called  thromboplastin.  It  is  a 
lipoid  compound,  probably  identical  with  cephalin. 

The  steps   in   the  process  might  thus  be  represented  as 
follows  :  — 

{From  Cells)  {In  Plasma) 


Prothrombin  .S       Calcium  Ions 

Thromboplastin^ 


Many  circumstances  influence  the  rapidity  of  clotting.  Tem- 
perature has  a  marked  effect,  a  low  temperature  retarding 
it,  a  shght  rise  of  temperature  above  the  normal  of  the 
particular  animal  accelerating  it.  If  a  trace  of  a  neutral 
salt  be  added  to  blood,  coagulation  is  accelerated  ;  but  if 
blood  be  mixed  with  strong  solutions  of  a  salt,  coagulation 
is  prevented  because  the  formation  of  thrombin  is  checked. 
Calcium  salts  have  a  marked  and  important  action,  and  if 
they  are  precipitated  by  the  addition  of  potassium  oxalate, 
blood  will  not  clot,  apparently  because  thrombin  cannot  be 
formed. 

The  injection  into  the  blood-vessels  of  a  living  animal  of 
commercial  'peptones,  which  consist  chiefly  of  proteoses, 
generally  prevents  the  blood  from  clotting  when  shed.  This 
appears  to  be  due  to  the  formation,  probably  in  the  liver,  of 
an  excess  of  anti-thrombin.  Hirudin,  an  extract  of  the 
head  of  the  medicinal  leech,  also  retards  clotting,  both  when 
injected  into  the  blood-vessels  and  when  added  to  the  blood 


BLOOD  479 

when  shed.  It  appears  to  be  of  the  nature  of  an  anti- 
thrombin. 

The  blood  does  not  coagulate  in  the  vessels  under  normal 
conditions  because  of  the  absence  of  thromboplastin  in  any 
quantity  and  the  presence  of  anti-thrombin.  Under  certain 
conditions  clotting  does  take  place.  (1)  If  inflammation  is 
induced  in  the  course  of  a  vessel,  coagulation  occurs  rapidly. 
(2)  If  the  inner  coat  of  a  vessel  be  torn,  as  by  a  ligature,  or 
if  any  roughness  occurs  on  the  inner  wall  of  a  vessel, 
coagulation  is  apt  to  be  set  up.  (3)  Various  substances 
injected  into  the  blood  stream  may  cause  the  blood  to 
coagulate,  and  thus  rapidly  kill  the  animal.  Among  such 
substances  are  extracts  of  various  organs — thymus,  testis, 
and  lymph  glands,  which  yield  thromboplastin — and  snake 
venom,  which  seems  to  contain  active  thrombin.  The 
injection  of  pure  thrombin  does  not  usually  cause  clotting, 
apparently  because  an  anti-thrombin  is  developed  to  neutral- 
ise it. 

The  blood  usually  clots  wlien  shed,  because  the  damaged 
tissues  yield  thromboplastin,  and  thus  thrombin  is  formed. 
This  acts  upon  the  fibrinogen  before  it  can  be  antagonised 
by  the  anti-thrombin.  If  blood  is  received  into  oil, 
or  into  a  vessel  anointed  with  vaseline  and  filled  with 
paraflSn  oil,  it  will  remain  fluid  for  a  considerable  time. 
Any  roughness  in  the  wall  of  the  blood-vessel  or  of  the 
vessel  in  which  the  blood  is  received  probably  serves  to 
catch  the  blood  platelets  (p.  484),  so  that  thromboplastin  is 
liberated  freely  as  they  disintegrate. 


III.  Plasma  and  Serum. 

These  may  be  considered  together,  since  serum  is  merely 
plasma  minus  fibrinogen.  As  serum  is  so  much  easier  to 
procure,  it  is  generally  employed  for  examination,  but 
plasma  may  be  readily  obtained  by  centrifuging  blood  which 
has  been  prevented  from  clotting  by  the  addition  of  an 
oxalate  or  a  citrate  (p.  477). 

Both  are  straw-coloured  fluids,  the  colour  being  due  to  a 
yellow   lipochrome.      Sometimes    they   are   clear  and   trans- 


480  VETERINARY   PHYSIOLOGY 

parent,  but,  after  a  fatty  diet,  they  become  milky.  They 
have  a  specific  gravity  of  about  1025,  and  contain  about 
90  per  cent,  of  water  and  10  per  cent,  of  solids.  The  chief 
solids  are  the  native  proteins — serum  albumin  and  serum 
globulin  (with,  in  the  plasma,  the  addition  of  fibrinogen). 
The  proportion  of  the  two  former  proteins  to  each  other 
varies  considerably  in  different  animals,  but  the  variations 
are  small  in  the  same  animal  at  diiTerent  times.  The 
globulin  probably  consists  of  at  least  two  bodies— euglobulin 
precipitated  by  weak  acid,  and  pseudoglobulin  not  so  pre- 
cipitated. The  amount  of  albumin  is  generally  greater 
when  the  body  is  well  nourished.  In  man  they  together 
form  about  7  per  cent,  of  the  serum.  In  virtue  of  the 
presence  of  these  proteins  the  plasma  is  colloidal,  and  it  has 
little  tendency  to  transude  through  the  walls  of  the  vessels. 
These  proteins  further  seem  to  have  a  small  osmotic 
pressure  (p.  574). 

The  other  constituents  of  the  serum  are  in  much  smaller 
amounts,  and  may  be  divided  into — 

1 .  Substances  to  be  used  by  the  tissues. 

Glucose  is  the  most  important  of  these.  It  occurs  only 
in  small  amounts — about  O'l  to  O'lo  per  cent.  Part  of  it 
is  free,  but  part  is  probably  in  combination.  It  is  present 
in  larger  amount  in  blood  going  to  muscles  than  in  blood 
coming  from  them,  and  this  difference  seems  to  be  more 
marked  when  the  muscles  are  active. 

Fats  occur  in  very  varying  amounts,  depending  upon  the 
amount  taken  in  the  food,  but  in  addition  to  these  true  fats 
there  is  also  a  small  amount  of  other  lipoids. 

2.  Substances  given  off  by  the  tissues. 

The  chief  of  these  is  urea,  which  occurs  constantly  in 
very  small  amounts  in  the  serum — about  '05  per  cent.  It 
will  afterwards  be  shown  that  it  is  derived  from  the  liver, 
and  that  it  is  excreted  in  the  urine  by  the  kidneys  (p.  559). 

Creatin  and  uric  acid,  and  some  allied  bodies,  appear  to 
be  normally  present  in  traces,  and  their  amount  may  be 
increased  in  diseased  conditions,  especially  of  the  kidneys. 

3.  Inorganic  constituents. — The  most  abundant  is  chloride  of 


BLOOD  481 

sodium,  to  which  the  osmotic  pressure  of  the  plasma  is  partly 
due,  and  which  is  present  in  the  proper  proportion  of 
sodium  with  the  other  cations,  potassium,  calcium,  and 
magnesium,  to  maintain  the  activity  of  the  tissues  (p.  218). 
Perhaps  the  most  important  salt  is  sodium  bicarbonate,  which 
maintains  the  reaction,  the  Cg  (see  Appendix  III.)  of 
the  blood,  and  of  the  body  fluids,  at  the  level  at  which 
their  chemical  activity  can  best  be  carried  on.  Sodium  pbos- 
pbate  is  also  present  in  very  small  quantities.  Calcium, 
potassium,  and  magnesium  occur  in  very  small  amounts. 

Sodium  bicarbonate  is  an  ideal  salt  for  maintaining  the 
balance  of  H  and  OH  ions  in  the  blood.  When  stronger 
acids,  such  as  sarcolactic  acid,  are  liberated  from  the  tissues, 
they  combine  with  some  of  the  sodium,  and  the  weak, 
slightly  dissociated  COo  is  set  free,  and  is  at  once  got  rid  of 
by  the  lungs  (p.  5  27).  When,  on  the  other  hand,  alkalies  are 
absorbed  and  added  to  the  blood,  there  is  available  in  the 
tissues  an  abundant  supply  of  CO,,  with  which  they  will 
combine  as  bicarbonates.  The  maintenance  of  the  propor- 
tion of  NaHCOg  in  the  blood  is  of  the  utmost  importance. 
It  may  be  termed  its  alkaline  reserve.  Only  when  this 
alkaline  reserve  is  drawn  upon  can  anything  like  a  real 
condition  of  acidosis,  a  real  increase  in  the  C^,  occur.  This 
has  been  termed  an  uncompensated  acidosis,  to  distinguish 
it  from  the  condition  in  which  an  increased  production  of 
acids  has  been  met  by  the  alkaline  reserve  {compensated 
acidosis). 

In  venous  blood  there  are  something  like  60  parts  of  CO., 
per  100  parts  of  blood.  Of  this,  at  the  temperature  of  the 
body  and  the  pressure  of  CO,  in  the  lungs  to  which  the 
blood  is  subjected  (p.  539),  some  3  parts  are  dissolved; 
the  remainder  is  in  combination  with  sodium  as  NaHCOg, 
so  that  the  balance  is 

H.COg     _  3  _  J^ 
NaHCOg  ~  60  ~  20' 

If  the  amount  of  CO.,  is  increased,  then  it  must  either  be 
got   rid  of  from   the  lungs,  or  the  proportion  of  Na  in  the 
denominator  must  be  increased,  while  if  the  Na  is  combined 
31 


482  VETERINARY   PHYSIOLOGY 

with  other  acids,  such  as  /3-oxybutyric,  then  the  amount  of 
CO2  must  be  decreased. 

The  proteins  of  the    blood  are  amphoteric,  and   it  has 

been    suggested     that    they    too    may    combine  with    COg. 

The  protein   of  the  pigment  of    the    red    cells  does    seem 

to    form    such     a    combination.       This    will    be  considered 
later. 

Behind  the  regulation  of  the  Cjj  of  the  blood  by 
the  NaHCOg  and  the  lungs  are  two  further  lines  of 
defence. 

1.  The  dissociated  HCl  of  the  NaCl  of  the  plasma  can, 
Avhen  the  Cjj  of  the  blood  increases,  pass  into  the  cells  and, 
seizing  upon  some  of  the  sodium  of  the  NajHPO^  turn  out 
NaH2P04  into  the  plasma  to  be  excreted  by  the  kidneys. 
Thus  some  of  tlie  excess  H  ions  is  got  rid  of. 

Plasma 


H2CO3  +  NaCll^NaHCOs  +  HCl 

Cell  Wall 


Cell 


HCl  +  Na„HP04^NaHoP0,  +  NaCl 

I 
Plasma 

In  fact,  the  kidneys  play  a  part  only  second  to  the  lungs 
in  regulating  the  Ch  of  the  blood  by  getting  rid  of  any 
excess  of  H  ions  in  acidosis  and  of  OH  ions  in  alkalosis. 

2.  With  any  increase  of  the  H  ions  and  the  development 
of  acidosis,  the  ammonia,  which  is  in  the  liver  normally 
converted  into  urea,  is  passed  into  the  blood  to  unite  with 
and  neutralise  the  acids  (p.  360). 


BLOOD 


488 


IV.  Cells  of  Blood. 
1.  Leucocytes— White  Cells. 

These  are  much  less  numerous  than  the  red  cells,  and 
their  number  varies  enormously  in  normal  conditions.  On 
an  average  there  are  about  7500  per  cubic  millimetre. 
{The  method  of  counting  must  be  studied  'practically.) 

They  are  soft,  extensile,  elastic,  and  sticky,  and  each 
contains  a  nucleus  and  a  well-developed  double  centrosonie. 
In  size  they  vary  considerably,  most  being  larger  than  the 
red    cells,    some    slightly    smaller. 


The    character    of    the 


Fio.  197.— Cells  of  the  Blood,     a,  erythrocytes  ;  b,  large,  and  c,  small  lym- 
phocyte ;  d,  polymorpho-nuclear  leucocyte  ;  e,  eosinophil  leucocyte. 

nucleus  varies  greatly,  and  from  this  and  from  variations  in 
the  protoplasm,  they  may  be  divided  into  three  classes. 

(1)  Lymphocytes. — Cells  with  a  clear  protoplasm  and  a 
more  or  less  circular  nucleus.  Some  are  very  small,  while 
others  are  larger.  They  constitute  about  20  to  25  per  cent, 
of  the  leucocytes  (fig.  197,  6  and  c). 

(2)  Polymorpho-nuclear  leucocytes  have  a  much-distorted 
and  lobated  irregular  nucleus  and  a  finely  granular  proto- 
plasm whose  granules  stain  with  acid  and  neutral  stains. 
These  constitute  about  70  to  7  5  per  cent,  of  the  leucocytes 
(fig.  197,  cZ).  ,.1 

(3)  Eosinophil  or  oxyphil  leucocytes  have  a  lobated  nucleus 


484  VETERINARY   PHYSIOLOGY 

like  the  last,  but  large  granules  in  the  protoplasm  which 
stain  deeply  with  acid  stains.  From  1  to  4  per  cent,  of  the 
leucocytes  are  of  this  variety  (fig.  197,  e). 

Basophil  leucocytes  are  practically  absent  from  normal 
blood.  They  have  a  lobated  nucleus  and  granules  in  the 
protoplasm  staining  Avith  basic  stains. 

Myelocytes  are  large  leucocytes  with  a  large  circular  or 
oval  nucleus  and  a  finely  granular  protoplasm.  They  are 
not  normal  constituents  of  the  blood,  but  appear  when  the 
activity  of  the  bone-marrow  is  increased  in  certain  patho- 
logical conditions. 

The  leucocytes  show — 

(a)  Amoeboid  movement. — Under  suitable  conditions  they 
undergo  changes  in  shajie,  as  may  be  readily  seen  in  the 
blood  of  the  frog  or  other  cold-blooded  animal.  The  motion 
may  consist  simply  of  the  pushing  out  and  withdrawal  of 
one  or  more  processes  (pseudopodia),  or,  after  a  process  is 
extended,  the  whole  corpuscle  may  follow  it  and  thus  change 
its  place,  or  the  corpuscle  may  simply  retract  itself  into  a 
spherical  mass.  As  a  result  of  these  movements  the 
corpuscles,  in  certain  conditions,  creep  out  of  the  capillary 
blood-vessels  between  the  endothelial  cells  and  wander  into 
the  tissues  {diapedesis).  The  amoeboid  movement  is  best 
marked  in  the  polymorpho-nuclear  leucocytes. 

(b)  Phagocyte  Action. — The  linely  granular  leucocytes 
and  the  lymphocytes  have  further  the  power  of  taking 
foreign  matter  into  their  interior,  and  of  digesting  it.  By 
this  devouring  action  useless  and  effete  tissues  are  removed 
and  dead  micro-organisms  in  the  body  are  taken  up  and  got 
rid  of.  This  scavenger  action  of  the  leucocytes  is  of  vast 
importance  in  pathology. 

2.  Blood  Platelets. 
These  are  small  circular  or  oval  discoid  bodies  about 
one-third  the  diameter  of  a  red  blood  corpuscle.  Some 
observers  have  stated  that  they  contain  a  central  nucleus. 
They  are  very  sticky  and  mass  together  when  blood  is  shed 
and  adhere  to  a  thread  passed  through  the  blood  or  to  any 
rough    point   in   the  lining  of  the  heart    or   vessels.      They 


BLOOD  485 

there  form  clumps,  and  in  these  they  disintegrate,  probably 
liberating  thromboplastin,  and  so  start  clotting.  They  are 
present  in  the  blood  of  mammals  only.  Their  source  is  not 
definitely  known,  but  it  has  been  suggested  that  they  are  the 
extruded  nuclei  of  developing  erythrocytes,  or  that  they  are 
derived  from  the  giant  cells  of  the  bone-marrow  (p.  500). 

3.  Erythrocytes— Red  Cells. 

1.  Characters. — All  mammals,  except  the  camels,  have 
circular,  biconcave,  discoid  erythrocytes,  which,  when  the 
blood  is  shed,  tend  to  run  together  like  piles  of  coins.  The 
camels  have  elliptical  biconvex  corpuscles.  The  fully 
developed  mammalian  erythrocytes  are  without  a  nucleus. 
In  birds,  reptiles,  amphibia  and  fishes,  the  corpuscles  are 
elliptical  biconvex  bodies,  with  a  well-marked  central  nucleus. 

2.  Size. — The  size  of  the  human  erythrocytes  is  fairly 
constant — on  an  average  5-5  micro-millimetres  in  diameter. 

3.  Number. — The  number  of  red  cells  in  health  is  about 
7,000,000  in  the  horse,  but  in  disease  it  is  often  decreased. 
The  number  of  corpuscles  per  cubic  millimetre  is  estimated  by 
the  Haemocytometer.  This  consists  of  (1)  a  pipette  by  which 
the  blood  may  be  diluted  to  a  definite  extent  with  a  salt 
solution  of  the  same  osmotic  equivalent  as  the  plasma,  and 
(2)  a  cell  of  definite  depth  ruled  in  squares,'  each  containing 
above  it  a  definite  small  volume  of  blood,  so  that  the  number 
of  corpuscles  in  that  volume  may  be  counted  under  the 
microscope  (Practical  Physiology). 

The  pale  yellow  colour  of  the  individual  corpuscles  is  due 
to  a  pigment  held  in  a  fine  sponge-like  stroma  which  seems 
also  to  form  a  capsule  round  the  cell. 

4.  Haemolysis. — This  pigment  may  be  dissolved  out  by 
various  agents,  and  the  action  is  termed  haemolysis.  It 
may  be  brouglit  about  in  different  ways  — 

1st.  By  placing  the  erythrocytes  in  a  fluid  of  lower 
osmotic  equivalent,  i.e.  of  lower  molecular  concentration, 
than  the  blood  plasma  and  corpuscles.  A  solution  of  0'9 
per  cent,  of  sodium  chloride  has  the  same  osmotic  equivalent 
as  the  plasma  and  preserves  the  corpuscles  unaltered  ;  in 
more    dilute    fluid    the    corpuscles    tend     to    swell     up     by 


486  VETERINARY   PHYSIOLOGY 

endosmosis,  the  capsule  bursts,  and  the  pigment  escapes. 
Erythrocytes  may  therefore  be  used  as  a  means  of  determin- 
ing the  osmotic  equivalent — the  molecular  concentration — 
of  a  fluid. 

2nd.  By  the  action  of  substances  which  dissolve  the 
lipoids  of  the  stroma,  e.g.  salts  of  the  bile  acids  (see  p.  324), 
chloroform,  ether,  etc. 

ord.  By  Hsemolysins.  («)  The  serum  of  each  species  of 
animal  contains  a  substance,  destroyed  by  heating  to  5  5'  C, 
which  is  hsemolytic  to  the  blood  of  animals  of  other  species, 
e.g.  the  serum  of  eels'  blood  contains  a  powerful  ha3molysin 
for  rabbits'  erythrocytes,  and  the  serum  of  the  dog  a  less 
powerful  one.  (6)  Further,  by  injecting  the  blood  or  the 
erythrocytes  of  one  species  of  animal  into  another  species,  a 
hsemolysin  is  developed  which  has  a  specific  action  on  the 
erythrocytes  of  the  first  species  (p.  614), 

Mh.  By  killing  the  erythrocytes  in  the  body  by  inject- 
ing substances  which  poison  them,  such  as  phenylhydrazin. 
They  are  subsequently  disintegrated  and  their  pigment 
removed.      This,  of  course,  is  not  a  true  hiemolvsis. 


5.  Chemistry. — (1)  The  stroma  of  the  erythrocytes  which 
is  left  after  the  pigment  is  washed  out  is  a  sponge  work 
made  up  of  a  globulin-like  substance,  in  which  lipoids, 
such  as  cholesterol  and  lecithin,  occur  in  considerable 
quantities,  and  seem  to  form  a  capsule  or  cell  membrane. 
Potassium  is  the  base  most  abundantly  present  in  man. 

(2)  Haemoglobin. — The  pigment  is  HaBmoglobin.  It  con- 
stitutes no  less  than  90  per  cent,  of  the  solids  of  the 
erythrocytes.  In  many  animals,  e.g.  the  rat,  when  dissolved 
from  the  corpuscles,  it  crystallises  very  readily  {Chemical 
Physiology).  The  crystals  prepared  from  human  blood 
are  rhombic  plates.  When  exposed  to  air  they  are  of  a 
bright  red  colour,  but  if  placed  in  the  receiver  of  an  air- 
pump  at  the  ordinary  temperature  they  become  of  a  purplish 
tint.  The  same  thing  occurs  if  the  haemoglobin  is  in 
solution,  or  if  it  is  still  in  the  corpuscles.  The  addition  of 
any  reducing  agent  such  as  ammonium  sulphide  or  a  ferrous 


BLOOD 


487 


salt  causes  a  similar  change.  This  is  due  to  the  fact  that 
hannoglohin  has  an  acuity  for  oxygen,  which  it  takes  up 
from  the  air,  forming  a  definite  compound  of  a  bright  red 
colour  in  which  one  molecule  of  haemoglobin  links  with  a 
molecule  of  oxygen,  HbO,.      This  is  known  as  oxy haemoglobin. 

Haemoglobin  is  closely  allied  to  the  proteins,  but  differs 
from  them  in  containing  0-42  per  cent,  of  iron  in  organic 
combination. 

When  light  from  the  sun  is  allowed  to  pass  through 
solutions  of  blood  pigments,  certain  parts  of  the  solar 
spectrum  are  absorbed,  and  when  the  spectrum  is  examined, 
dark  bands — the  absorp- 
tion bands — are  seen.  In 
a  weak  solution  of  oxy- 
haemoglobin  in  a  thin 
layer,  a  dark  band  is 
seen  in  the  green  and 
another  in  the  yellow  part 
of  the  spectrum  between 
Frauenhofer's  lines  D  and 
E,  while  the  violet  end  of 
the  spectrum  is  absorbed 
(fig.  199).  These  bands 
may  be  broadened  or 
narrowed  by  strengthening 
or  weakening  the  solution, 
or  varying  the  thickness  of 
the  layer.  In  stronger  solutions  they  become  broader  and 
finally  run  together,  while  more  and  more  of  the  violet  end 
of  the  spectrum  is  absorbed,  until,  with  a  solution  of 
sufhcient  strength,  only  the  red  end  of  the  spectrum  is 
visible  (fig.  198). 

When  the  oxygen  is  taken  away  and  the  dark  reduced 
hsemoglobin  is  formed,  a  single  broad  band  between  D  and 
E  takes  the  place  of  the  two  bands  (fig.  199).  If  the 
solution  is  again  shaken  up  with  air,  oxygen  is  taken  up  and 
the  bands  of  oxyhsemoglobin  reappear  {Chemical  Physiology). 
The  property  of  taking  oxygen  from  the  air  and  of  again 
giving  it  up  at  a  moderate  temperature  and  under  a  low 


aCB    D       Eb     F  Oh 

Fic.  198. — The  parts  of  the  specirum 
absorbed  by  solutions  of  oxyhfemo- 
globin  of  different  percentage  strengths 
in  a  layer  of  1  cm.  thick. 


488 


VETERINARY   PHYSIOLOGY 


pressure  of  oxygen  is  the  great  function  of  the  blood  pigment 
in  the  body.  The  hcemoglobin  plays  the  part  of  a  middle- 
man betiueen  the  air  and  the  tissues,  taking  oxygen  from 
the  one  and  handing  it  on  to  the  other  (Chemical  Physi- 
ology). 

Amount- — Haemoglobin  constitutes  about   13    or   14 
cent,    of  the  blood,   but  in  various 
decreased. 


or   i  4   per 
its   amount  is 


Carbon-monoxide  ~| 

Hsemoglobiu    .  V 

Osyhaemoglobin  .  J 


Haemoglobin. 


Methasmoglobin . 

Acid  Hsematin    . 

Carbon  -  dioxide 

Hasmodobin    . 


Reduced  Alkali 
Haematin    . 


Yellow. 
D 


Gkekn. 


Blue. 

m 

il 

;i^Pi^ 


Fig.  199. —Spectra  of  the  more  important  Blood  Pigments  and  their  more 
important  derivatives.  (The  spectra  of  oxyhaemoglobin  and  carbon 
monoxide  haemoglobin  and  those  of  acid  hiematin  and  methrerno- 
globin  are  not  identical.)  The  arrows  indicate  that  oxyhemoglobin 
and  meth:iimoglobin  are  changed  to  htemoglobin  by  reducing  agents. 


The  best  method  of  estimatmg  its  amount  is  by  Haldane's 
Hsemoglobinometer.  This  consists  of  two  tubes  of  uniform 
calibre,  one  filled  with  a  1  per  cent,  solution  of  normal  blood 
saturated  with  carbon  monoxide,  and  another  containing 
water  in  which  20  c.mm.  of  the  blood  to  be  examined, 
measured  in  a  pipette,  are  placed,  mixed  with  coal  gas  to 
saturate  with  CO,  and  then  diluted  till  it  has  the  same  tint 
as  the  standard  tube.  The  percentage  of  haemoglobin,  in 
terms  of  the  normal,  is  indicated  by  the  mark  on  the  tube 
at  which  the  fluid  stands  {Chemical  Physiology). 


BLOOD  489 

Derivatives  of  Haemoglobin. — The  following  pigments  are 
derived  from  luemoglobin  : — 

(1)  Methsemoglobin. — Haemoglobin  forms  another  com- 
pound with  oxygen — methtemoglobin  ;  a  substance  which 
must  be  acted  on  by  strong  reducing  agents  before  it 
will  part  with  its  oxygen.  When,  therefore,  this  pigment  is 
formed  in  the  body,  the  tissues  die  from  want  of  oxygen. 
It  may  be  produced  by  the  action  of  various  substances  on 
oxyhemoglobin.  Among  these  are  ferricyanides,  nitrites, 
and  permanganates.  It  crystallises  in  the  same  form  as 
oxyhasmoglobin,  but  it  has  a  chocolate  brown  colour.  Its 
spectrum  is  also  different  from  haemoglobin  and  oxyhaemo- 
globin,  showing  a  narrow  sharp  band  in  the  red  part  of  the 
spectrum,  with  two  or  more  bands  in  other  parts  according 
to  the  reaction  of  the  solution  in  which  it  is  dissolved  (fig. 
199).  It  is  of  importance,  since  it  occurs  in  the  urine 
in  such  pathological  conditions  as  'paroxysmal  methcemo- 
glohinuria. 

(2)  Carboxyhaemoglobin. — Hemoglobin  also  combines  with 
some  other  gases.  Among  these  is  Carbon  monoxide,  CO. 
Haemoglobin  has  a  greater  affinity  for  this  gas  than  it  has 
for  oxygen,  so  that,  when  carbon  monoxide  haemoglobin  is 
once  formed  in  the  body,  the  blood  has  little  power  of  taking 
up  oxygen,  and  the  animal  dies.  Carbon  monoxide  is 
evolved  freely  in  the  fumes  from  burning  charcoal,  is  present 
in  coal  gas,  and  is  found  in  the  air  of  coal  mines  after 
explosions.  Carbon  monoxide  hemoglobin  forms  crystals 
like  oxyhemoglobin,  and  has  a  bright  pinkish  red  colour, 
without  the  yellow  tinge  of  oxyhemoglobin.  Since,  after 
death  it  does  not  give  up  its  carbon  monoxide  and  become 
changed  to  purple  hemoglobin,  the  bodies  of  those  poisoned 
with  the  gas  maintain  the  florid  colour  of  life.  Its  spectrum 
is  very  like  that  of  oxyhemoglobin,  the  bands  being  slightly 
more  to  the  blue  end  of  the  spectrum  (fig.  199).  It  maybe 
at  once  distinguished  by  the  fact  that  when  gently  warmed 
with  ammonium  sulphide  it  does  not  yield  reduced  hemo- 
globin {Chemical  Physiology). 

(3)  Nitric  oxide,  NO,  has  even  a  greater  affinity  for  Hb 
than    has    CO.      The    compound  is  very    similar  in  all    its 


490  VETERINARY  PHYSIOLOGY 

characters  to  the  last.      Some  of  it  is  generally  found,  along 
with  methgemoglobin,  after  poisoning  with  uitrites. 

(4)  Carbon  dioxide  Hsemoglobin. — By  bubbling  CO2  through 
a  solution  of  hsemoglobin,  in  the  absence  of  oxygen,  a  two- 
banded  spectrum  resembling  methsemoglobin  has  been 
produced,  and  on  evacuating  the  gas  in  an  air-pump  the 
single  band  of  haemoglobin  has  been  found  to  appear.  This, 
again,  gives  place  to  the  two  bands  when  COo  is  passed  through 
the  solution.  It  appears  from  this  that  haemoglobin  can  carry 
CO2  as  a  definite  compound.  Probably  it  is  the  globin  part 
of  the  molecule  which  acts  in  this  way,  while  the  htematin 
part  carries  the  oxygen. 

Decomposition  of  Hsemoglobin. — Haemoglobin  is  a  somewhat 
unstable  body,  and,  in  the  presence  of  acids  and  alkalies,  it 
splits  up  into  about  96  per  cent,  of  a  colourless  protein — globin, 
belonging  to  the  group  of  histones  (Appendix  II.),  and 
about  4  per  cent,  of  a  substance  of  a  brownish  colour  called 
hsematin  {Chemical  Physiology). 

(1)  Haematin, — The  spectrum  and  properties  of  haematin 
are  different  in  acid  and  alkaline  media,  (a)  In  acid  media 
it  has  a  spectrum  closely  resembling  methjemoglobin,  but  it 
can  at  once  be  distinguished  by  the  fact  that  it  is  not 
changed  by  such  reducing  agents  as  ferrous  salts.  It  is 
sometimes  important  to  distinguish  between  these  pigments, 
since  both  may  appear  in  the  urine,  methaemoglobin  occurring 
in  paroxysmal  methsemoglobinuria  and  acid  htematin  as  the 
result  of  the  action  of  the  acid  salts  of  the  urine  upon 
hfemoglobin  present  as  the  result  of  kidney  disease.  (6) 
Haematin,  in  alkaline  solution,  can  take  up  and  give  off 
oxygen  in  the  same  way  as  hfemoglobin  does.  Reduced 
alkaline  haematin  has  a  very  definite  spectrum  (fig.  198), 
and  its  preparation  affords  a  ready  means  of  detecting  old 
blood  stains  (Chemical  Physiology). 

Haematin  contains  the  iron  of  the  hemoglobin,  and  it  is 
this  pigmented  iron-containing  part  of  the  molecule  which 
has  the  affinity  for  oxygen.  It  is  the  presence  of  iron  which 
gives  it  this  property,  1  grm.  of  iron  being  able  to  carry 
400  c.cm.  of  oxygen. 


BLOOD 


491 


(2)  Haematoporphyrin. — If  hsemoglobin  is  broken  down 
and  the  iron  removed  from  the  htematin  by  means  of 
sulphuric  acid,  a  purple-coloured  substance,  iron-free 
hcematin,  hsematoporphyrin,  is  formed,  which  has  no  affinity 
for  oxygen.  This  pigment  occurs  in  the  urine  in  some 
pathological  conditions  (Chemical  Physiology). 

One  point  of  great  interest  in  the  chemistry  of  hieniatin 
and  its  derivatives  is  that  they,  like  the  green  chlorophyll  of 
plants,  yield  upon  decomposition  very  similar  bodies  belong- 
ing to  the  pyrrol  group  (see  Appendix). 

(oj  Bilirubin  and  Haematoidin. — In  the  liver,  hEemoglobin 
IS  broken  down  to  form  bilirubin  and  the  other  bile  pigments 
(p.  324).  These  are  iron-free,  and,  like  haematoporphyrin, 
do  not  take  up  and  give  off  oxygen.  But  not  only  are  these 
iron-free  pigments  formed  from  haemoglobin  in  the  liver,  but 
they  are  produced  in  the  cells  of  other  parts  of  the  body,  and 
thus  in  blood-extravasations  a  yellow  pigment  hiematoidin 
is  formed  which  is  really  the  same  as  bilirubin. 

(4)  Haemin — the  hydrochloride  of  hcematin — is  formed 
when  blood  is  heated  with  sodium  chloride  and  glacial  acetic 
acid.  It  crystallises  in  small  steel-black  rhombic  crystals, 
and  its  formation  is  sometimes  used  as  a  test  for  blood 
stains  (Chemical  Physiology). 

The  following  table  shows  the  relationship  of  these  pig- 
ents  to  one  another  : — 


Relationship  of  Hb  and  its  Derivatives. 

Methffimogiobin HbO., HI  .CO  \ 

I 


Hh 


Hsemati 


I 
Acid  Haematin 


Alkaline  Haematin 

I 

I 


Oxidised 


Iron-free  Haematin 
^  HcBmatoporphy  rin) 


Reduced  / 

I 
Haematoidin 

Bilirubin 


Globin 
,  Contain  Iron 

Iron-free 


492 


VETERINARY  PHYSIOLOGY 


V.   Gases  of  the  Blood. 
A.  The  Oxygen  of  the  Blood. 

The  study  of  the  pigments  of  the  blood  has  shown  that 
the  function  of  h;emoglobin  is  to  carry  oxygen  from  the 
lungs  and  to  give  it  off  to  the  tissues.  It  has  been  shown 
that  it  is  the  coloured  iron-containing  hsematin,  constituting 
only  about  4  per  cent,  of  the  molecule  which  acts  as  the 
carrier. 

Haemoglobin  carries  oxj^gen  in  virtue  of  the  fact  that, 
when  it  is  exposed  to  a  high  partial  pressure  of  the  gas  in 
the  lungs  it  takes  it  up,  while  it  gives  it  off  when  exposed  to 
a  low  pressure  in  the  tissues. 

The  partial  pressure  of  a  gas  in  an  atmosphere  is  got 
by  multiplying  its  percentage  amount  by  the  atmospheric 
pressure  and  dividing  by  100.  Thus,  taking  the  oxygen  at 
20  per  cent,  of  atmospheric  air,  at  normal  pressure  at  sea- 
level  of  7  GO  mm.  Hg  the  partial  pressure  of  the  oxygen  is — 

20x760      _^  ^ 

=  1  o  2  mm.  Hg. 

100  "^ 

The  tension  of  a  gas  in  a  fluid,  i.e.  its  tendency  to  escape, 
may  be  measured  by  finding  the  partial  pressure  of  the  gas 
in  the  atmosphere  to  which  the  fluid  is  exposed  at  which  the 
gas  is  neither  given  otf  nor  taken  up.  Thus,  if  three  vessels 
containing  oxygen  in  blood  had  over  the  fluid  2,  5,  10 
per  cent,  of  0,,  i-e.  Oo  at  partial  pressures  of  15'2,  38"0, 
and  76"0  mm.  Hg,  and  it  were  found  that  Oo  came  otY  in  the 
first  and  was  taken  up  in  the  third,  but  remained  constant 
in  the  second,  we  should  say  that  the  tension  of  the  gas  was 
38  mm.  Hg. 

0, 


Beginning 
in  Air 

2% 
15 -2  mm.  Hg. 

t 

5% 
38-0  mm.  Hg. 

^0% 
76-0  mm.  Hg. 

J.' 

End_ 
in  Air 

5% 
3S-0  mm.  Hg. 

5% 
38-0  mm.  Hg. 

5% 
38-0  mm.  Hg. 

BLOOD 


493 


The  amount  of  oxygen  taken  up  and  given  off  is  not 
proportionate  to  the  partial  pressure  of  the  gas  to  which  the 
Hb  is  exposed. 

This  has  been  ascertained  by  exposing  solutions  of  Hb  to 
atmospheres  with  different  percentages  of  oxygen,  i.e.  to 
oxygen  at  different  pressures. 

Starting  from  an  atmosphere  containing  no  oxygen — with 


28 


45 


63 


100 


^ 

Dissociation  Curve 

in     '20       30       4 

or  aCMOGLOBlN  AT  37°  C. 
n       f)0        (^0        70       80       9 

94 
87 

72. 
55 
2.7 

. . 

--^ 

^ 

/ 

/ 

/ 

/ 

; 

^ 

1 

\/ 

/ 

1 

/ 

1 

1 

1 

y 

/; 

OXY 

_— — 

GEN  ft 

E5SUR 

:iNTn 

m.op 

Hg. 

80   . 

H 

o 

70  i 

o 
50  1 


40  % 


30 


20  ( 


10 


Dissociation  Curve  ofH/EMOglobin  inthe3lood. 


Fig.  200. — The  Dissociation  Curve  of  Haemoglobin  in  pure  solution  (continu- 
ous line)  and  in  blood  (broken  line).  The  oxj-gen  pressure  in  mm.  is 
indicated  by  the  ordinates,  and  the  percentage  saturation  is 
indicated  by  the  abscissas.  Note  the  more  rapid  dissociation  under  50 
mm.  Hg  in  blood  than  in  pure  solution  of  Hb. 

no  partial  pressure  of  oxygen — it  is  found  that  the  Hb  is 
entirely  reduced — carries  no  oxygen.  When  exposed  to 
atmospheres  containing  higher  and  higher  percentages  of 
oxygen  it  is  found  that  the  amount  taken  up  rapidly 
increases  till  at  30  mm.  Hg,  equivalent  to  about  6  per 
cent,  of  oxygen  in  the  atmosphere  at  sea-level,  the  Hb  is 
saturated  to  about  80  per  cent. 


494 


VETERINARY   PHYSIOLOGY 


Further  increasing  the  proportion  and  pressure  of  oxygen 
in  the  air  brings  about  only  a  slightly  increased  taking 
up. 

Conversely,  if  Hb  saturated  with  oxygen  is  exposed  to 
lower  and  lower  pressures  of  the  gas,  it  gives  up  its  oxN^gen 
slowly  till  a  pressure  of  30  mm.  Hg  is  reached  and  then 
more  rapidly. 

This  is  shown  in  fig.  200. 

In  blood    the    curve    given    is    different    because    of  the 


'r 

-^ 

:::= 

■ — 

// 

/ 

^ 

^ 



__^ 

/ 

/ 

X 

^ 

^..^^ 

/ 
/ 

/ 
/ 

/^ 

■■■/■ 

1 

/ 

i 

/ 

f 

1/ 

\l 

I 

Fig.  201. Dissociation    curves  of  oxyhivmoglobin  to  show  the  influence  of 

temperature  i.,  at  16'  ;  ii.,  at  25' ;  in.,  at  32'  ;  iv.,  at  3S'  ;  and  v.,  at 
49'  C.  Oxygen  pressure  along  abscissa  percentage  of  reduced  h;emoglobin 
on  vertical  line. 

presence  of  CO.,  and  of  electrolytes.  The  taking  up  of 
oxygen  rises  rapidly  to  50  mm.  Hg,  equivalent  to  10  per 
cent,  of  oxygen  in  the  air,  when  the  hremoglobin  is  saturated 
to  about  80  per  cent.  The  giving  olT  takes  place  in  the  same 
ratio  (fig.  200). 

This  association  of  Hb  and  O2  and  the  dissociation  are 
modified  by — 

1st.  Temperature. — Fig.  201  gives  the  results  at  iv. 
about   the  temperature   of    the    body.      If  the   temperature 


BLOOD 


495 


is  raised  the   dissociation  curve   is   lowered  v.,    and  if  the 
temperature  is  lowered  the  curve  is  raised  i.,  ii.,  m. 

'2nd.  The  H  ion  concentration  of  the  blood. — This,  as 
already  shown  (p.  481),  is  controlled  by  the  sodium  bicar- 
bonate of  the  plasma,  and  is  chiefly  determined  by  the 
amount  of  dissociated  HgCOg  in  the  blood.  Any  increase 
of  the  Ch  alters  the  form  of  the  curve,  tending  to  bring 
about  dissociation  of  HbOo  at  higher  pressures,  as  is  shown 
infis.  202. 


20     30 


50     60 


Fig.  202. — To  show  the  effect  of  the  tension  of  COo  in  the  blood  upon  the 
giving  off  of  oxygen.  The  pressures  of  oxygen  are  given  as  the  abscissa; 
in  mm.  Hg,  and  the  saturation  of  the  htenioglobin  as  the  ordinates.  Note 
the  marked  difference  at  20  mm.  Hg  of  oxygen  with  5  and  with  40 
ram.  Hg  pressure  of  COo- 

In  fact,  the  CO..  of  the  blood  plays  a  most  important  part 
in  setting  free  the  0.,  for  the  tissues,  since  it  raises  the  C^  of 
the  blood. 

Zrd.  The  presence  of  electrolytes  also  lowers  the  curve. 


B.  The  Carbon  Dioxide  in  the  Blood. 

The  carriage  of  CO.2  in  the  blood  has  already  been  dealt 
with.  It  has  been  shown  that  it  exists  to  a  large  extent  as 
NaHCOg  and  to  a  small  extent  in  solution  (p.  481). 

By  subjecting  blood  to  different  pressures  of  CO2,  it  is 
found   that   the   amount   carried   practically   varies   directly 


496  VETERINARY   PHYSIOLOGY 

with  the  partial  pressure  in  the  air  to  which  the  blood  is 
exposed. 

As  already  indicated,  the  proteins  of  the  blood  plasma 
and  of  the  red  cells,  notably  the  globin,  which  is  the  chief 
constituent  of  ha?moglobin,  may  combine  with  CO.,. 

Attempts  have  been  made  to  determine  the  amounts  of 
CO2  in  the  various  combinations,  but  at  present  our  knowledge 
is  too  defective  to  allow  of  definite  figures  being  given. 

It  has  been  maintained  that,  since  NaHCOg  is  not  dis- 
sociated at  the  temperature  of  the  body  with  a  partial 
pressure  of  COg  such  as  occurs  in  the  lungs,  therefore  the 
bicarbonate  does  not  play  the  part  of  carrying  the  CO.2  from 
the  tissues  to  the  lungs,  and  that  the  main  carrier  is  the  ha^mo- 
globin,  the  HbCOo  being  more  unstable  in  the  presence  of  0,. 

This    theory    seems    to    ignore    the    significance    of   the 

.       ,  H,COo         1  ,  .    \.  , 

proportion  between  „  unr)  =  ^  ^^^  ^^^  adjustment  under 

various  conditions. 

It  is  certain  that  the  amount  of  CO.,  which  leaves  the 
blood  in  the  lungs  is  a  very  small  part  of  the  amount  held 
in  the  blood  (p.  498). 

The  actual  quantities  of  oxygen  and  of  carbon  dioxide  in 
the  blood  are  of  much  less  importance  than  their  tension. 

Method  of  Determining. 
(i.)  They  may  be  extracted  by  subjecting  the  blood  to 
the  Torricellian  vacuum  over  the  barometric  colunm  of 
mercury.  Many  forms  of  mercury  gas  pumps  have  been 
devised.  One  is  shown  in  fig.  203.  By  raising  the  mercury 
ball  M.B.,  air  may  be  driven  out  of  the  blood  bulbs  a-h  by 
filling  them  with  mercury.  On  clamping  at  a  and  lowering 
M.B.,  a  TorricelHan  vacuum  is  produced.  The  bulbs  are 
then  detached  and  weighed,  and  blood  is  collected  in  them 
from  a  vessel.  The  blood  bulbs  are  then  connected  with  the 
apparatus  and  a  vacuum  produced  in  G.B.,  where  the  gases 
are  collected.  By  turning  the  two-way  tap  T.,  they  can  be 
passed  into  the  eudiometer  tube  E.,  and  then  analysed,  the 
carbon  dioxide  being  absorbed  by  caustic  soda  and  the 
oxygen  by  alkaline  sodium  pyrogallate. 


BLOOD 


497 


The  fact  that  the  CO2  can  be  completely  removed  from 
blood  containing  the  red  cells  but  not  from  the  plasma 
without  the  addition  of  a  weak  acid  seems  to  show  that  the 
haemoglobin  acts  as  an  acid. 

(ii.)  Haldane  and  Barcroft  have  devised  a  convenient 
method,  which  depends  upon  the  fact  that  the  oxygen  can 


Fig.  203. — Diagram  of  one  Form  of  Mercury  Pump  for  Collecting  the  Gases 
of  the  Blood.  M.B.,  the  mercury  bulb  which  can  be  raised  so  as  to 
fill  G.B.  and  a-h  with  mercury,  and  lowered  so  as  to  produce  a  Torri- 
cellian vacuum  in  them,  a  and  h,  clamps  by  which  the  blood  bulbs 
may  be  shut  off,  to  be  weighed  and  to  receive  the  blood.  G.B.,  the 
bulb  in  which  the  gases  are  collected.  T.,  the  three-waj-  tap  by 
which  the  gas  bulb  G.B.  is  connected,  either  with  the  blood  bulb,  or 
with  the  eudiometer  tube,  E.  B.  is  the  bath  of  mercury  in  which 
the  tube  filled  with  mercury  is  set. 

be  driven  otF  from  blood  treated  with  dilute  ammonia,  by 
the  addition  of  potassium  ferricyanide,  and  that  the  carbon 
dioxide  is  liberated  by  adding  an  acid.  The  amount  of  gas 
may  be  (a)  directly  measured  in  a  Duprd's  apparatus,  or  (6) 
determined  by  measuring  the  increased  pressure  in  the  tube 
in  which  the  gas  has  been  given  otf,  by  means  of  Barcroft's 
apparatus  {Chemical  Physiology). 
32 


498  VETERINARY   PHYSIOLOGY 

(iii.)  Van  Slyke  has  devised  an  apparatus  for  the  liberation 
of  the  CO2  of  the  blood  by  weak  sulphuric  acid  and  of  the 
Oo  by  potassium  ferricyanide.  The  gases  are  then  collected 
in  a  Torricellian  vacuum  and  measured  at  atmospheric 
pressure.  The  method  may  be  carried  out  in  a  few  minutes 
and  is  of  use  in  clinical  work. 

Amounts  of  Gases. — The  amount  of  gases  which  may  be 
extracted  varies  considerably.  About  60  c.c.  of  gas, 
measured  at  0°  C.  and  760  mm.  pressure,  from  100  c.c. 
of  blood  may  be  taken  as  a  rough  average.  The  proportion 
of  the  sases  varies  in  arterial  and  venous  blood. 


Average  Amount  of  Gases  per  Hundred  Volumes 
OF  Blood. 


Arterial  Blood. 

Venous  Blood. 

Oxygen    _     _. 

20 

8-12 

Carbon  dioxide 

40 

46-60 

Recently  a  series  of  analyses  of  the  arterial  and  venous 
blood  in  normal  men  has  been  made,  and  it  has  been  found 


that  the  average  content  is  about 


Arterial  Blood. 

Venous  Blood. 

Oxygen 

21 

14 

Carbon  dioxide      . 

50 

55 

In  the  lungs  the  blood  gains  about  5   per  cent,  of  oxygen 
and  loses  about  5  per  cent,  of  carbon  dioxide. 

While  there  is  an  exchange  of  something  like  36  per 
cent,  of  the  oxygen,  the  exchange  of  carbon  dioxide  amounts 
to  only  between  8  and  9  per  cent,  of  the  total  amount  in  the 
blood. 

In  the  tissues  there  is,  of  course,  a  reversal  of  the  changes 
that  go  on  in  the  lungs. 

VI.  Source  of  the  Blood  Constituents. 

A.  Plasma. — The  water  of  the  blood  is  derived  from  the 
water  ingested.  But  there  is  a  free  interchange  of  water 
between  the  blood  and  the  tissues  so  that  after  bleeding 
water  rapidly  passes  into  the  blood  to  make  up  the  original 


BLOOD  499 

volume,  and  after  the  injection  of  hyjjotonic  saline  and  other 
non-colloidal  fluids  into  the  blood-vessels,  water  passes  out 
into  the  tissues.      The  volume  of  the  blood  is  thus  regulated. 

The  origin  of  the  proteins  is  unknown,  although 
ultimately  they  must  come  from  the  food  ;  very  probably 
they  are  in  part  derived  from  the  tissues.  But  the  signifi- 
cance of  the  two  proteins,  albumin  and  globulin,  and  of  their 
variations  has  not  yet  been  elucidated. 

The  glucose  is  derived  from  the  carbohydrates  and  from 
the  proteins  of  the  food,  and  during  starvation  it  is  constantly 
produced  in  the  liver  and  poured  into  the  blood  (p.  354). 

The  fats  are  derived  from  the  fats  and  carbohydrates  of 
the  food  and  tissues  (p.  351). 

The  urea  and  other  waste  constituents  are  derived  from 
the  various  tissues.  A  transference  of  sodium  phosphate 
from  the  tissues  to  the  blood  occurs  when  acidosis  is 
threatened,  probably  by  the  dissociated  HCl  of  the  NaCl  of  the 
blood  passing  into  the  cells  and  turning  out  the  phosphate 
as  NaHoPO^  to  be  excreted  (p.  482). 

B.  Cells- — I.  Leucocytes. — 1.  In  the  embryo  these  are  lirst 
developed  from  the  mesoderm  cells  generally.  In  extra- 
uterine life  they  are  formed  in  the  lymph  tissue  and  in  the 
red  marrow  of  bone. 

2,  Lymph  Tissue  is  very  widely  distributed  in  the  body, 
occurring  either  in  patches  of  varying  shape  and  size,  or  as 
regular  organs,  the  lymphatic  glands  (fig.  205).  These  are 
placed  on  the  course  of  lymphatic  vessels,  and  consist  of  a 
sponge-work  of  fibrous  tissue,  in  the  interstices  of  which  are 
set  patches  of  lymph  tissue,  in  which  multiplying  lymphocj^tes 
are  closely  packed  together.  Each  mass  of  lymphatic  tissue 
is  surrounded  by  a  more  open  network,  the  sinus,  through 
which  the  lymph  flows,  carrying  away  the  lymphocytes  from 
the  germ  centres.  In  the  sinuses  are  foimd  many  cells  with 
a  marked  phagocytic  action,  and,  when  erythrocytes  are 
destroyed  by  htemolytic  agents,  the  pigment  and  the  iron 
derived  from  the  haemoglobin  are  often  found  abundantly  in 
the  cells  in  the  sinuses  of  lymph  glands. 

Round  some  of  the  lymphatic  glands  of  certain  animals 
large  blood  spaces  or  sinuses  are  seen,  and  these  glands  are 


500 


VETERINARY   PHYSIOLOGY 


called  hsemolymph  glands  (fig.  205).      They  are   intermediate 
between  lymphatic  glands  and  the  spleen. 

3.  Bone  Marrow  (fig.  204). — Young  leucocytes  or  leuco- 
blasts,  in  the  condition  of  mitosis,  are  abundant  in  this  tissue, 
often  in  patches,  the  leucoblastic  areas,  and  they  pass  away 
in  the  blood  stream.  They  are  of  all  varieties.  In  digestion 
leucocytosis  and  in  certain  pathological  conditions  the  for- 
mation of  these  cells  is  increased  and  a  leucocytosis  results. 


.  ^ 

*' 

I   'i 

^ 

< 

■ 

Fig.  20-4. — Section  of  Red  Mairuw  of  Bone,  u,  lymphocyte;  h,  fat  cell;  c, 
erythroblast ;  (/,  giant  cell ;  e,  erythrocyte  ;  /,  erythroblast  in  mitosis  ; 
g,  neutrophil  myelocyte;  h,  eosinophil  myelocyte;  k,  eosinophil 
leucocyte  ;  I,  polymorpho-nuclear  leucocyte. 


II.  Erythrocytes. — In  the  embryo,  these  cells  seem  to  be 
formed  by  a  process  of  budding  from  the  mesoderm  cells, 
which  become  vacuolated  to  form  the  primitive  blood-vessels. 
The  primitive  red  cells  are  larger  than  those  of  later  life,  and 
they  have  a  very  distinct  nucleus.  In  extra-uterine  life  they 
occur  in  the  blood  as  megalohlasU  in  some  blood  diseases. 

A  new  set  of  nucleated  red  cells  next  develops  in  the  liver, 
and  later  in  the  spleen  and  bone  marrow.  They  are  smaller 
than  the  megaloblasts,  and  they  are  known  as  normoblasts 
when  they  appear  in  the  blood  in  extra-uterine  life,  as  they 


BLOOD  501 

do  in  certain  pathological   conditions.      After  birth,  erythro- 
cytes are  formed  in  the  red  marrow  of  bone  (fig.  204). 

Marrow  consists  of  a  fine  fibrous  tissue  with  large  blood 
capillaries  or  sinusoids  running  in  it.  In  the  fibrous  tissues 
are  numerous  fat  cells  (clear  spaces  h  in  fig.  204)  and 
generally  a  considerable  number  of  multi-nucleated  giant 
cells  (d)  and  myelocytes  (g).  In  addition  t,o  these  are  the 
young  leucocytes,  leucoblasts  (a.g.h.),  and  lastly,  young 
nucleated  red  cells,  the  erythrohlasts  (c./.).  After  hemorrhage, 
the  formation  of  these  becomes  unusually  active,  and  may 
implicate  parts  of  the  marrow  not  generally  concerned  in  the 
process,  and  hence,  the  red  marrow  may  spread  from  the  ends 
of  the  long  bones,  where  it  is  usually  situated,  towards  the 
middle  of  the  shaft. 

After  haemorrhage,  when  the  process  of  regeneration  is 
very  active,  red  cells  with  nuclei,  normoblasts,  escape  into  the 
blood.  Young  erythrocytes,  even  after  they  have  lost  their 
nucleus,  may  be  distinguished  by  a  peculiar  reticulated 
appearance  when  they  are  stained  with  brilliant  cresyl  blue. 

The  nuclei  of  the  erythrohlasts  atrophy,  and  the  cells 
escape  into  the  blood  stream. 

The  red  marrow  has  the  power  of  retaining  the  iron 
of  disintegrated  erythrocytes,  which,  in  different  stages  of 
disintegration,  are  found  enclosed  in  large  modified  leucocytes 
or  phagocytes.  The  iron  is  often  very  abundant  after  a 
destruction  of  erythrocytes. 

VII.  Total  Amount  of  Blood  in  the  Body. 

1.  Welcker's  method  consists  in  (1)  bleeding  an  animal, 
measuring  the  amount  of  blood  shed,  and  determining  the 
amount  of  haemoglobin  contained  in  it;  (2)  then  washing  out 
the  blood-vessels,  and,  after  measuring  the  amount  of  fluid 
used,  determining  the  amount  of  haemoglobin  in  it  to 
ascertain  the  amount  of  blood  it  represents.  By  this  method 
the  amount  of  blood  was  found  to  be  about  xV — 7  7  per  cent, 
of  the  body  weight. 

2.  Haldane  and  Lorrain  Smith  have  devised  a  method 
which  can  be  applied  to  the  living  animal.      It  depends  upon 


502  VETERINARY   PHYSIOLOGY 

the  fact  that,  after  an  animal  or  person  has  inhaled  carbon 
monoxide,  it  is  possible  to  determine  to  what  proportion  the 
gas  has  replaced  ox3'gen  in  the  oxyhsemoglobin.  If,  then, 
an  individual  breathes  a  given  volume  of  carbon  monoxide, 
and  if  a  measured  specimen  of  blood  is  found  to  contain  a 
definite  percentage  of  the  gas,  the  rest  of  the  gas  must  be 
equally  distributed  through  the  blood,  and  thus  the  amount 
of  blood  may  be  deduced.  If,  for  instance,  50  c.c.  have  been 
taken  up,  and  there  is  1  per  cent,  in  the  blood,  the  whole  blood 
holding  the  50  c.c.  must  be  5000  c.c.  They  conclude  that 
the  blood  is  about  ^tt'  ^  P®^'  cent.,  of  the  weight  of  the 
body  in  the  human  subject.  This  method  has  been 
adversely  criticised  on  the  ground  that  the  CO  may  be  taken 
up  by  the  tissues  of  the  body  as  well  as  by  the  blood. 

3.  Dreyer  has  devised  another  method  for  the  living 
animal.  After  bleeding,  the  volume  of  blood  is  restored  in  a 
a  few  minutes  by  the  passage  of  fluid  from  the  tissues  (p.  450). 

The  number  of  red  cells  per  c.mm.  is  determined.  A 
definite  amount  of  blood  is  drawn  ;  and  after  a  few  minutes 
the  number  of  corpuscles  is  again  counted.  The  reduction 
indicates  the  dilution  of  the  blood.  Thus,  suppose  the  first 
count  gave  5,000,000  per  c.mm.,  and  that  400  c.c.  of  blood 
were  taken,  and  that  the  second  count  gave  4,500,000 — a 
fall  of  500,000  or  10  per  cent. — the  400  c.c.  must  be  10  per 
cent,  of  the  whole  blood  which  is  thus  4000  c.c. 

4.  The  vital  red  method.  Vital  red  is  a  non-toxic 
pigment,  which  forms  a  colloidal  solution  in  the  blood,  and 
does  not  readily  transude  from  the  vessels.  By  injecting  a 
measured  quantity  into  a  vein  and  determining  its  dilution 
in  the  blood  plasma,  the  total  amount  of  plasma  may  be 
calculated,  and  if  the  volume  of  cells  is  determined  by 
centrifuging  in  an  ha^uiatocrit,  the  total  volume  of  blood 
may  be  calculated.  These  last  two  methods  give  results 
corresponding  to  Welcker's  (Practical  Physiology). 

As  has  been  shown  in  the  study  of  circulation  (p.  4  50), 
the  total  amount  of  blood  in  the  body  does  not  always 
correspond  with  the  amount  in  effective  circulation.  In 
such  conditions  as  wound  or  operation  shock  considerable 
amounts  of  blood  may  stagnate  in  the  capillaries,  and  thus 


'    BLOOD  503 

reduce  the  volume  in  circulation  to  such  an  extent  that  the 
supply  of  oxygen  to  the  tissues  is  seriously  interfered  with. 

VIII.  Distribution  of  the  Blood.  y 

Roughly  speaking,  the  blood  is  distributed  somewhat  as 
follows  : — 

Heart,  lungs,  large  vessels     ■  .         .  ^ 

Muscles        ......  ^ 

Liver ^ 

Other  organs i- 


IX.  Fate  of  the  Blood  Constituents. 

A.  Of  the  Plasma. — The  water  of  the  blood  is  got  rid  of  by 
the  kidneys,  skin,  lungs,  and  bowels. 

About  the  fate  of  the  proteins  we  know  nothing. 

The  glucose  and  fat  are  undoubtedly  used  up  in  the 
tissues. 

The  urea  and  waste  products  are  excreted  by  the  kidneys. 

As  already  indicated,  the  salts  of  the  blood  play  the  triple 
part  (1)  of  supplying  the  tissues  with  the  necessary  cations; 
(2)  of  maintaining  the  osmotic  pressure  of  the  blood;  (3) 
of  regulating  the  C^  of  the  blood.  Their  due  proportion  is 
maintained  chiefly  by  the  action  of  the  kidneys,  which 
respond  at  once  to  any  change  in  the  C^  of  the  blood  by 
eliminating  the  excess  of  anions  or  of  cations. 

B.  Of  the  Cells. — (1)  The  leucocytes  break  down  in  the 
body — but  when  and  how  is  not  known.  They  are  greatly 
increased  in  number  after  a  meal  of  proteins  (digestion 
leucocytosis,  p.  348),  and,  since  the  increase  lasts  only  for  a 
few  hours,  they  are  probably  rapidly  broken  down,  possibly  to 
liberate  amino-acids.  But  it  is  also  possible  that  they  may 
return  to  the  bone-marrow  and  lymph  tissue,  from  which 
they  emerged  during  digestion. 

(2)  The  erythrocytes  also  break  down.  How  long  they 
live  is  not  known.  It  is  found  that,  after  injecting  blood 
from  another  animal  of  the  same  species,  the  original  number 
of  corpuscles  is  not  reached  for  about  a  fortnight ;  and  hence 
it   has   been   concluded   that   the    corpuscles    live    for    that 


504  VETERINARY  PHYSIOLOGY 

period.  Advantage  has  been  taken  of  the  fact  that  the 
blood  of  different  individuals  belongs  to  one  of  four  groups  as 
regards  power  of  agglutinating  cells  of  other  groups.  If  a  man 
be  transfused  with  a  blood  which  does  not  belong  to  his  group, 
specimens  of  his  blood  treated  with  a  serum  which  agglutinates 
his  corpuscles  leaves  the  transfused  corpuscles  unagglutinated. 
It  has  been  found  that  this  condition  may  last  for  over  a 
month,  indicating  that  the  corpuscles  have  continued  in  the 
blood  for  this  period  of  time.  The  methods  are  unsatisfactory, 
and  the  results  must  be  accepted  with  reservation. 

The  method  of  disposal  of  old  and  effete  erythrocytes  in  the 
body  has  been  studied  by  poisoning  them  with  various 
reagents  such  as  phenylhydrazin.  It  is  then  found  that 
they  are  removed  from  the  circulation  more  especially  by 
two  organs. 

1.  The  Liver. 

In  the  endothelial  cells  lining  the  capillaries,  erythrocytes 
in  all  stages  of  disintegration  may  be  seen,  and  the  iron  of 
the  haemoglobin  in  a  compound,  or  series  of  compounds, 
generally  called  haemosiderin,  may  be  demonstrated  by  the 
green  colour  developed  by  treating  with  hydrochloric  acid 
and  potassium  ferrocyanide.  When  haemoglobin  is  set  free 
in  the  plasma,  it  is  chiefly  taken  up  by  the  true  liver  cells. 
That  the  pigment  is  split  up  and  the  iron-free  part  excreted 
is  shown  by  the  presence  of  the  bile  pigments  in  the  bile. 

2.  The  Spleen. 
1.  Structure. — This  organ  is  composed  of  a  fibrous 
capsule  containing  visceral  muscle,  and  of  a  sponge-work  of 
fibrous  and  muscular  trabecular,  in  the  interstices  of  which  is 
the  spleen  pulp.  The  branches  of  the  splenic  artery  run  in 
the  trabeculse,  and  twigs  pass  out  from  these  trabecule,  and 
are  covered  with  masses  of  lymph  tissue  forming  the 
Malpighian  corpuscles.  Beyond  these,  the  vessels  open  into 
a  series  of  complex  sinusoids,  lined  by  large  prominent 
endothelial  cells.  From  these  capillary  sinuses  the  blood  is 
collected  into   channels,  the  venous  sinuses,   which  carry  it 


BLOOD 


505 


back  to  branches  of  the  splenic  vein  in  the  trabeculae.  The 
pulp  may  thus  be  compared  with  the  blood  sinuses  of  the 
hffimolymph  glands,  and  the  spleen  may  be  considered  as 
being  a  still  further  development  of  the  hsemolymph  gland 
from  the  lymph  gland  (fig.  205). 

2.    Functions — 

A.  Blood  Formation. — (1)  Lymphocytes  are  undoubtedly 
formed  in  the  Malpighian  corpuscles.  But  their  formation 
is  probably  unimportant.  Their  number  is  not  greater  in 
the  blood  of  the  splenic  vein  than  in  that  of  the  artery,   and 


H.EMOLYMPH 


Fig.  205.— To  show  the  Relationship  of  the  Spleen  to  Lymph  Glands  and 
Hremolymph  Glands.  The  black  indicates  lymphoid  tissue ;  the 
coarsely  spotted  part,  lymph  sinuses,  and  the  finely  dotted  part, 
blood  sinuses.     (Lewis.  ) 

removal  of  the  spleen  causes  no  change  in  the  number  in 
the  blood. 

f2)  Erythrocytes. — That  the  spleen  is  not  a  seat  of  the 
formation  of  erythrocytes  in  normal  extra-uterine  life  is 
indicated — (i)  by  there  being  no  greater  number  in  the 
blood  of  the  splenic  vein  than  in  that  of  the  splenic  artery  ; 
(ii)  by  the  fact  that  removal  of  the  spleen  causes  no 
decrease  in  the  total  number  of  erythrocytes  ;  and  (iii)  by 
the  equally  rapid  regeneration  of  erythrocytes  in  animals 
from  which  the  spleen  has  been  removed  and  in  normal 
animals. 

B.  Blood  Destruction. — 1.  That  it  takes  no  active  part  in 


506  VETERINARY  PHYSIOLOGY 

the  destruction  of  erythrocytes  is  shown  by  the  facts  (i) 
that  injections  of  extracts  of  the  spleen  cause  no  change  in 
the  number  of  corpuscles ;  (ii)  that  removal  causes  no 
increase  in  the  number  of  erythrocytes  ;  (iii)  that,  when 
blood  is  injected,  the  added  corpuscles  are  not  removed 
more  quickly  in  the  normal  than  in  the  spleenless  animal. 

2.  The  spleen  is  rather  to  be  regarded  as  a  scavenger, 
which  removes  dead  erythrocytes  from  the  blood.  This  is 
indicated  by  the  facts  (i)  that,  after  injecting  h^emolytics, 
such  as  phenylhydrazin,  there  is  less  marked  anaemia  in  the 
spleenless  animals  on  the  fourth  day,  because  the  dead 
corpuscles  are  not  removed  from  the  blood  ;  (ii)  that  the 
remains  of  the  corpuscles  may  be  seen  in  the  cells  of  the 
spleen,  and  chiefly  in  the  endothelial  cells  of  the  sinuses  ; 
(iii)  that  iron  compounds  from  the  hemoglobin  accumulate 
in  the  spleen  after  haemolysis.  A  result  of  this  is  that  if 
rabbits  are  fed  on  a  food  poor  in  iron  such  as  rice  they 
become  anaemic  more  rapidly  if  the  spleen  has  been  removed, 
because  they  have  a  smaller  store  of  iron  to  draw  upon. 

C.  Digestion. — It  has  been  suggested  that  the  spleen 
manufactures  a  kinase  which  activates  the  pancreatic 
juice,  but  the  evidence  is  against  this  theory. 

The  spleen  probably  acts  as  a  blood  reservoir  regulating 
the  supply  of  blood  to  the  digestive  organs. 

On  the  purin  metabolism  it  may  have  an  effect  (see  p. 
556)  ;  on  the  general  metabolism  it  has  no  action. 

D.  Movements. — The  visceral  muscle  in  the  framework 
of  the  spleen  undergoes  rhythmic  contraction  and  relax- 
ation, and  the  organ  thus  contracts  and  expands  at  regular 
intervals  of  about  a  minute.  In  the  dog  the  movements  are 
very  marked. 

These  movements  may  be  recorded  by  enclosing  the 
organ  in  an  oncometer,  a  closed  capsule  connected  with  some 
form  of  recording  apparatus.  They  are  controlled  by  nerve 
fibres,  from  the  true  sympathetic  system,  leaving  tlie  spinal 
cord  chiefly  in  the  6  th,  7th,  and  8th  dorsal  nerves  of  both 
sides.  Strong  stimulation  of  these  causes  contraction, 
which  is  also  caused  by  the  intravenous  injection  of 
adrenalin. 


LYMPH  507 

B.   LYMPH. 

Lymph  is  the  fluid  which  plays  the  part  of  middleman 
between  the  blood  and  the  tissues.  It  fills  all  the  spaces  in 
the  tissues  and  bathes  the  individual  cell  elements.  Those 
who  maintain  that  the  lymphatics  are  shut  off  from  the 
tissue  spaces  prefer  to  call  the  fluid  filling  these  spaces 
tissue  fluid.  They  consider  that  it  is  separated  from 
the  lymph  in  the  lymphatics  by  a  layer  of  endothelium. 
These  spaces  in  the  tissues  open  into  vessels — the  lymph 
vessels — in  which  the  lymph  flows  away  and  is  conducted 
through  lymph  glands  and  back  to  the  blood  through  the 
thoracic  duct  (see  fig.  162,  p.  383). 

1.  Characters  of  Lymph. — Lymph  varies  in  character 
according  to  the  situation  from  which  it  is  taken  and  accord- 
ing to  the  condition  of  the  animal. 

(1)  Lymph  taken  from  the  lymph  spaces — e.g.  the  peri- 
cardium, pleura,  or  peritoneum — is  a  clear  straw-coloured 
fluid.  It  has  little  or  no  tendency  to  coagulate.  Microscopic 
examination  shows  that  it  contains  few  or  no  cells — any  cells 
which  may  exist  being  lymphocytes.  It  has  the  same  Cjj 
as  the  blood  plasma.  The  specific  gravity  varies  according 
to  its  source,  being  lowest  when  from  the  limbs  and  highest 
when  from  the  liver. 

Apparently  the  cause  of  the  non-coagulation  of  such 
lymph  is  the  absence  of  cells  from  which  thromboplastin 
may  be  set  free.  If  blood  or  leucocytes  be  added  to  it,  a 
loose  coagulum  forms. 

(2)  Lymph,  taken  from  lymphatic  vessels  after  it  has 
passed  through  lymphatic  glands,  is  found  to  contain  a 
number  of  lymphocytes  and  to  coagulate  readily. 

Chemically,  lymph  resembles  blood  plasma,  but  the 
proteins  are  generally  in  smaller  amount,  while  the  inorganic 
salts  are  in  the  same  proportion  as  in  the  blood.  The 
amount  of  proteins  varies  in  lymph  from  difterent  organs. 

Lymph  of  Proteins. 

Limbs       ....        About  2-3  per  cent. 
Intestines  ...  ,,      4-6       ,, 

Liver        ....  ,,      6-8       ,, 


508 


VETERINARY   PHYSIOLOGY 


In  the  lymphatics,  coming  from  the  alimentary  canal, 
after  a  meal  containing  fat,  the  lymph  has  a  milky  appear- 
ance and  is  called  chyle.  This  appearance  is  due  to  the 
presence  of  fats  in  a  very  fine  state  of  division,  forming  what 
is  called  the  molecular  basis  of  the  chyle. 

Lymph,  in  various  diseases,  tends  to  accumulate  as  serous 
eflfusions  in  the  large  lymph  spaces — e.g.  the  pleura,  peri- 
toneum, pericardium — and  these  effusions  behave  differently 
as  regards  coagulation.  The  following  table  helps  to  explain 
this  (S.A.  is  Serum  Albumin,  S.G.  Serum  Globulin) : — 


CoAGULABlLlTy    01 

Lymph,  Sef.um, 

AND  Effusions. 

Plasma  and 
Lymph. 

Serous  Effusion. 

Serum. 

Coag. 

Coag.  with 
Thrombin. 

Uncoag. 

Uncoag. 

S.A. 

S.A. 

S.A. 

S.A. 

S.A. 

S.G. 

.S.G. 

S.G. 

S.G. 

S.G. 

Fibrinogen. 

Fibrinogen. 

Fibrinogen. 

.. 

Prothrombin 

Prothrombin 

Thrombin. 

and  Throm- 

and ThroTii- 

boplastin. 

boplastin. 

Ca 

Ca 

Ca 

Ca 

Ca 

2.  Formation  of  Lymph. — The  amount  of  lymph  formed  is 
measured  by  opening  the  thoracic  duct  in  the  neck  and 
collecting  the  lymph  which  flows  from  it. 

Lymph  is  derived  partly  from  the  blood  and  partly  from 
the  tissues.  Two  processes  may  be  involved — (1)  Filtration, 
the  forcing  of  fluid  and  of  substances  dissolved  in  it  through 
the  pores  of  a  membrane  under  pressure.  (2)  Osmosis,  the 
passage  of  water  through  semi-permeable  membranes — 
membranes  allowing  the  passage  of  water,  but  not  of 
substances  in  solution — to  a  point  of  higher  molecular  con- 
centration. Diffusion,  or  the  passage  of  dissolved  substances 
through  a  membrane  from  a  point  of  high  to  a  point  of  low 
concentration,  can  play  only  a  small  part. 

If  the  formation  of  lymph  cannot  be  explained  in  terms 
of  these  purely  physical  processes,  then  some  unknown 
action    of    the  cells  of  the  capillaries  must   be  invoked   to 


LYMPH  509 

explain  it.  The  most  careful  study  seems  to  show  that 
these  physical  factors  are  adequate  and  that  filtration  plays 
the  most  prominent  part. 

(1)  The  formation  of  lymph  from  the  blood  depends  largely 
upon  the  permeability  of  the  walls  of  the  capillaries  and 
the  pressure  of  blood  in  the  blood-vessels.  Thus,  although 
the  pressure  in  the  blood-vessels  of  the  limbs  is  much  higher 
than  the  pressure  in  the  vessels  of  the  liver,  hardly  any 
lymph  is  usually  produced  in  the  former,  while  very  large 
quantities  containing  a  high  percentage  of  proteins  are 
produced  in  the  latter — apparently  because  of  the  slight 
permeability  of  the  limb  capillaries  and  the  great  permeability 
of  the  hepatic  capillaries.  The  permeability  may  be  increased 
bv  the  injection  of  hot  water  or  of  proteoses,  probably  because 
these  injure  the  capillary  walls,  but  possibly  because  they 
increase  the  activity  of  the  organ. 

While  the  permeability  of  the  vessel  wall  is  the  most 
important  factor  controlling  lymph  formation,  any  increase 
of  the  intravascvda.r  'pressure  of  a  region  may  increase  the 
flow  of  lymph,  and  for  this  reason  any  obstruction  to  the 
free  flow  of  blood  from  a  part  leads  to  increased  lymph 
production  from  that  area. 

Asher  has  pointed  out  that  the  flow  of  lymph  from 
the  salivary  glands  and  from  the  liver  is  increased  with  the 
increased  activity  of  the  organ  irrespective  of  changes  in 
blood  pressure.  Even  after  death,  the  flow  of  lymph  may 
go  on.  He  explains  this  by  supposing  that  the  activity  of  a 
gland  leads  to  the  breaking  down  of  larger  into  smaller 
molecules,  which  increases  the  osmotic  j^ressure  of  the  fluid 
in  the  lymph  spaces  and  thus  causes  an  osmosis  of  water  and 
an  increased  lymph  formation.  After  death  the  disintegra- 
tive changes  may  produce  these  smaller  molecules  and 
have  the  same  effect. 

A  method  of  washing  out  wounds  by  causing  a  flow  of 
lymph  has  been  based  upon  this  action  of  osmosis.  The 
application  to  the  w^ound  of  a  hypertonic  saline  brings  it 
about. 

(2)  That  lymph  is  also  formed  from  the  tissues  is  indicated 
by  the  fact  that  the  injection  of  substances  of  high  osmotic 


ilO 


VETERINARY   PHYSIOLOGY 


equivalent  into  the  blood — such  as  sugar  or  sodium  sulphate 
— leads  to  a  flow  of  fluid  into  the  blood  by  a  process  of 
osmosis  so  that  it  becomes  diluted,  and  also,  to  an  increased 
formation  and  flow  of  lymph.  This  increase  of  water  in 
both  blood  and  lymph  can  be  explained  only  by  its  with- 
drawal from  the  tissues  (fig.  206). 

(3)  The  amount  of  lymph  formed  is  not  great. 
In  man  probably  only  about  100  c.cm.  of  lymph  pass  into 
the  blood  per  hour.  Hardly  any  of  this  comes  from 
the     muscles,    although    these   hold     about    70    per     cent. 

of  the  water  in  the 
body  ;  nearly  all  comes 
from  the  abdominal 
organs,  chiefly  from  the 
liver,  although  these  hold 
only  about  7  per  cent, 
of  the  water  of  the 
body. 


LYMPH  VESSEL 


mam    exchange   of 

is   directly  between 


TISSUE      FLUID 


206. — Diagram  to  illustrate  the 
formation  of  lymph  and  the  inter- 
change between  tlie  blood  and  the 
issue  fluids. 


CAPILLARIES 

The 
water 

the  blood  and  the  tissues 
through  the  fluid  in  the 
tissue   spaces   and  not  by 

Fig.  206.— Diagram     to     illustrate     the       the        lymphatic         vessels. 

Fluid  injected  into  the 
blood-vessels  very  rapidly 
transudes  to  the  tissues, 
and,  when  blood  is  withdrawn  from  the  vessels,  the  water  of 
the  tissues  very  rapidly  passes  into  the  vessels  to  make  up  the 
original  volume.  It  is  by  way  of  the  blood-vessels  that  serous 
effusions  into  the  pleura,  peritoneum  and  other  serous 
cavities  are  removed,  as  is  indicated  by  the  fact  that 
methylene  blue,  injected  into  the  pleural  cavity,  appears  in 
the  urine  in  about  10  minutes,  but  is  not  found  in  the 
lymph  for  from  20  to  120  minutes.  Such  facts  as  these 
rather  favour  the  view  that  the  true  lymph  is  separated  from 
the  tissue  spaces  by  a  layer  of  endothelium  (fig.  206). 


CEREBRO-SPINAL  FLUID  511 


C.   THE  CEREBRO-SPINAL  FLUID. 

1.  Distribution. — This  fluid  fills  the  pericellular  spaces 
so  that  the  nerve  cells  lie  bathed  in  it,  the  perivascular 
spaces,  the  subarachnoid  space,  the  ventricles  of  the  brain 
and  the  central  canal  of  the  spinal  cord.  At  the  base  of  the 
brain  the  subarachnoid  spaces  filled  with  fluid  are  large  and 
they  protect  this  important  part  of  the  nervous  system 
against  injury  by  acting  as  a  water  cushion. 

2.  Characters. — The  cerebro-spinal  fluid  is  clear  and 
transparent,  with  a  specific  gravity  of  1006  to  1008.  It  is 
devoid  of  cells  and  contains  only  traces  of  proteins.  Its 
Ch  is  practically  the  same  as  that  of  the  blood  plasma. 
Its  principal  constituent  is  sodium  chloride  with  sodium 
bicarbonate  and  traces  of  phosphates,  urea  and  dextrose. 
It  has  much  the  composition  of  Locke's  modification  of 
Ringer's  solution,  and  it  contains  oxygen  in  considerable 
amounts. 

8.  Source- — It  was  formerly  supposed  to  be  formed  like 
lymph  by  filtration  from  the  blood.  The  amount  formed 
may  be  measured  by  inserting  a  cannula  into  the  sub- 
cerebellar  space,  (i)  In  this  way  it  is  found  that,  while  the 
amount  produced  does  vary  with  the  blood  pressure,  it  is 
not  proportional  to  it,  (ii)  On  the  other  hand,  any  increase 
of  CO.,  in  the  blood,  the  administration  of  such  anesthetics  as 
chloroform,  and,  above  all,  the  injection  of  extracts  of  the 
choroid  plexus  increase  its  production. 

While  diftusible  substances  pass  readily  into  the  lymph 
they  do  not  all  pass  from  the  blood  to  the  cerebro-spinal  fluid. 
Some,  like  urethane  and  alcohol,  do  pass,  but  salvarsan  is  held 
back  and  is  thus  of  little  use  in  the  treatment  of  syphilitic 
affections  of  the  nervous  system  in  man. 

The  fluid  thus  seems  to  be  a  secretion  from  the  choroid 
plexus.  This  is  covered  by  cubical  vacuolated  cells,  and  is, 
in  fact,  an  inverted  gland,  which  passes  on  from  the  blood 
the  inorganic  constituents  and  oxygen,  but  which  holds  back 
the  proteins  and  many  toxic  substances. 

On  the  other  hand,  diftusible  substances  pass  out  readily 


512  VETERINARY   PHYSIOLOGY 

from  the  cerebro-spinal  fluid  of  the  brain  to  the  blood  ;  but 
in  the  lower  part  of  the  spinal  cord  they  do  not  pass  out  freely, 
and  therefore  anaesthetics  like  cocaine  may  be  injected  into  the 
subarachnoid  space  in  this  region  to  anaesthetise  the  cord. 

This  outward  passage  into  the  blood  seems  to  show  that 
the  fluid  secreted  by  the  choroid  plexus  is  carried  away  in 
the  blood  stream.  It  may  escape  along  certain  of  the  cranial 
nerves,  more  especially  along  the  olfactory  nerve.  This 
channel  is  of  importance  in  allowing  the  entrance  of  certain 
micro-organisms  such  as  those  of  infective  poliomyelitis  and 
of  cerebro-spinal  meningitis  in  man. 

It  is  also  probably  passed  into  the  blood  of  the  dural 
sinuses  by  the  arachnoidal  villi  which  project  into  these,  and 
which  are  covered  by  curious  collections  of  cells,  and  from 
the  perivascular  spaces  into  the  capillaries  which  they 
surround. 

Excessive  secretion  or  blocking  of  the  lateral  ventricles 
may  cause  an  increase  in  the  fluid,  a  rise  of  pressure  and 
cerebral  symptoms.  The  fluid  naturally  escapes  from  the 
fourth  ventricle  by  the  foramen  of  Magendie  and  the  foramen 
of  Luschka  into  the  subarachnoid  space. 

4.  Quantity. — The  quantity  of  fluid  contained  is  small, 
in  man  about  130  c.cms. 

5,  Functions. — (i)  The  cerebro-spinal  fluid,  filling  all  the 
spaces  in  the  brain,  equalises  pressure  throughout  the 
cerebro-spinal  system  and  acts  as  a  water  cushion,  especially  at 
the  base  of  the  brain,  protecting  the  medulla  against  shock, 
(ii)  In  the  perivascular  lymphatics  it  acts  as  a  support  to 
the  thin- walled  blood-vessels.  (iii)  It  also  acts  as  an 
adjusting  mechanism  in  variations  of  blood  supply  to  the 
central  nervous  system,  (iv)  It  plays  the  part  of  a  middle- 
man between  the  blood  and  the  nerve  cells. 


SECTION    VI. 

RESPIRATION. 

The  study  of  the  metabolism  of  muscle  has  taught  that  a 
process  of  oxidation  is  constantly  going  on  in  the  living 
tissues  for  which  oxygen  is  constantly  required,  and  by 
which  carbon  dioxide  is  constantly  being  produced. 

In  the  lowliest  animals  a  direct  exchange  of  gases  takes 
place  between  the  cells  and  the  surrounding  medium. 

In  the  higher  and  more  complex  animals  a  special 
mechanism  has  been  evolved  for  carrying  the  oxygen  from 
outside  to  the  tissues,  and  of  transporting  the  carbon  dioxide 
from  the  tissues  to  the  exterior. 

This  is  the  Respiratory  Mechanism. 

In  mammals  it  consists  of  arrangements  by  which — 

1.  Air  is  brought  into  relationship  with  the  blood. 

2.  The  exchange  of  gases  between  air  and  blood  takes 
place. 

3.  The  blood  carries  the  oxygen  to  the  tissues,  and  the 
carbon  dioxide  from  the  tissues. 

4.  The  oxygen  is  passed  from  the  blood  to  the  tissues, 
and  the  carbon  dioxide  from  the  tissues  to  the  blood. 

5.  The  oxygen  brings  about  combustion  in  the  tissues. 

The  first  two  constitute  the  process  of  External  Respira- 
tion, the  third  and  fourth  that  of  Intermediate  Respiration, 
and  the  last  that  of  Internal  Respiration.  This  last  has  been 
already  dealt  with  in  the  study  of  muscle  (p.  255)  and  of  meta- 
bolism, and  it  dominates  the  other  parts  of  the  process. 

A.   EXTERNAL  RESPIRATION. 

I.   STRUCTURE   OF   THE   RESPIRATORY 
MECHANISM. 

In  aquatic  animals  the  mechanism  by  which  this  process 
is  carried  on  is  a  gill  or  gills.     Each  consists  of  a  process 
33  513 


514  VETERINARY   PHYSIOLOGY 

from  the  surface  covered  by  a  very  thin  layer  of  integument, 
just  below  which  is  a  tuft  of  capillary  blood-vessels.  The 
oxygen  passes  from  the  water  to  the  blood ;  the  carbon 
dioxide  from  the  blood  to  the  water. 

A  lung  is  simply  a  gill  or  mass  of  gills,  turned  outside 
in,  with  air,  instead  of  water,  outside  the  integument. 
While  in  aquatic  gill-bearing  animals  there  is  constantly  a 
fresh  supply  of  water  passing  over  the  gills,  in  lung-bearing 
animals  the  air  in  the  lung  sacs  must  be  exchanged  by  some 
mechanical  contrivance. 

{The  structure  of  the  various  parts  of  the  respiratory 
tract  must  be  studied  practically.) 

The  lungs  consist  of  myriads  of  small  thin-walled  air  sacs 
attached    round     the    funnel-like    expansions     (infundibular 

passages)  in  which  the  air 
passages  terminate.  These 
infundibula  are  the  most  ex- 
pansile structures  in  the 
lung,  and  they  are  largest 
where  the  expansion  of  the 
lung  is  greatest  (fig.  207). 
Each  air  sac  is  lined  by  a 
Fig.  207.— Scheme  of  the  Distribu-  layer  of  simple  squamous 
S:;:r„7Attcs'o°/rLut"  ep'theUum.  with  smaller,  more 
granular  cells  between  them. 
The  cells  are  readily  stimulated  to  proliferate  by  the  action  of 
irritant  substances,  and  the  cells  so  produced  take  upon  them- 
selves a  phagocytic  action.  The  epithelium  is  placed  upon  a 
framework  of  elastic  fibrous  tissue  richly  supplied  with  blood- 
vessels. It  has  been  calculated  that,  if  all  the  air  vesicles 
in  the  lungs  of  a  man  were  spread  out  in  one  continuous 
sheet,  a  surface  of  about  100  square  metres  would  be  pro- 
duced and  that  the  blood  capillaries  would  occupy  about  75 
square  metres  of  this.  Through  these  vessels  about  5000 
litres  of  blood  pass  in  twenty-four  hours,  and  during 
muscular  exercise  the  flow  may  be  increased  some  sevenfold 
(p.  413). 

The  larger  air  passages  are  supported  by  pieces  of  hyaline 
cartilage  in  their  walls,  but  the  smaller  terminal  passages, 


RESPIRATION  515 

the  bronchioles,  are  without  this  support,  and  are  surrounded 
by  a  specially  well-developed  circular  band  of  non-striped 
muscle — the  bronchial  muscle — which  governs  the  admission 
of  air  to  the  infundibula  and  air  sacs. 


II.    PHYSIOLOGY. 

I.  Physical  Considerations. 

The  lungs  are  packed  in  the  thorax  round  the  heart, 
completely  filling  the  cavity. 

They  may  be  regarded  as  two  compound  elastic-iualled, 
sacs,  which  completely  fill  an  air-tight  box  with  movable 
walls  — the  thorax — and  which  communicate  with  the  exterior 
by  the  windpipe  or  trachea. 

No  space  exists  between  the  lungs  and  the  sides  and  base 
of  the  thorax,  so  that  the  so-called  pleural  cavity  is  simply  a 
potential  space. 

The  lungs  are  kept  in  the  distended  condition  in  the 
thoracic  cavity  by  the  atmospheric  pressure  within  them. 

Their  elasticity  varies  according  to  whether  they  are 
stretched  or  not.  As  they  collapse,  their  elastic  force 
naturally  become  less  and  less,  as  they  are  expanded,  greater 
and  greater.  Taken  in  the  average  condition  of  expansion 
in  which  they  exist  in  the  chest,  the  elasticity  of  the  excised 
lungs  of  a  man  is  capable  of  supporting  a  column  of  mercury 
of  about  30  mm.  in  height,  so  that  they  are  constantly 
tending  to  collapse  with  this  force. 

But  the  inside  of  the  lungs  freely  communicates  with  the 
atmosphere,  and  this,  at  the  sea-level,  has  a  pressure  of 
about  760  mm.  Hg.  During  one  part  of  respiration,  this 
pressure  becomes  a  few  mm.  less,  during  another  part  a  few 
mm.  more  ;  but  the  mean  pressure  of  760  mm.  of  mercury 
is  constantly  expanding  the  lung,  and  acting  against  a 
pressure  of  only  30  mm.  of  mercury,  tending  to  collapse  the 
lung  (fig.  208> 

Obviously,  therefore,  the  lungs  must  be  kept  expanded 
and  in  contact  with  the  chest  wall. 

When   a  pleural  cavity    is    opened,   the    distribution    of 


516 


VETERINARY    PHYSIOLOGY 


forces  is  altered,  for  now  the  atmospheric  pressure  tells  also 
on  the  outside  as  well  as  on  the  inside  of  the  lung  and  acts 
along  with  the  elasticity  of  the  organ  ;  so  that  now  a  force 
of  760  mm. +  80  mm.  =  790  mm.  acts  against  760  mm., 
causing  a  collapse  of  the  lung,  which  comes  to  occupy  a 
small  space  posteriorly  round  the  bronchus  and  pulmonary 
vessels  (fig.  208). 

In  the  surgery  of  the  thorax,  as  well  as  in  the  physiology 
of  respiration,  these  points  are  of  great  importance. 

It  is  possible  that  a  small  opening  may  not  immediately 
lead  to  collapse,  because  the  surface   tension    between    the 


1^^ 


Fig.  208. — Shows  the  Distribution  of  Pressure  in  the  Thorax  with  the  Chest 
Wall  Intact,  and  with  an  Opening  into  the  Pleural  Cavity,  (j) 
indicates  the  atmospheric  pressure  of  760  mm.  of  mercury  ;  30  is  the 
elasticitj'  of  the  lungs,  also  in  mm.  of  mercury. 


parietal  and  pulmonary  pleura  may  be  sufficient  to  overcome 
the  atmospheric  pressure. 

II.  The  Passage  of  Air  into  and  out  of  the  Lungs. 

This  is  brought  about — 

\st.  By  the  movements  of  respiration — breathing. 

Ind.   By  diffusion  of  gases. 

The  air  is  made  to  pass  into   and  out  of  the  lungs  by 
alternate  insjnratlon  and  expiration. 


1.  Movements  of  Respiration. 
1.   Inspiration. — During  this  act,   the    thoracic    cavity  is 
increased    in    all    directions — lateral,    vertical,    and    antero- 


RESPIRATION  517 

posterior.  As  the  thorax  expands,  the  air  pressure  inside  the 
lungs  keeps  them  pressed  against  the  chest  wall,  and  the 
lungs  expand  with  the  chest.  As  a  result  of  this  expansion 
of  the  lungs,  the  pressure  inside  becomes  less  than  the 
atmospheric  pressure,  and  air  rushes  m  until  the  pressures 
inside  and  outside  again  become  equal.  This  can  be  shown  by 
placing  a  tube  connected  with  a  water  manometer  in  the  mouth, 
closing  the  nostrils,  and  breathing  (Practical  Physiology). 

This  expansion  of  the  lungs  can  readily  be  determined  in 
the  antero-posterior  direction  by  percussion,  and  in  the  trans- 
verse planes  by  measurement.  By  tapping  the  chest  with 
the  finger  over  the  lung  in  the  intercostal  spaces,  a 
resonant  note  is  produced,  while  if  the  percussion  is  per- 
formed below  the  level  of  the  lung,  a  dull  note  is  heard. 
If  the  lower  edge  of  this  resonance  be  determined  before  an 
inspiration,  and  again  during  it,  the  lung  will  be  found  to 
have  expanded  backwards  (Practical  Physiology).  The 
change  from  before  backwards  cannot  be  seen  directly,  but 
it  is  indicated  by  a  downward  movement  of  the  wall  of  the 
abdomen.  It  will  be  further  described  when  considering  the 
mechanism  by  which  it  is  brought  about. 

The  expansion  of  the  chest  in  inspiration  is  a  muscular  act 
and  is  carried  out  against  the  following  forces  : — 

1st.  The  Elasticity  of  the  Lungs.— To  expand  the  lungs 
their  elastic  force  has  to  be  overcome,  and  the  more  they  are 
expanded  the  greater  is  their  elasticity.  This  factor  therefore 
plays  a  smaller  part  at  the  beginning  than  towards  the  end  of 
inspiration. 

2nd.  The  Elasticity  of  the  Chest  Tr«^^.— The  resting  position 
of  the  chest  is  that  of  expiration.  To  expand  the  chest  the 
costal  cartilages  have  to  be  twisted. 

Srd.  The  Elasticity  of  the  Abdominal  Wall— As  the  cavity 
of  the  thorax  increases  downwards,  the  abdominal  viscera  are 
pushed  against  the  muscular  abdominal  wall,  which,  in  virtue 
of  its  elasticity,  resists  the  stretching  force. 

The  changes  in  inspiration  are  brought  about  by — 

1st.   An  Increase  in  the  Thorax  from  before  backwards. 

This  is  due  to  the  contraction  of  the  diaphragm  (fig.  209)  ; 
see  also  fig.  168,  p.  392. 


518  VETERINARY   PHYSIOLOGY 

In  expiration  this  dome-like  muscle,  rising  (a)  from  the 
vertebral  column,  and  (6)  from  the  lower  costal  margin, 
arches  forwards,  lying  for  some  distance  along  the  inner 
surface  of  the  ribs,  and  then  curving  inwards  to  be  inserted 
into  the  flattened  central  tendon,  to  which  is  attached  the 
pericardium  containing  the  heart. 

In  inspiration  the  muscular  fibres  contract.  But  the 
central  tendon  being  fixed  by  the  pericardium  does  not 
undergo  extensive  movement.     The  result  of  the  muscular 


Fig.  209. — Vertical- tangential,  Transverse,  and  Vertical  Mesial  Sections 
of  the  Thorax  in  Inspiration  and  Expiration  in  man.  Similar 
changes  occur  in  quadrupeds. 

contraction  is  thus  to  flatten  out  the  more  marginal  part  of 
the  muscle  and  to  withdraw  it  more  or  less  from  the  chest 
wall — thus  opening  up  a  space,  the  complemental  pleura,  into 
which  the  lungs  expand  (fig.  209). 

The  vertebral  part  of  the  diaphragm  pulls  downwards, 
the  costal  part  pulls  backwards,  and  together  they  act  like  a 
piston  extending  the  vertical  diameter  of  the  thorax  (fig.  168). 

^nd.  An  Increase  in  the  Thorax  in  the  transverse 
and  vertical  diameters. 

This  is  brought  about  by  the  pulling  forwards  of  the 
ribs  which  rotate  round  the  axes  of  their  attachments  to  the 
vertebral  column. 

To  understand  this,  the  mode  of  connection  of  the  ribs 


RESPIRATION 


il9 


to  the  vertebral  column  must  be  borne  in  mind.  The  head 
of  the  rib  is  attached  to  the  bodies  of  two  adjacent  vertebrae. 
The  tubercle  of  the  rib  is  attached  to  the  transverse  process 
of  the  hinder  of  these  vertebrae.  From  this,  the  shaft  of  the 
rib  projects  outwards,  downwards,  and  backwards,  to  be 
attached  in  front  to  the  sternum  by 
the  costal  cartilage  running  for- 
wards. If  the  rib  is  made  to 
rotate  round  its  two  points  of 
attachment,  its  lateral  margin  is 
elevated  and  carried  outwards,  while 
its  sternal  end  is  carried  downwards 
and  forwards. 

The  first  pair  of  ribs  does  not 
undergo  this  movement  ;  the  motion 
of  the  second  pair  of  ribs  is  slight, 
but  the  range  of  movement  becomes 
greater  and  greater  as  we  pass  down- 
wards. This  greater  movement  is 
simply  due  to  the  greater  length  of 
the  muscles  moving  the  ribs.  The 
muscles  are  the  external  intercostal 
muscles,  and  they  may  be  considered 
as  acting  from  the  fixed  first  rib. 
Now,  if  the  fibres  of  the  first  inter- 
costal muscle  are  one  inch  in 
length,  the  second  rib  can  be  pulled 
forwards,  say,  half  an  inch.  The 
first  and  second  intercostals  acting 
on  the  third  rib  will  together  be  two 
inches  in  length,  and,  in  contract- 
ing, they  can  move  the  third  rib 
through,  say,  half  of  two  inches 
— i.e.  one  inch.  The  first,  second, 
and  third  intercostals,  acting  on 
three  inches  in  length,  and  can 
rib  half  of  three,  or  one  and  a  half  inches.  The  floating 
ribs  are  fixed  by  the  abdominal  muscles,  and  limit  the  move- 
ment of  the  ribs  next  above  them. 


Fig.  210.— Rib  and  vertebral 
column,  A.,  in  an  anterior 
rib  segment,  and  B.,  in  a 
posterior  rib  segment  to 
show  the  difference  in  the 
obliquity  of  articulation 
and  the  resulting  differ- 
ence in  the  expansion  of 
the  chest,  which  is  greater 
from  side  to  side  in  the 
more  posterior  part  of  the 
chest. 


the    fourth     rib, 
therefore    move 


are 
this 


520  VETERINARY    PHYSIOLOGY 

The  expansion  of  the  lungs  is  very  unequal,  being 
most  marked  at  the  anterior  and  lower  margins  and  much 
less  marked  posteriorly,  especially  round  the  roots. 

The  ventilation  of  the  air  vesicles  is  thus  much  greater 
in  the  former  than  in  the  latter  parts,  where,  if  the  breathing 
be  shallow,  the  exchange  of  gases  comes  to  depend  more 
largely  upon  diffusion  from  the  air  of  the  dead  space 
(p.  523). 

In  these  parts  the  proportion  of  carbon  dioxide  in  the 
vesicles  will  be  greater  and  the  proportion  of  oxygen  less 
than  in  the  other  parts  of  the  lungs. 

When  the  diaphragm  takes  the  chief  part  in  inspiration 
the  breathing  is  said  to  be  abdominal  in  type — when  the 
intercostal s  take  the  chief  part  it  is  said  to  be  thoracic. 

The  diaphragm  and  the  external  intercostals  are  the 
essential  muscles  of  inspiration,  but  other  muscles  also 
participate  in  the  act.  The  nostrils  dilate  with  each  inspira- 
tion. The  nostrils  expand  due  to  the  action  of  the 
dilatores  narium,  which  contract  synchronously  with  the 
other  muscles  of  inspiration.  Again,  if  the  larynx  be 
examined,  it  will  be  found  that  the  vocal  cords  slightly 
diverge  from  one  another  during  inspiration.  This  is 
brought  about  by  the  action  of  the  posterior  crico-arytenoid 
muscles  (p.  551),  and  this  movement  is  interfered  with  in 
"  roarers  "  (p.  553). 

Forced  Inspiration — This  comparatively  small  group  of 
muscles  is  sufficient  to  carry  out  the  ordinary  act  of  inspira- 
tion. But,  in  certain  conditions,  inspiration  becomes  forced. 
A  forced  inspiration  may  be  made  voluntarily ;  often  it  is  pro- 
duced involuntarily.  Every  muscle  which  can  act  upon  the 
thorax  to  expand  it  is  brought  into  play.  The  body  and  spinal 
column  are  fixed.  The  head  is  thrown  back  and  fixed  by 
the  posterior  spinal  muscles.  The  forelimbs  and  shoulders  are 
fixed,  and  every  muscle  which  can  act  from  the  fixed  spine, 
head  and  shoulder  girdle  upon  the  thorax  is  brought  into  play. 
Normally,  these  act  from  the  thorax  upon  the  parts  into  which 
they  are  inserted  ;  now  they  act  from  their  insertions  upon 
their  points  of  origin.  The  sterno-mastoids,  sterno-thyroids, 
and  sterno-hyoids  assist  in  expansion  of  the  thorax.    The  serratus 


RESPIRATION  521 

magnus,  pectoralis  minor,  and  posterior  fibres  of  the  pectoralis 
major,  and  the  part  of  the  latissinius  dorsi  which  passes  from 
the  humerus  to  the  posterior  ribs,  also  pull  these  structures 
forwards.  The  facial  and  laryngeal  movements  also  become 
exaggerated. 

2.  Expiration  is  a  return  of  the  thorax  to  the  position  of 
rest.  The  various  muscles  of  inspiration  cease  to  act,  and  the 
forces  against  which  they  contended  contract  the  thorax  in  its 
three  diameters — 

The  elasticity  of  the  lungs  is  no  longer  overcome  by 
the  muscles  of  inspiration,  and  the  external  atmospheric 
pressure  acting  along  with  it  drives  the  chest  wall  in- 
wards. 

The  elasticity  of  the  costal  cartilages  tends  to  bring  the 
chest  back  to  the  position  of  rest;  the  elasticity  of  the  abclomninal 
wall  drives  the  abdominal  viscera  against  the  relaxed  diaphragm 
and  again  arches  it  towards  the  thorax,  bringing  its  marginal 
portion  in  contact  with  the  ribs  and  occluding  the  complemental 
pleura.  By  this  constriction  of  the  thorax,  the  air  in  the  lungs  is 
compressed  and  the  pressure  is  raised  above  the  atmospheric 
pressure  outside,  and  so  the  air  is  driven  out. 

Experimental  evidence  shows  that  the  internal  intercostals 
contract  with  each  expiration,  and  help  to  draw  the  ribs 
backwards. 

Ordinary  expiration  is  thus  normally  mainly  a  passive  act, 
being  simply  a  return  of  the  thorax  to  the  position  of  rest.  But 
voluntarily,  and,  in  certain  conditions,  involuntarily,  expiration 
may  be  forced. 

Forced  expiration  is  due  to  the  action  ot  muscles.  Every 
muscle  which  can,  in  any  way,  diminish  the  size  of  the  thorax 
comes  into  play.  Chief  of  these  are  the  abdominal  muscles, 
which,  by  compressing  the  viscera,  push  them  upwards  and  press 
the  diaphragm  further  up  into  the  thorax.  At  the  same  time, 
by  acting  from  the  pelvis  to  pull  back  the  ribs,  they  decrease 
the  thorax  from  side  to  side  and  from  below  upwards. 
The  serratus  posticus  inferior  and  part  of  the  sacro-lumbalis 
pull  downwards  the  lower  ribs,  and  the  triangularis  sterni  also 
assists  in  this. 


522  VETERINARY  PHYSIOLOGY 

3.  Special  Respiratory  Movements. — There  are  several  peculiar 
and  special  reflex  actions  of  the  respiratory  muscles,  each  caused 
by  the  stimulation  of  a  special  region,  and  each  having  a  special 
purpose.  They  are  generally  protective  reflexes  in  response  to 
nocuous  stimuli. 

Coughing. — This  consists  of  an  inspiration  followed  by  a 
strong  expiratory  effort  during  which  the  glottis  is  constricted 
but  is  forced  open  repeatedly  by  the  current  of  expired  air. 
It  is  generally  due  to  irritation  of  the  respiratory  tract,  and 
its  object  is  to  expel  products  of  inflammation  or  foreign 
matter. 

Sneezing. — This  is  generally  produced  by  irritation  of  the 
nasal  mucous  membrane,  and  its  object  is  to  expel  irritating 
matter.  It  consists  in  an  inspiratory  act  followed  by  a  forced 
expiration  during  which,  (a)  by  contraction  of  the  pillars  of 
the  fauces  and  descent  of  the  soft  palate,  and  (b)  by  the  tip 
of  the  tongue  being  pressed  against  the  hard  palate,  the 
air  is  compressed  and  finally  forced  through  the  nose  and 
mouth. 

Hiccough  consists  in  a  sudden  reflex  contraction  of 
the  diaphragm  causing  a  sudden  inspiration  which  is 
interrupted  by  a  spasmodic  contraction  of  the  glottis. 
It  is  allied  to  vomiting.  Abdominal  irritation  is  its  chief 
cause. 

Sighing  and  Yawning  are  deep  involuntary  inspirations 
which  serve  to  accelerate  the  circulation  of  the  blood  in  the 
brain  when,  from  any  cause,  it  becomes  less  active.  They  are 
probably  due  to  cerebral  anemia,  which  they  help  to  correct  by 
increasing  the  general  arterial  pressure,  and  they  are  the  result 
of  direct  chemical  stimulation  of  the  respiratory  centre  rather 
than  reflex  actions  (p.  527). 


II.  Amount  of  Air  Respired. 

The  amount  of  air  respired  is  difl"erent  in  ordinary  and 
in  forced  respiration  (fig.  211). 

In  an  ordinary  respiration  in  the  horse  about  3000  ccms.  of 
air  enter  and  leave  the  chest.  That  is  called  the  tidal  air- 
Its  amount  varies  with  the  size  and  muscular  development 
of  the  chest. 


RESPIRATION 


523 


Vihcl 
Cooac.h 


'0/ 


By  a  forced  inspiration  a  much  larger  quantity  of  air  may 
be  made  to  pass  into  the  lungs— a  quantity  varying  with  the 
size  and  strength  of  the  individual — but 
on  an  average  about  14,000  c.cms. 

This  is  called  the  complemental  air. 

By  forced  expiration,  an  amount  of  air 
much  larger  than  the  tidal  can  be  expelled, 
an  amount  usually  about  the  same  as  the 
complemental  air,  and  called  the  reserve 
air. 

The  total  amount  of  air  which  an 
individual  can  draw  into  and  drive  out 
of  his  lungs  is  a  fair  measure  of  the 
size  and  muscular  development  of  the 
thorax,  and  it  has  been  called  the  vital 
capacity  of  the  thorax,  and  in  the  horse  it 
amounts  to  something  like  25,000  to  30,000  c.cms. 

Even  after  the  whole  of  the  reserve  air  has  been  driven  out 
of  the  chest,  a  considerable  quantity  still  remains  in  the  air 
vesicles,  its  amount  depending  upon  the  size  of  the  chest,  but 
averaging  about  10,000  c.cms.     This  is  called  the  residual  air. 

This  very  important  point  must  always  be  remembered, 
that  the  air  taken  into  the  chest  never  fills  the  air  vesicles, 
and  that  air  is  never  driven  completely  out  of  them.  The  air 
in  them  is  thus  not  changed  by  the  movements  of  respiration, 
hut  by  the  process  of  diffusion. 

A  fairly  reliable  conclusion  as  to  the  vital  capacity  may  be 
arrived  at  by  measuring  the  circumference  of  the  chest  in 
expiration  and  inspiration  {Practical  Physiology). 


Fig.  211. — The  Amount 
of  Air  Respired  in 
Ordinary  Respira- 
tion, and  in  Forced 
Inspiration  and 
Expiration. 


III.  Interchange  of  Air  in  the  Lungs  by  Diffusion  of  Gases. 

Since,  in  ordinary  breathing  in  the  horse,  a  residue  of  almost 
24,000  c.cms.  of  air  remains  in  the  lungs  while  only  3000  c.cms. 
pass  into  and  out  of  them,  the  question  whether  any  of  this  gets 
to  the  air  vesicles  must  be  considered.  The  trachea  and  bronchi 
have  a  capacity  of  probably  about  1400  c.cms.  and  they  constitute 
a  "  dead  space,"  so  that,  after  filling  these,  about  1600  c.cms.  are 
available  to  reach  the  vesicles.     But,  so  large  is  the  capacity  of 


524  VETERINARY  PHYSIOLOGY 

these  vesicles,  that,  if  this  air  were  uniformly  distributed,  it  would 
add  only  about  -fth  to  the  volume  of  each.  The  exchange  of 
gases  depends,  in  fact,  largely  upon  the  process  of  diffusion 
(fig.  212). 

Oxygen  is  constantly  being  removed  by  the  blood  from  the 
air  in  the  air  vesicles,  and  carbon  dioxide  is  constantly  being 
added  to  it.  Hence,  the  pressure  of  oxygen  is  lower  and  the 
pressure  of  carbon  dioxide  higher  than  in  the  air  breathed,  and 
hence,  a  diffusion  of  oxygen  to  the  air  in  the  vesicles  and  a 
diffusion  of  carbon  dioxide  from  it  are  constantly  going  on.  In 
the  less  expansile  parts  of  the  lung  the  exchange  of  gases  will 


Fig.   212. — To  show  the  exchanges  of  gases  by  diffusion  between 
the  tidal  air  and  the  reserve  and  residual  air. 

depend  more  largely  upon  diffusion  than  in  the  more  expansile 
parts. 

IV.  Breath  Sounds- 

The  air,  as  it  passes  into  and  out  of  the  lungs,  produces 
sounds  that  may  be  heard  on  listening  over  the  thorax.  The 
character  of  the  breath  sounds  is  of  the  utmost  importance  in 
the  diagnosis  of  diseases  of  the  lungs,  and  must  be  studied 
practically  (Practical  Physiology). 

On  listening  over  the  trachea  or  over  the  bifurcation  of  the 
bronchi  behind  (between  the  4th  and  5th  dorsal  vertebrae),  a 
harsh  sound,  something  like  the  guttural  oh  (German  ich),  may 
be  heard  with  inspiration  and  expiration.  This  is  called  the 
bronchial  sound. 

If  the  ear  be  applied  over  a  spot  under  which  a  mass  of  air 
vesicles  lies,  a  soft  sound,  somewhat  resembling  the  sound  of 


RESPIRATION  525 

centle  wind  among  leaves,  may  be  heard  throughout  inspira- 
tion, and  for  a  third  or  less  of  expiration.  This  is  called  the 
vesicular  sound. 

When  the  air  vesicles  become  consolidated  by  disease,  the 
vesicular  sound  is  lost,  and  the  bronchial  sound  takes  its  place. 
The  cause  of  the  vesicular  character  is  therefore  to  be  sought  in 
the  vesicles,  infundibula,  or  small  bronchi. 

The  cause  of  the  bronchial  sound  has  been  determined  by 
experiments  on  horses.  In  the  study  of  the  cardiac  circulation, 
it  was  shown  that  a  column  of  fluid  moving  along  a  tube  of 
uniform  calibre,  or  with  the  calibre  only  slowly  changing,  pro- 
duces no  sound.  The  same  is  true  of  a  column  of  air.  Any 
sudden  alteration  in  calibre  produces  vibration  and  a  musical 
sound,  as  explained  on  p.  409.  The  first  sharp  constriction  of 
the  respiratory  tract  is  at  the  glottis,  and  it  is  here  that  the 
bronchial  sound  is  produced.  If  the  trachea  be  cut  below  the 
larynx  and  drawn  freely  outwards,  the  bronchial  sound  at 
once  stops  and  the  vesicular  sound  becomes  lower  and  less 
distinct. 

The  cause  of  the  vesicular  sound  is  not  so  satisfactorily 
explained.  It  is  in  part  due  to  propagation  of  the  bronchial 
sound,  altered  by  passing  through  vesicular  tissue  ;  but  it  is 
also  probably  due  to  the  expansion  and  contraction  of  the 
infundibula  drawing  in  and  expelling  air.  The  reason  why  the 
sound  is  best  heard  during  inspiration  may  be  that  the  sound  is 
best  conducted  in  the  direction  of  the  air  stream. 


V.  Rhythm  of  Respiration. 

The  movements  of  respiration  are  carried  on  in  a  regular 
rhythmic  manner.     They  may  be  recorded — 

1.  By  recording  the  movement  of  the  chest  wall  by  some 
form  of  stethograph. 

2.  By  recording  the  movements  of  the  column  of  air  by 
placing  a  glass  tube  in  one  nostril  and  connecting  it  with  a 
recording  tambour  {Practical  Physiology). 

3.  In  lower  animals,  by  connecting  a  strip  of  the  diaphragm 
to  a  lever. 

Their  rate  varies  with  many  factors;  but  the  average 
number  of  respirations  per  minute  in  the  adult   horse  is  about 


526  VETERINAKY   PHYSIOLOGY 

ten  to  twelve,  or  about  one  to  every  four  or  five  beats  of  the 
heart.  Deep  breathers  are  slow  breathers,  and  shallow  breathers 
are  quick  breathers,  and  in  the  latter  a  smaller  part  of  the  lung 
is  ventilated. 

The  most  important  factor  modifying  the  rate  of  respiration 
is  muscular  exercise.  After  galloping  the  respirations  may  be 
over  60  per  minute. 

The  other  modifications  in  the  rate  of  breathing  will  be 
better  understood  after  studying  the  nervous  mechanism  of 
respiration. 

Inspiration  is  more  rapid  than  expiration  (see  fig.  216). 
As  soon  as  it  is  completed,  a  reverse  movement  occurs,  which 
is  at  first  rapid,  but  gradually  becomes  slower,  and  may  be 
followed  by  a  pause,  during  which  the  chest  remains  in  the 
collapsed  condition.  The  existence  and  duration  of  this  pause 
varies  much,  and  it  may  really  be  regarded  as  the  terminal 
period  of  expiration.  Considering  it  in  this  light,  we  may  say 
that  inspiration  is  to  expiration  as  6  is  to  7. 

VI.  The  Nervous  Control  of  Respiration. 

The  rhythmic  movements  of  respiration  require  the  har- 
monious action  of  a  number  of  muscles,  and  this  is  directed  by 
the  nervous  system. 

The  Respiratory  Centre. 
If  the  spinal  cord  be  cut  above  the  third  cervical  nerves  the 
movements  of  respiration    at   once    stop.     Obviously  there  is 
some  nervous  mechanism  above  this  level  presiding  over  these 
muscles. 

A.  Position. — Removal  of  the  brain  above  the  medulla 
oblongata  does  not  stop  the  respiratory  rhythm.  The  mechan- 
ism must  therefore  be  situated  in  the  medulla  oblongata. 

If  the  medulla  be  split  into  two  by  an  incision  down  the 
middle  line,  respiration  continues,  but  the  two  sides  do  not 
always  act  at  tlie  same  rate.  The  mechanism,  then,  is  bi- 
lateral. Normally  the  two  parts  are  connected,  and  thus  act 
together. 

Destruction  of  the  part  of  the  medulla  lying  near  the  root 


RESPIRATION  527 

of  the  vagus  arrests  respiration,  and  it  may  therefore  be  con- 
cluded that  the  nervous  mechanism  presiding  over  this  act  is 
situated  there. 

This  centre  sends  fibres  down  the  lateral  column  of  the  cord 
to  act  upon  the  outgoing  neurons  to  the  muscles  of  respiration, 
and  it  is  by  influencing  the  activity  of  these  that  the  respiratory 
centre  controls  the  act  of  respiration. 

Outgoing  Nerves. — The  diaphragm  is  supplied  by  the 
phrenic  nerves  arising  from  the  third  and  fourth,  and  partly 
from  the  fifth  cervical  nerves.  The  intercostals  are  supplied  by 
branches  from  their  corresponding  dorsal  nerves. 

If  the  spinal  cord  be  cut  or  the  neck  broken  below  the  fifth 
cervical  nerves,  the  intercostal  muscles  cease  to  act.  If  the 
section  be  made  above  the  third  cervical  nerves,  the  diaphragm, 
too,  is  paralysed,  and  the  animal  dies  of  suffocation. 

B.  Mode  of  Action. — The  respiratory  centre  is  under  the 
control  of  higher  nerve  centres,  and,  through  these,  it  may  be 
thrown  into  action  at  any  time,  or  prevented  from  acting  for 
the  space  of  over  a  minute  and,  under  certain  conditions,  for  two  or 
three  minutes.  But,  sooner  or  later,  the  respiratory  mechanism 
acts  in  spite  of  the  most  powerful  attempts  to  prevent  it. 

Pearl  divers  rarely  are  able  to  stay  under  water  for  more 
than  85  seconds  ;  the  record  period  of  submergence  is  4  minutes 
45  seconds. 

1.  Chemical  Regulation. 
A.  Carbon  dioxide. 

The  activity  of  the  centre  is  chiefly  regulated  by  the  tension  of 
CO2  in  the  blood  going  to  it  (p.  495),  and  everything  which  leads 
to  an  increase  in  the  COg  increases  the  activity  of  respiration, 
while  everything  which  decreases  it  decreases  the  activity  of 
breathing. 

The  tension  of  C0„  in  the  blood  is  directly  proportional  to 
the  partial  pressure  of  COj  in  the  medium  to  which  the  blood  is 
exposed  (p.  496).  The  amount  and  tension  of  COo  in  the  blood 
and  therefore  its  action  upon  the  respiratory  centre,  depend 
upon  its  production  in  the  muscles  on  the  one  hand,  and  on  its 
tension  in  the  alveolar  air  on  the  other.  The  latter  may  be 
raised  by  breathing  air  with  an  increased  amount  of  C0„,  e.g. 
5  per  cent. 


528 


VETERINARY   PHYSIOLOGY 


With  any  increase  in  the  amount,  the  respiratory  centre  is 
stimulated  and  the  respirations  increased  Qiyperpncea),  till  the 
tension  of  COo  in  the  blood  becomes  normal.  The  power  of  ad- 
justment is  extraordinarily  efficient,  so  efficient  that  the  tension 
of  C0„  in  the  alveolar  air  is  maintained  at  a  constant  level  of 
about  40  mm.  Hg  under  wide  variations  of  atmospheric  pressure. 
It  has  been  found  that  an  increase  of  0*2  per  cent,  in  the  COo 
of  the  alveolar  air,  i.e.  a  rise  of  tension  from  40  to  41  6  mm.  Hg 
is  sufficient  to  double  the  ventilation  of  the  lungs. 

By  forced  breathing  the  tension  of  COo  in  the  alveolar 
air  may  be  so  lowered  that  the  tension  in  the  blood  is 
markedly  reduced,  and  the  stimulus  to  the  respiratory  centre 
so  decreased,  that  a  long  period  without  breathing,  an  ajjnoea, 
may  occur.  In  fact,  in  some  cases  in  man,  the  face  may  become 
livid  from  want  of  oxygen  before  breathing  is  re-established. 


Alveolar  Air 

COo  tension 
40  mm.  Hg. 

\ 

45 

40 

\ 
35 

Hy 

Ten 

aerpnoea 

sion    of    COo    in    Bl 
in  mm.  Hg. 

Normal  Breathing 

ood 
Apnoea 

B.  Oxygen. 

The  influence  of  the  oxygen  content  of  the  blood  upon  the 
respiratory  centre  is  not  so  manifest. 

The  amount  and  tension  of  Oo  in  the  blood,  and  its  transit 
to  the  tissues  and  the  respiratory  centre,  are  in  some  respects 
more  complex  than  in  the  case  of  CO^. 

As  already  explained  (p.  498)— (1)  the  amount  in  the  blood  is 
not  directly  proportional  to  the  partial  pressure  of  the  gas  to 
which  it  is  exposed.  As  the  latter  rises  from  zero  the  amount  in 
the  blood  rapidly  increases,  till  at  50  mm.  Hg  the  hemoglobin 


RESPIRATION  529 

is  saturated  to  the  extent  of  about  80  per  cent.,  and  a  further 
rise  causes  only  a  slight  increase  in  the  saturation. 

(2)  The  giving  off  of  O.,  from  the  haemoglobin  depends 
upon  the  hydrogen  ion  concentration  of  the  blood  which  is 
mainly  determined  by  the  tension  of  COo.  With  a  pressure 
of  80  mm.  Hg  of  COo  75  per  cent,  of  the  oxygen  is  given  off 
at  an  oxygen  pressure  of  20  mm. ;  with  5  mm.  Hg  pressure  only 
about  30  per  cent,  is  given  at  the  same  pressure  (fig.  202). 
With  a  low  C0„  tension  the  blood  may  pass  through  the  tissues 
and  the  respiratory  centre  and  remain  of  a  bright  red  colour 
because  it  has  not  parted  with  its  oxygen,  and  the  tissues  and 
the  centre  may  thus  have  an  inadequate  supply. 

A  due  proportion  of  CO  2  in  the  blood  is  therefore  of 
importance  in  securing  an  adequate  transference  of  Oo  to  the 
centre. 

When  oxygen  deficiency  is  sufficiently  marked,  the  re- 
spiratory centre  not  only  becomes  more  sensitive  to  the  stimu- 
lating action  of  CO..,  but  may  be  stimulated  quite  apart 
from  any  action  of  COo.  This  is  seen  in  the  oxygen- want 
experienced  in  high  aviation  and  in  re-breathing  air  from  which 
the  COo  is  removed  by  passing  over  soda  lime.  The  rate 
rather  than  the  depth  of  breathing  is  increased. 

This  increase  may  lead  to  an  increased  intake  of  oxygen 
although,  on  account  of  the  rapid  shallow  breathing,  it  does  not 
necessarily  do  so  (p.  526).  It  may  thus  fail  to  relieve  the 
condition,  because  at  the  same  time  the  CO.  is  driven  out 
of  the  blood  and  thus  the  giving  off  of  oxygen  to  the  respiratory 
centre  may  be  decreased. 

The  result  is  that,  under  the  continued  want  of  oxygen,  the 
respiratory  centre  may  fail,  the  breathing  stop,  and  death 
supervene,  as  is  seen  in  death  from  asphyxia  (p.  548). 

Fortunately,  the  decreased  supply  of  O2  to  the  muscles 
leads  to  a  failure  to  oxidise  the  sarcolactic  acid,  which  may 
accumulate  in  the  blood  and  by  causing  an  acidosis — an  increase 
in  the  Ch  of  the  blood — may  stimulate  the  respiratory  centre, 
and  at  the  same  time  facilitate  the  giving  off  of  Oo  to  the 
tissues. 

The  primary  increase  of  breathing  with  oxygen-want 
appears  to  be  due  to  a  direct  action  on  the  centre,  and  not 
as  was  formerly  supposed  to  the  development  of  an  acidosis.  It 
34 


530 


VETERINARY   PHYSIOLOGY 


occurs   under  conditions  when  such  an   acidosis   could   hardly 
develop. 

While  an  increase  of  free  CO.,  increases  the  depth,  and  also 
generally  the  rate  of  breathing,  and  thus  secures  a  more 
thorough  ventilation  of  all  parts  of  the  lung,  oxygen 
deficiency  seems  chiefly  to  cause  an  acceleration  of  rate.  In 
consequence  of  this  quicker,  more  shallow  breathing  only 
the  more  expansile  parts  of  the  lung  are  ventilated  (p.  523).  The 
result  is  that  a  greater  proportion  of  the  blood  passes  through 
parts    of    lung    imperfectly    ventilated,    from    the    alveoli    of 


Giving  off  of  COj  in  hyper- 
ventilated part  of  lung. 


Giving  off  of  COj  normal 
and  actual.  •  ^ 

Taking  up  of  O2  by  the  blood 
in  hyper- ventilated  part  of  lung. 
Taking    up  of  O2  normal.   .^ 


Taking    up    of   Oj  actual. 


Giving  off  of 
CO,  in  non- 
ventilated 
part  of  lung. 

Taking  up  of 
O3  in  same. 


Fig.  2lo. — To  show  effects  of  shallow  breathing  and  imperfect  ventilation 
uf  the  lungs  upon  the  taking  up  of  Og  and  giving  oft'  of  COo.  The 
shaded  part  of  the  square  represents  the  badly  ventilated  part. 

which  the  oxygen  gets  used  up  and  reduced  to  a  low  partial 
pressure.  If  the  pressure  falls  below  50  mm.  Hg  the  blood  will  be 
imperfectly  oxygenated.  The  rest  of  the  blood  passing  through 
the  well-ventilated  expansile  part  of  the  lung  cannot  take  up 
much  more  oxygen  than  it  does  at  a  pressure  of  a  little  over 
50  mm.  Hg  (p.  493),  so  that  any  rise  in  the  partial  pressure  as  the 
result  of  better  ventilation  produces  only  a  small  effect.  Thus 
imperfectly  oxygenated  blood  is  mixed  with  a  smaller  quantity 
of  normally  oxygenated  blood,  and  thus  the  total  blood  leaving 
the  lungs  carries  less  oxygen  than  normally  (fig.  213). 

On  the  other  hand,  since  the  C0„  tension  varies  directly 
with  the  partial  pressure,  the  decreased  giving  off  of  C0„  in 
the  badly  ventilated  parts  of  the  lung  may  be  compensated  for 


RESPIRATION  531 

by  the  increased  giving  off  in  the  well-ventilated  parts,  and 
thus  the  amount  of  COo  in  the  blood  may  not  be  raised,  and 
so  the  normal  stimulus  to  the  respiratory  centre  may  not  come 
into  play  (fig.  213). 

It  has  been  suggested  that  in  these  conditions  the  addition 
of  COo  to  the  air  breathed  may  have  an  even  more  beneficial 
effect  than  the  addition  of  ox3'gen.  Possibly  a  combination 
would  be  more  efficacious. 

There  is  some  evidence  that  a  sufficient  clearing  out  of 
COo  from  the  blood  by  forced  breathing  may  so  decrease  the 
Ch  of  the  blood,  may  produce  so  marked  an  alkalosis,  that  the 
HbOo  is  not  dissociated,  and  the  central  nervous  system  may 
be  so  imperfectly  supplied  with  oxygen  that  consciousness  may 
be  lost,   the    arterioles  may  contract  and  the  heart  fail  as  in 


Fig.  214.— To  show  the  Characters  of  Cheyne-Stokes  Breathing  and  the 
factors  producing  it  (see  text). 

asjjJiyxia.       This    has    been    termed    by   Yandell    Henderson 
acajynia. 

In  some  people  and  under  some  conditions  a  deficient 
supply  of  oxygen  to  the  respiratory  centre  leads  to  a  periodic 
type  of  breathing.  The  patient  stops  breathing  for  a  time 
(ajmcBa),  then  begins  to  breathe,  first  quietly,  then  more 
forcibly  Qiyperpnoea),  and,  after  several  respirations,  again  with 
decreasing  depth  till  the  respirations  stop.  In  these  cases,  the 
respiratory  centre  is  less  excitable  than  usual,  and  it  is  called 
into  action  only  when  CO 2  has  accumulated  in  the  blood. 
After  this  accumulation  has  been  got  rid  of  by  the  forcible 
respirations,  the  activity  of  the  centre  again  wanes.  Since  the 
forced  breathing  tends  to  produce  excessive  clearing  out  of 
COo,  it  may  lead  to  a  decreased  dissociation  of  HbOo  and  to  a 


532 


VETERINARY   PHYSIOLOGY 


limitation  of  the  free  supply  to  the  tissues  of  the  oxygen  which 
has  been  taken  up  by  the  blood.  This  has  been  called  Cheyne- 
Stokes  breathing  (fig.  214). 


2.  Reflex  Regulation- 

The  i'espiratory  centre  is  also  acted  upon  by  various  ingoing 
nerves. 

(1)  The  Vagus. — Since  the  vagus  is  the  ingoing  nerve  of  the 
respiratory  tract,  we  should  expect  it  to  have  an  important 
influence  on  the  centre  (fig.  215). 

Section  of  one  vagus  generally  causes  the  respirations  to 
become  slower  and  deeper ;  but,  after  a  time,  the  effect  wears 


Dyaph 


Fig.  215. — Nervous  Mechanism  of  Respiration.  B.C.,  respiratory  centre; 
Citt.,  cutaneous  nerves  ;  Ph.,  phrenics  ;  In.C,  intercostal  nerves  ;  P., 
pulmonarj-  branches  of  vagus  ;  S.L.,  superior  laryngeal  branch  of 
vagus;  La.,  the  larynx;  G.Ph.,  glossopharyngeal  nerve;  Diaph., 
diaphragm. 

off  and  the  previous  rate  and  depth  of  respiration  are  regained 
(fig.  216). 

Section  of  both  vagi  causes  a  very  marked  slowing  and 
deepening  of  the  respiration,  which  persists  for  some  time,  and 
passes  ofi"  slowly  and  incompletely.  But  if,  after  the  vagi 
have  been  cut,  the  connection  of  the  respiratory  centre  with 
the  wpjper  brain  tracts  is  severed,  the  mode  of  action  of  the 
centre  changes.  Instead  of  discharging  rhythmically,  it  may 
discharge  irregularly  (fig.  216,  c). 

To  investigate  further  this  influence  of  the  vagus  it  is  neces- 
sary to  study  the  effect  of  stimulating  the  nerve. 

Strong   stimulation   of    the   pulmonary    branches  of    one 


RESPIRATION  533 

vagus,  below  the  origiu  of  the  superior  laryngeal,  generally 
causes  the  respirations  to  become  more  rapid,  the  inspiratory 
phase  being  chiefly  accentuated.  Weak  stimuli,  on  the  other 
hand,  may  cause  inhibition  of  the  respirations. 

Such  experiments  prove  that  impulses  are  constantly  travel- 
ling from  the  lungs  to  the  centre  to  regulate  its  rhythmic 
activity. 

Positive  and  Negative  Ventilation,  i.e.  passively  inflating 
and  deflating  the  lungs,  shows  that  two  sets  of  fibres  come  into 
play  in  normal  respiration. 

If  the  lungs  be  forcibly  inflated,  the  inspirations  become 
feebler  and  finally  stop.     The  nature  of  the  gas,  if  non-irritant, 


a 

■V     L 


d  d' 

Fif4.  216.— Tracings  of  the  Respiratiou— Downstroke  is  inspiration  ;  Upstroke 
is  expiration.  At  a  one  vagus  nerve  was  cut ;  at  h  the  second  was 
divided  ;  at  c  the  upper  brain  tracts  also  were  cut  off;  d  and  d'  show 
the  effect  of  stimulating  the  glossopharyngeal  nerve. 

with  which  this  inflation  is  carried  out,  is  of  no  consequence. 
If,  on  the  other  hand,  air  is  sucked  out  of  the  lungs,  inspira- 
tions become  more  dominant,  and  may  end  in  a  spasm  of  the 
inspiratory  muscles. 

Inspiration  is  thus  checked  by  one  set  of  fibres  and  expira- 
tion by  another,  and  tlie  vagus  thus  regulates  the  action  of  the 
respiratory  nervous  mechanism,  much  as  the  pendulum  regu- 
lates the  action  of  a  clock.  Under  some  conditions,  e.g.  after 
gassing,  the  activity  of  this  reflex  may  be  increased  so  that 
inspiration  and  expiration  are  checked  too  soon,  and  the  breath- 
ing may  thus  be  made  shallow  and  quick. 

(2)  Other  Ingoing  Nerves. — (a)  Section  of  the  superior  laryn- 


534  VETERINARY   PHYSIOLOGY 

geal  branch  of  the  vagus,  the  sensoiy  nerve  of  the  larynx,  does 
not  alter  the  rhythm  of  respiration.  Stimulation  of  the  upper 
end  of  the  cut  nerve  causes  first  an  inhibition  of  inspiration, 
and,  if  stronger,  produces  forced  expiratory  acts.  This  is  well 
illustrated  by  the  very  common  experience  of  the  effect  of  a 
foreign  body,  such  as  a  crumb,  in  the  larynx.  The  fit  of  cough- 
ing that  ensues  is  a  series  of  expiratory  acts  reflexly  produced 
through  this  nerve. 

(b)  When  the  splanchnics  in  the  abdomen  are  stimulated, 
inspiration  is  inhibited.  Every  one  has  experienced  the  "  loss 
of  wind  "  as  the  result  of  a  blow  on  the  abdomen. 

(c)  The  glossopharyngeal,  which  supplies  the  back  of  the 
tongue,  when  stimulated,  as  by  the  passage  of  food  in  the  act 
of  swallowing,  causes  an  instant  arrest  of  the  respiratory  move- 
ments either  in  inspiration  or  expiration.  The  advantage  of  this 
in  preventing  the  food,  as  it  is  swallowed,  from  passing  into  the 
trachea  is  obvious  (fig.  216,  d  and  d'). 

(d)  Stimulation  of  the  cutaneous  nerves  stimulates  the  in- 
spiratory centre  and  causes  a  deep  inspiration.  This  is  seen 
when  cold  water  is  dashed  upon  the  skin.  Stimulation  of  the 
skin  by  slapping  is  sometimes  used  to  establish  breathing  in  the 
newly  born  infant.  The  reaction  is  most  clearly  demonstrated  in 
animals  with  the  vagi  cut  when  an  inspiratory  movement  may 
often  be  liberated  by  merely  touching  the  skin. 

3.  Influence  of  Temperature- — The  temperature  of  an  animal 
also  acts  on  the  respiratory  centre.  Increase  in  temperature 
accelerates  the  rate  of  the  heart  and  it  also  accelerates  the  rate 
of  the  respirations  in  about  the  same  proportion.  This  is  seen 
in  feverish  attacks,  where  pulse  and  respirations  are  proportion- 
ately quickened  so  that  their  ratio  remains  unaltered.  When 
the  respiratory  rate  rises  out  of  proportion  to  the  rate  of  the 
pulse  it  is  usualh'  an  indication  that  some  pulmonary  irritation 
is  present. 

4.  Postural  Control. — In  diving  birds  the  respirations  are 
controlled  by  a  postural  reflex  from  the  labyrinths  and  neck. 
When  the  head  is  put  in  the  diving  position  respirations  are 
stopped.  The  respiratory  centre  in  these  birds  does  not  respond 
to  the  Ch  of  the  blood,  but  rather  to  the  want  of  oxygen. 


RESPIRATION  535 

III.  Interaction  of  Circulation  and  Respiration. 

The  lungs  aud  heart,  being  packed  tightly  together  in  the 
air-tight  thorax,  and  both  undergoing  periodic  changes,  neces- 
sarily influence  one  another.  At  the  same  time,  the  close 
proximity  of  the  respiratory  and  cardiac  centres  in  the 
medulla  seems  to  lead  to  the  activity  of  one  influencing  the 
other. 

A.  Influence  of  Respiration  on  Circulation. — The  circulation 
is  modified  in  two  ways  by  respiration.  First,  the  pulse,  and 
second,  the  arterial  blood  pressure  undergo  alterations. 

l.sf.  Pulse. — (a)  Rate. — If  a  sphygmographic  trace,  giving 
the  pulse  waves  during  the  course  of  tw^o  or  three  respirations, 
be  examined,  it  Avill  be  found  that  during  inspiration  the  heart 
is  acting  more  rapidly,  while  during  expiration  its  action  is 
slower. 

If  the  vagus  be  cut,  these  changes  are  not  seen,  showing 
that  the  inspiratory  acceleration  is  not  the  result  simply  of 
the  larger  amount  of  blood  which  enters  the  heart  during 
inspiration,  but  is  really  due  to  changes  in  the  cardio-motor 
centre — the  accelerating  part  of  which  has  its  activity  in- 
creased during  inspiration,  while  the  inhibitory  part  is  more 
active  during  expiration.  This  is  therefore  partly  a  reflex  effect 
from  the  lung  through  the  vagus,  although  it  may  be  in  part 
due  to  the  proximity  of  the  centres  in  the  medulla. 

(b)  Volume. — Not  only  is  the  rate  of  the  pulse  altered  by 
respiration,  but  the  waves  are  smaller  during  inspiration  and 
larger  during  expiration.  This  is  simply  due  to  there  being 
more  time  for  diastolic  filling  when  the  heart  is  beating  more 
slowly. 

•2rtd.  Blood  Pressure.— J..  If,  in  an  anaesthetised  animal 
tracings  of  the  arterial  pressure  and  of  the  respiratory  movements 
are  taken  at  the  same  time,  it  is  found  that  there  is  a  general 
rise  of  pressure  during  inspiration  and  a  general  fall  during 
expiration,  but  that  at  the  beginning  of  inspiration  the  pressure 
is  still  falling,  and  at  the  beginning  of  expiration  it  is  still  ri.sing. 
This  influence  of  respiration  on  arterial  pressure  is  chiefly  a 
mechanical  one,  depending  on  the  variations  in  the  pressure  in 
the  pericardium,  which  is  decreased  during  inspiration,  allowing 


)36 


VETERINARY   PHYSIOLOGY 


greater  diastolic   filling.      By  allowing  access   of  air  into   the 
pericardial  sac  the  differences  are  abolished. 

B.  In  man,  and  this  is  probably  also  the  case  in  the 
unansesthetised  animal,  in  thoracic  breathing  the  arterial 
pressure  falls  during  inspiration  and  rises  during  expiration, 
on  account  of  the  retention  of  blood  in  the  distended 
thorax  in  inspiration  and  its  expulsion  in  expiration,  and 
that,  in  abdominal  breathing,  the  reverse  is  the  case ; 
inspiration,  by  pressing  on  the  abdominal  vessels  and 
sending  more  blood  on  into  the  arteries,  increasing  the 
pressure. 

B.  Influence  of  the  Action  of  the  Heart  on  Respiration. — The 

heart  lies  in  the  thorax  sur- 
rounded by  the  elastic  lungs. 
As  it  contracts  and  dilates  it 
must  alternately  pull  upon  and 
compress  the  lungs,  and  thus 
tend  to  cause  an  inrush  and  an 
outrush  of  air — the  cardio-pneu- 
matic  movements. 

If  a  simultaneous  tracing 
of  the  heart-beat  and  of  the 
movements  of  the  air  column  be 
taken,  it  will  be  seen  that  (1) 
at  the  beginning  of  ventricular 
systole  there  is  a  slight  outrush 
of  air  from  the  lungs,  probably 
caused  by  the  blow  given  to  the 
lungs  by  the  suddenness  of  the  systolic  movement.  (2)  This  is 
followed  by  a  marked  inrush  of  air  corresponding  to  the  out- 
flow of  blood  from  the  ventricles,  and  caused  by  the  fact  that 
the  contracting  ventricles  draw  on  and  expand  the  lungs. 
(3)  Succeeding  this  is  a  slower  outrush  of  air  corresponding  to 
the  active  filling  of  the  ventricles  during  the  beginning  of 
ventricular  diastole.  (4)  Lastly,  during  the  period  of  passive 
diastole,  the  cardio-pneumatic  movements  of  air  are  in 
abeyance  (fig.  217). 

These  cardio-pneumatic  movements  are  of  importance  in 
two   ways.     (1)  In   animals   during    hibernation,   the    ordinary 


Fig.  217.  — To  show  Relations  of 
Cardio-pneumatic  Movements, 
A.,  to  the  Cardiac  Cycle,  B. 
In  A.  the  upstrokes  are  expira- 
tory, the  downstrokes  inspira- 
tory. 


RESPIRATION 


>37 


respirations  almost  stop,  but   a  sufficient  gaseous  interchange 
is  kept  up  by  these  cardio-pneumatic  movements. 

(2)  If,  as  is  often  the  case  in  bronchitis,  there  is  a  plug  of 
mucus  in  a  small  bronchus  near  the  heart,  the  rush  of  air  past 
it  may  give  rise  to  a  murmuring  sound,  in  character  very  like 
a  cardiac  murmur  and  synchronous  with  the  heart's  action. 


IV.  Interchange  between  the  Air  breathed  and  the  Blood 
in  the  Lung  Capillaries. 

I.  Effects  of  Respiration  upon  the  Air  breathed. — 1.  Method 


Air 

PerCe  NTT 


PerCent 
OF  Gases 


Inspired 

Expired 

Hi 

N19          0^21 

Venous 

N19        ai6 

Arterial 

0.22 

0.3E 

N1-2 

co,u 

N1-2 

CO.  6(9 

4 


F  iG.  218. — .4.  Shows  the  Composition  of  Inspired  and  Expired  Air.  B. 
Shows  the  Difference  in  the  percentage  Composition  of  the  Gas  of 
Venous  and  Arterial  Blood. 


of  Investigation. — A  measured  quantity  of  air  is  collected  in 
a  graduated  burette.  It  is  then  forced  into  a  chamber  contain- 
ing caustic  potash,  by  which  the  COg  is  absorbed,  and  the 
volume  of  air  is  again  measured.  It  is  next  forced  into  a 
chamber  containing  sodium  pyrogallate  in  caustic  soda,  which 
absorbs  the  0„,  and  is  again  measured.  The  residue  is 
nitrogen.  In  this  way  the  amount  of  the  gases  present  is 
determined  {Practical  Physiology). 


538  VETERINARY   PHYSIOLOGY 

2.  Results. — (1)  Gases — The  following  table  shows  the  average 
percentage  composition  of  the  air  inspired  and  the  air  expired 
(fig.  218):- 


Percent,  of 

N. 

o„. 

CO, 

[nspired  air 

79 

21 

0 

Expired  air 

SO 

16 

4 

i.e.  about  5  per  cent,  of  oxygen  is  taken  from  the  air,  and 
about  4  per  cent,  of  carbon  dioxide  is  added  to  it.  In  man 
the  amount  of  carbon  dioxide  given  off  is  smaller  than  the 
amount  of  oxygen  taken  up,  and  hence,  as  already  explained 

(p.  258),  the  Respiratory  Quotient^^^^^^^  is  generally  less 

than  unity — usually  about  0'8  to  0'9 — and  the  percentage  of 
nitrogen  in  expired  air  is  increased. 

(2)  Expired  air  is  saturated  with  watery  vapour,  and  there- 
fore it  usually  contains  more  water  than  inspired  air. 

(3)  Expired  air  also  contains  small  amounts  of  organic 
riiatter,  which  may  give  it  an  offensive  odour.  These  are  not 
derived  from  the  lungs,  but  are  produced  by  putrefactive 
changes  in  the  mouth  and  nose.  The  injurious  effects  of  the 
"foul  air"  in  overcrowded  spaces  are  chiefly  due  to  bad 
ventilation  with  imperfect  movements  of  the  air,  which  result 
in  an  increased  humidity  and  a  decreased  elimination  of  heat, 
and  at  the  same  time  to  the  accumulation  of  the  volatile 
products  from  dirty  skins. 

(4)  Expired  air  is  usually  warmer  than  inspired  air,  because 
usually  the  body  is  warmer  than  the  surrounding  atmosphere. 
When,  however,  the  temperature  of  the  air  is  higher  than  that 
of  the  body,  the  expired  air  is  cooler  than  the  inspired. 

II.  Effects  of  Respiration  on  the  Blood- — To  understand  these 
changes  in  the  air,  we  must  refer  to  the  changes  in  the  gases 
of  the  blood  in  passing  through  the  lungs.  These  have  already 
been  partly  considered  when  dealing  with  the  gases  of  the 
blood  (p.  492).  Analyses  show  that  the  blood  going  to  the 
lungs  is  poorer  in  oxygen  and  richer  in  carbon  dioxide  than 
the  blood  coming  from  the  lungs  (fig.  218). 

The  following  table  gives  not  the  percentage  composition  of 


RESPIRATION  539 

the  gas  extracted  from  blood,  but  the  amount  of  each  gas  per 
100  parts  of  blood  : — 

Amount   in    100  /lajV*'   of  Blood   {Human). 


CO,. 

0,. 

Venous    . 

.     55 

15 

Arterial  . 

.     50 

20 

Oxygen  is  taken  by  the  blood  from  the  air,  carbon  dioxide 
is  given  by  the  blood  to  tJte  air. 

III.  The  Causes  of  the  Respiratory  Exchange- — How  is  this 
effected  ?  The  extensive  capillary  network  in  the  walls  of  the 
air  vesicles  in  man,  if  spread  out  in  a  continuous  sheet,  would 
present  a  surface  of  about  75  square  metres.  Between  the  blood 
in  the  capillaries  and  the  air  in  the  air  vesicles  are  two  layers  of 
living  cells — 

1st.  The  endothelium  lining  the  capillaries. 

2nd.  The  flattened  cells  lining  the  air  vesicles. 

Through  these  cells  the  interchange  of  gases  must  take 
place. 

The  interchange  might  take  place  in  either  or  both  of  two 
ways — 

1st.  By  simple  diffusion. 

2nd.  By  some  special  action  of  the  cells. 

If  the  process  follows  strictly  the  laws  of  diffusion,  it  is 
unnecessary  to  invoke  the  activity  of  the  cells  as  playing 
a  part.  But,  if  the  gaseous  interchange  does  not  strictly 
follow  these  laws,  we  must  conclude  that  the  cells  do  play 
a  part. 

Diffusion  tal:es  place  from  the  point  of  higher  partial 
pressure  to  the  point  of  lower  pressure  till  equilibrium  is 
established. 

To  determine  if  the  process  can  be  accounted  for  by 
diffusion,  it  is  therefore  necessary  to  know — 

1.  The  partial  pressure  of  the  gases  in  the  air  in  the  vesicles 
of  the  lungs. 

2.  The  partial  pressure  or  tension  of  the  gases  in  the  blood 
going  to  and  coming  from  the  lungs. 

1.  Partial  Pressure  of  Gases  in  the  Air  Vesicles.— The  method 


540  VETERINARY    PHYSIOLOGY 

of  determining  the  partial  pressure  or  tension  of  a  gas  in  the 
atmosphere  has  been  described  on  p.  492,  and  it  has  been  shown 
that  at  sea-level,  with  an  atmospheric  pressure  of  760  mm.  Hg, 
the  tension  of  oxygen  is  about  152  mm.  Hg. 

The  air  in  the  vesicles  or  alveoli  is  renewed  partly  by 
direct  ventilation  from  without,  and  partly  by  a  process  of 
diffusion  (p.  492).  For  this  reason  the  amount  of  oxygen  in 
the  vesicles  must  be  smaller,  the  amount  of  carbon  dioxide 
larger,  than  in  the  air  respired. 

Haldane  has  devised  a  method  of  procuring  samples  of  the 
alveolar  air  for  analysis.  A  wide  tube  is  fitted  with  a  measured 
glass  bulb  near  one  end,  and  this  bulb  is  made  a  vacuum.     The 


Mouthpiece 


SsLmpling  Tube 


Fig.  219. — Haldane's  Apparatus  for  Determining  the  Composition  of 
Alveolar  Air. 

end  of  the  tube  near  the  bulb  is  put  in  the  mouth  or  fitted 
to  a  mask,  and  the  person  under  observation  breathes  through 
it.  At  the  end  of  an  ordinary  inspiration  he  expires  deeply 
through  the  tube,  closes  the  mouthpiece  with  his  tongue, 
and  by  opening  the  upper  stop-cock  collects  a  sample  of 
the  expired  air.  A  second  sample  is  taken  in  the  same  way 
at  the  end  of  a  normal  expiration.  The  mean  of  these 
samples  represents  the  average  composition  of  the  alveolar 
air  (fig.  219). 

By  the  use  of  this  method,  it  has  been  found  that  the 
partial  pressure  of  the  0„  varies  within  wide  limits,  while  tlie 
partial  pressure  of  the  CO  3  remains  very  constant. 

Thus,  at  the  top  of  Ben  Nevis  the  tension  of  oxygen  in 
the  air  vesicles  was  76  mm.  Hg,  at  the  bottom  of  a  mine 
it  was  111  mm.;  while  in  both  jolaces    the  tension  of  carbon 


RESPIRATION  541 

dioxide  was  about  42  mm.,  the  amount  varying  from  about  4 
to  5'5  per  ceut. 

At  sea-level  the  partial  pressure  or  tension  in  the  alveoli 
may  be  taken  as  about — 

O,  =  100  mxn.  Hg. 
00^=42  mm.  Hg. 

From  the  great  inequality  in  the  expansion  of  the  lungs  in 
different  parts  of  the  thorax  and  the  resulting  differences  in  the 
ventilation  of  the  air  vesicles,  the  samples  of  alveolar  air  taken 
by  Haldane's  method  will  tend  to  give  too  low  a  carbon  dioxide 
and  too  high  an  oxygen  figure,  since  the  sample  is  chiefly  derived 
from  the  better  ventilated  part  of  the  lung. 

This  means  that  in  the  less  expanding  parts  of  the  lung  the 
blood  is  subjected  to  a  higher  partial  pressure  of  CO.-,,  and  a 
lower  partial  pressure  of  0„. 

2,  The  Partial  Pressure  or  Tension  of  the  Gases  in  the  Blood. — 
The  tension  of  Oo  and  C0„  in  the  blood  has  been  already 
considered  (p.  492).  Whether  a  gas  is  simply  dissolved,  or 
whether  it  be  held  in  loose  chemical  combination,  its  amount 
will  depend  upon  the  temperature  of  the  fluid  and  upon  the 
'pressure  of  the  gas  over  the  fluid.  If  the  temperature  is 
raised,  the  fluid  will  hoM  less  of  the  gas. 

If  the  pressure  of  a  gas  over  a  fluid  is  increased,  some  will 
be  taken  up  by  the  fluid  ;  if  it  be  decreased,  the  gas  will  tend 
to  come  off  from  the  fluid,  as  occurs  when  a  bottle  of  soda 
water  is  opened. 

Thus,  for  every  temperature,  there  is  a  certain  pressure  of 
the  gas  in  the  atmosphere  at  which  the  solution  or  chemical 
combination  exposed  to  it  will  neither  give  off  nor  take  up 
more  of  the  gas,  and  this  gives  the  measure  of  the  tension  of 
the  gas  in  the  fluid. 

Theoretically,  the  determination  of  the  tension  of  a  gas  in 
a  fluid  is  simple  (p.  492).  But  when  it  has  to  be  carried  out  in 
circulating  blood  it  becomes  extremely  difficult.  It  must  be 
carried  out  without  marked  disturbance  of  the  circulation, 
and  a  thorough  exposure  of  the  air  to  the  blood  must  be 
secured.      The  trouble  of  clotting  has  also  to  be  faced. 

The  best  results  have  been  obtained  by  the  aerotonometer 
of  Krogh,  in  which  a  bubble  of  air  is  exposed  to  the  blood. 


542 


VETERINARY   PHYSIOLOGY 


and,  after  equilibrium  has  been  established,  is  withdrawn  and 
analysed.      Its  volume  is  measured  in  a  line  graduated  tube. 

It  is  then  forced  through 
caustic  soda  solution  till 
all  the  CO2  is  absorbed, 
again  drawn  into  the 
tube  and  measured,  the 
decrease  in  volume  giv- 
ing the  amount  of  CO2. 
Xext,  it  is  passed  through 
a  solution  of  sodium 
pyrogallate  in  caustic 
soda    to    absorb    the    Og 

____>    and   is    again  measured. 

^,  The  residual  gas  is  nitro- 

:20.-Kroglvs  Microtonometer.  B.,  g^U,  a  _  small  amount  of 
chamber  attached  to  blood-vessel  whicll  is  always  dissolved 
with  bubble  of  air  ;  -1  fine  graduated  J^  the  blood.  From  the 
tube    in    water   jacket  tor  analysis   of 

gases.  "  percentage  of  these  gases 

in  the  bubble  their  ten- 
sion in  the  circulating  blood  in  which  the  bubble  of  air  lay 
is  calculated.      The  apparatus  is  shown  in  fig.  220. 

It  has  been  found  that  the  tension  of  00^  in  arterial  blood 
is  identical  with  that  in  the  air  in  the  vesicles  of  the  lungs. 
When  the  amount  in  the  air  is  altered,  the  tension  in  the  blood 
follows  the  variation. 

The  Og  tension  in  venous  blood  is  in  all  cases  lower  (by  1  to 
4  per  cent.)  than  that  in  the  air  of  the  lung  vesicles,  and  it  also 
follows  any  alteration  in  the  latter. 

Krogh  has  also  shown  that,  by  modifying  the  air  breathed, 
0„  may  be  made  to  come  off  from  the  blood,  and  COo  to 
be  taken  up  by  the  blood. 

The  difference  in  the  pressure  of  these  gases  in  the  alveolar 
air  and  in  the  blood  may  be  represented  as  follows  in  mm.  Hg  : — 

Oxygen.  Carbon  Dioxide. 


Alveolar  Air     . 
Blood  from  Lungs 


100 
96 


RESPIRATION  543 

The  exchange  of  gases  between  the  air  of  the  lungs  and  the 
blood  may  therefore  be  explained  by  simple  diffusion. 

So  perfect  is  the  exchange  that  the  tension  of  a  gas  in  the 
air  of  the  alveoli  may  be  taken  as  a  measure  of  the  tension  in 
the  blood  flowing  through  the  lungs.  Brodie  has  suggested 
that  a  dead  lung,  through  the  vessels  of  which  the  blood  is 
allowed  to  flow,  might  be  used  a  tonometer. 

Haldane  maintains  that  at  low  partial  pressures  of  oxygen, 
the  passage  of  the  gas  from  the  alveoli  to  the  blood  cannot 
be  explained  by  diffusion  and  that  it  must  be  due  to  some 
as  yet  unknown  factor. 

Certainly  the  accumulation  of  gas  in  the  swim  bladder  of 
fishes  cannot  be  explained  by  the  laws  of  diffusion  of  gases,  and 
it  seems  to  be  dependent  on  the  activity  of  the  cells  lining  tlie 
bladder.  It  may  be  arrested  by  section  of  the  nerves  supplying 
the  bladder. 

A.  The  Effects  of  Decreased  Almosplieric  Pressure. — The  fact 
that  the  hEemoglobin  in  the  blood  is  so  nearly  fully  oxygenated 
at  a  pressure  of  only  50  mm.  Hg  (p.  493),  explains  why  the 
pressure  of  this  gas  in  the  atmosphere  may  fall  to  about 
one-half  of  its  normal  152  mm.  Hg  without  interfering  with 
the  supply  of  oxygen  to  the  blood,  why  men  and  animals  can 
live  at  high  altitudes,  and  why  aviation  to  such  enormous 
heights  is  possible.  The  record  height  is  probably  30,500  feet, 
or  about  10,000  metres. 

The  following  table  shows  the  relationship  of  the  height, 
partial  pressure  of  oxygen  in  the  alveolar  air,  and  the  per- 
centage saturation  of  the  haemoglobin  with  oxygen,  and  it  shows 
that  at  about  5000  metres  (16,000  feet)  the  marked  decrease  in 
the  oxygen  carrying  capacity  of  the  blood  begins  (consult  p.  545). 

When  an  animal  is  suddenly  subjected  to  a  very  marked 
decrease  of  pressure,  especially  if  it  has  to  do  muscular  work, 
as  in  climbing,  the  decreased  supply  of  oxygen  leads  to  shortness 
of  breath,  palpitation,  and  even  to  sickness  (mountain  sickness). 
These  symptoms  generally  pass  off,  increased  pulmonary  venti- 
lation and  increased  heart's  action  augmenting  the  intake  of 
oxygen.  Hence,  residence  in  high  altitudes  tends  to  increase 
the  power  of  the  respiratory  muscles  and  the  strength  of  the 
heart.  It  also  increases  the  richness  of  the  blood  in  erythro- 
cytes and  in  haemoglobin. 


544 


VETERINARY   PHYSIOLOGY 


If  a  markedly  deficient  supply  of  oxygen  is  long 
continued,  permanent  damage  may  be  done  to  many 
important  structures,  such  as  the  heart  and  the 
central  nervous  system,  and  the  respiratory  centre  may 
be  so  seriously  modified  that  it  may  fail  to  act.  The  early 
implication  of  the  higher  centres  in  the  brain  may  prevent  the 


level          1 

2 

3 

4 

6 

7 

800 

"•^.^ 

/ 

"x 

" 

62          / 

85 
54 

^0 

48 

76 

41 

i  / 

N 

/ 

V 

/ 

^^ 

/ 

^ 

Fig.  221.— To  show  the  effects  of  altitudes  from  sea-level  to  SLKJO  metres 
upon  the  pressure  of  oxygen  in  the  alveoli  of  the  lungs  —  —  — 
and  the  saturation  of  the  blood  with  oxygen  

individual  from  noticing  the  onset  of  the  symptoms  till 
consciousness  is  lost.  It  is  therefore  most  important  to 
administer  oxygen  as  early  as  possible  in  such  cases. 

B.  The  Effects  of  Increased  Atmospheric  Pressure- — On  the  other 
hand,  the  atmospheric  pressure  may  be  enormously  increased 
without  any  change  in  the  respirations  being  produced.  The 
haemoglobin  will  not  take  up  more  than  a  definite  amount  of 
oxygen,  and  any  increase  is  due  to  the  gas  dissolved  in  the  plasma. 


RESPIRATION  545 

In  a  diving  bell,  200  feet  under  water,  a  pressure  of  seven 
atmospheres — 5120  mm.  Hg — is  sustained.  As  a  result  of 
the  high  pressure  of  the  gases  of  such  an  atmosphere,  they  are 
dissolved  in  large  quantities  in  the  blood  and  tissues,  and  there 
is  great  danger  in  a  too  sudden  relief  of  pressure,  since  this  may 
cause  bubbles  of  gas  to  be  given  off  in  the  vessels,  and  these 
may  lead  to  air  embolism  and  a  plugging  of  the  smaller  vessels 
(Caisson  disease)- 


B.  INTERMEDIATE   RESPIRATION. 

1.  The  Carriage  of  Gases  in  the  Blood  has  been  already 
considered  (p.  492). 

2.  The  Passage  of  Gases  between  Blood  and  Tissues- 

1.  Oxygen. — In  studying  the  metabolism  of  muscle  (p.  254), 
which  may  be  taken  as  a  type  of  all  the  active  tissues,  it  was 
seen  that  oxygen  is  constantly  being  used  by  the  muscle.  The 
living  tissues  have  such  an  affinity  for  oxygen  that  they  can 
split  it  off  from  such  pigments  as  alizarin  blue.  The  tension  of 
oxygen  in  muscle  is  therefore  always  very  low.  We  have  seen 
that  the  tension  of  oxygen  in  arterial  blood  is  nearly  100  mm. 
Hg,  and  that,  below  a  pressure  of  50  mm.,  the  oxygen  is  rapidly 
given  off,  till  at  10  or  20  mm.  Hg  the  oxyhsemoglobin  is 
largely  reduced.  The  influence  of  the  €„  of  the  blood  and  of 
temperature  upon  the  process  has  already  been  considered 
(p.  495).  Hence,  when  the  blood  is  exposed  to  a  low  tension 
of  oxygen  in  the  capillaries,  the  oxygen  comes  off  from 
the  blood  and  passes  into  the  tissues  by  the  ordinary  laws  of 
diffusion. 

The  process  takes  place  in  three  stages.  The  "  head  of 
oxygen,"  as  it  may  be  called  in  arterial  blood,  i.e.  the  difference 
of  tension  between  that  of  the  HbOj  and  that  of  the  tissues 
is,  in  normal  conditions,  far  in  excess  of  the  requirements. 
The  haemoglobin  is  generally  saturated  to  about  95  per  cent., 
but  in  various  pathological  conditions  of  the  lungs,  e.g.  pneu- 
monia, it  may  fall  to  as  low  as  50  per  cent,  saturation  and  still 
the  same  amount  of  oxygen  per  unit  of  volume  of  blood  may  be 
given  off  to  the  tissues,  the  saturation  of  the  venous  blood 
falling  proportionately,  i.e.  the  difference  between  them  remain- 
ing at  about  5  per  cent.  (fig.  222). 
35 


546 


VETERINARY   PHYSIOLOGY 


The   tissues   must   be   able   to   take  up  the  oxygen   they 
require  from  a  wide  range  of  pressure  in  the  blood. 


CYAN  05 15 

0 

+H- 

+ 

4> 

^ 

^ 

nX^ 

-- 

4 

y 

J 

< 

f 

A' 

f 

/ 

/ 

PER  CENT  10         20       20        40         50        60 

JO       60       9 

Fig.  222. — To  show  the  relationship  between  the  .saturation  or  unsaturation 
with  oxygen  of  arterial  and  venous  blood,  with  variations  in  the 
unsaturation  of  arterial  blood  from  5  to  50  per  cent.,  in  cases  of 
pneumonia,  with  different  degrees  of  cyanosis.     (Stadie.) 

Such  results  cannot  fail  to  raise  the  question  of  the 
beneficial  effects  of  administering  oxygen  in  pneumonia  in  order 
to  increase  the  supply  to  the  tissues. 

When  a  stagnation  of  blood  in  the  capillaries  occurs,  it  is 
very  probable  that  the  removal  of  oxygen  is  so  complete  that 
a  true  oxygen  starvation  of  the  tissues  exists. 

(1)  The  tissue  elements  are  always  taking  up  oxygen  from 
the  tissue  fluids,  because  of  the  very  low  tension  of  oxygen  in 
the  protoplasm. 

(2)  As  a  result  of  this,  the  oxygen  pressure  in  the  fluids 
falls  and  becomes  lower  than  the  oxygen  pressure  of  the  blood 
plasma,  and  thus  the  gas  passes  from  the  blood,  through  the 
capillary  walls,  to  the  fluids. 

(3)  As  a  result  of  the  withdrawal  of  oxygen  from  the  plasma, 
the  partial  pressure  round  the  erythrocytes  is  diminished, 
a  dissociation  of  oxyhsemoglobin  takes  place,  and  the  oxygen 
passes  out  into  the  plasma,  leaving  some  of  the  ha?moglobin  in 
the  erythrocytes  in  a  reduced  condition. 

2.  Carbon  Dioxide. — The  tissues  are  constantly  producing 
carbon  dioxide,  so  that  it  is  at  a  high  tension  in  them — about 
60  mm.  Hg.  In  the  blood,  as  already  indicated,  it  is  partly 
dissolved  and  partly  combined  with  sodium  as  the  bicarbonate. 
Possibly  it  is  partly  combined  with  the  proteins  of  the  plasma, 
and  probably  in  part  with  the  globin  of  haemoglobin  (p.  496).     It 


RESPIRATION  547 

is  at  a  tension  of  a  little  over  40  mm.  Hg  in  venous  blood. 
Hence  there  is  a  constant  passage  of  carbon  dioxide  from  the 
tissues  to  the  blood.  The  amount  of  C0„  Avhich  the  blood 
carries  from  the  tissues  to  the  lungs  to  be  eliminated  is  a  very 
small  proportion  of  the  total  amount  carried — only  between 
8  and  9  per  cent,  of  the  whole.  Possibly  this  amount  may  be 
carried  by  the  hasmoglobin  (p.  496). 

C.   INTERNAL  RESPIRATION. 

This  has  been  already  considered  under  muscle  (p.  254 
et  seq.). 

The  rate  of  internal  respiration  depends  upon  the  activity 
of  the  tissues,  and  not  upon  the  amount  of  oxygen  in  the  blood. 
It  has  been  shown  that,  when  a  tissue  is  stimulated,  the 
increased  activity  precedes  the  increased  taking  up  of  oxygen 
and  giving  off  of  carbon  dioxide,  thus  confirming  the  view  that 
the  evolution  of  energy  is  not  due  to  a  direct  oxidation 
(p.  249  et  seq.). 

When  the  oxidation  processes  in  the  tissues  are  decreased 
as  in  poisoning  with  cyanides,  the  oxygen  tension  in  the 
tissues  is  not  lowered  and  COo  is  not  evolved.  Under 
these  conditions  the  sarcolactic  acid  liberated  passes  into 
the  blood  and  increases  the  0^.  But  without  the  low 
tension  in  the  tissues  the  partial  pressure  of  the  oxygen  in  the 
blood  plasma  remains  so  high  that  dissociation  of  HbOg  does 
not  take  place  and  bright  red  blood  passes  on  to  the  veins. 


D.  EXTENT  OF  RESPIRATORY  EXCHANGE. 

This  has  been  studied  under  metabolism  of  muscle  (p.  254 
et  seq.). 

The  extent  of  the  respiratory  interchange  in  the  lungs  is 
governed  by  the  extent  of  the  internal  respiratory  changes, 
i.e.  by  the  activity  of  the  tissues  and  chiefly  of  muscle. 
Merely  increasing  the  number  or  depth  of  the  respirations  has 
only  a  transient  influence  on  the  amount  of  the  respiratory 
interchanges.  Every  factor  which  increases  the  activity  of  the 
metabolic  changes  in  the  tissues  increases  the  intake  of  oxygen 
and  the  output  of  carbon  dioxide  by  the  lungs. 


548  VETERINARY   PHYSIOLOGY 


E.  VENTILATION. 

The  rate  of  gaseous  exchange  governs  the  necessary  supply 
of  fresh  air.  The  subject  is  considered  under  Stable  Manage- 
ment. In  byres,  some  800  cubic  feet  are  generally  allowed  per 
cow. 

The  bad  effects  of  breathing  in  a  crowded,  close,  badly- 
ventilated  space  is  dealt  with  on  p.  538. 


F.  ASPHYXIA. 

This  is  the  condition  caused  by  any  interference  with  the 
supply  of  oxygen  to  the  blood  and  to  the  tissues,  (a)  It  may  be 
induced  rapidly  and  in  an  acute  form  by  preventing  the  entrance 
of  air  to  the  lungs,  as  in  drowning  or  suflfocation,  or  by  causing 
the  animal  to  breathe  air  deprived  of  oxygen,  or  by  interfering 
with  the  flow  of  blood  through  the  lungs,  or  with  the  oxygen- 
carrying  capacity  of  the  blood,  as  in  CO  poisoning,  or  with  the 
processes  of  oxidation  in  the  tissues,  as  in  poisoning  with 
cyanides.  (6)  It  is  slowly  induced,  in  a  less  acute  form,  when 
the  muscles  of  respiration  fail  as  death  approaches. 

In  acute  asphyxia  there  is  (1)  an  initial  stage  of  increased 
respiratory  effort  due  to  the  accumulation  of  C0„,  the  breath- 
ing becoming  panting,  and  the  expirations  more  and  more 
forced.  The  pupils  are  contracted,  and  the  heart  beats  more 
slowly  and  more  forcibly,  while  the  arterioles  are  strongly 
contracted,  and  a  rise  in  the  arterial  pressure  is  generally 
produced.  In  some  animals  this  is  very  transitory.  When 
the  vagi  are  cut,  the  slowing  of  the  heart  does  not  occur, 
and  the  rise  of  blood  pressure  may  be  more  marked.  (2) 
Usually  within  a  couple  of  minutes,  a  general  convulsion, 
involving  chiefly  the  muscles  of  expiration,  occurs.  The 
intestinal  muscles  and  the  muscles  of  the  bladder  may  be 
stimulated,  and  faeces  and  urine  may  be  passed  involuntarily. 
(3)  Then,  as  the  result  of  oxygen  want,  the  respirations  stop, 
deep  gasping  inspirations  occurring  at  longer  and  longer 
intervals.  The  pupils  are  dilated,  and  consciousness  is 
abolished.     The  heart  fails,  and  thus,  although  the  arterioles 


RESPmATION  549 

are  still  contracted,  the  pressure  in  the  arteries  falls.  (4) 
Finally,  the  movements  of  the  heart  cease  and  death 
supervenes. 

Before  the  heart  has  stopped,  recovery  may  be  brought  about 
by  artificial  respiration,  which  may  be  performed  by  slow 
rhythmic  compressions  of  the  thorax  and  abdomen  at  a  rate 
not  exceeding  the  normal  rate  of  breathing  in  the  animal  or 
by  connecting  the  trachea  to  a  respiration  pump. 


VOICE. 

In  connection  with  the  respiratory  mechanism  of  many 
animals,  an  arrangement  for  the  production  of  sound  or  voice 
is  developed.  This  is  constructed  on  the  principle  of  a  wind 
instrumeat,  and  it  consists  of  (1)  a  bellows,  (2)  a  windpipe, 
(3)  a  vibrating  reed,  and  (4)  resonating  chambers.  In  man 
and  other  mammals  the  bellows  is  formed  by  the  lungs  and 
thorax  ;  the  windpipe  is  the  trachea ;  the  vocal  cords  in 
the  larynx  are  the  vibrating  reeds;  and  the  resonating 
chambers  are  the  pharynx,  nose,  and  mouth. 


the  structure  of 


A.  Structure  of  the  Larynx. 
The  points  of  physiological  importance  in 
the  larynx  are  the  following : — 

1.  Cartilages  (figs.  223,  224).— The  ring-like  cricoid  (Cr.),  at 
the  top  of  the  trachea,  is  thickened  from  below  upwards  at 
its  posterior  part  and  carries  on  its  upper  border  two  pyramidal 
cartilages,  triangular  in  section— the  arytenoids  (Ar.).  These 
articulate  with  the  cricoid  by  their  inner  angle.  At  the  outer 
angle,  the  posterior  and  lateral  crico-arytenoid  muscles  are 
attached.  The  vocal  cords  arise  from  their  anterior  angles  and 
run  forward  to  the  thyreoid.^  The  thyreoid  cartilage  (Th.)  forms 
a  large  shield,  which  articulates  by  its  posterio-iaferior 
processes  with  the  sides  of  the  cricoid,  so  that  it  moves  round  a 
horizontal  axis.  The  epiglottis,  or  cartilaginous  lid  of  the 
larynx,  is  fixed  to  its  upper  and  anterior  part. 

2.  Ligaments. — The  articular  ligaments  require  no  special 
attention.  The  true  vocal  cords  are  fibrous  ligamentous 
structures  which  run  from  the  anterior  angle  of  the  arytenoids 
forward  to  the  posterior  aspect  of  the  middle  of  the  thyreoid. 
They  contain  many  elastic  fibres  and  are  covered  by  a  stratified 
squamous  epithelium,  and  they  appear  white  and  shining. 

The  vocal  cords  increase  in  length  as  the  larynx  grows  ; 
in  adult  life,  they  are  generally  longer  in  the  male  than  in  the 

1  The    name    thyroid,    instead    of  thyreoid,  is    based  upon  a  mistake  ; 
Ovpeoi  is  a  shield,  while  dupos  is  a  door  or  aperture. 
550 


VOICE 


551 


female  and  the  whole   larynx   is  larger.     The  cleft   between 
them  is  the  rima  glottidis, 

3.  Muscles. — The  crico-tJtyreoidei  take  origin  from  the 
antero-lateral  aspects  of  the  cricoid,  and  are  inserted  into  the 
inferior  part  of  the  lateral  aspect  of  the  thyreoid.  In  contract- 
ing  they  approximate    the  two 

cartilages  anteriorly,  and  render 
tense  the  vocal  cords  (fig. 
223). 

The  crico-arytenoidei  postici 
arise  from  the  back  of  the  cricoid 
and  pass  outwards  to  be  inserted 
into  the  external  or  muscular 
process  of  tlie  arytenoids.  In 
contracting,  they  pull  these  pro- 
cesses inwards,  and  thus  diverge 
the  anterior  processes  and  open 
the  glottis  (fig.  224). 

The  crico-arytenoidei  later- 
ales  take  origin  from  the  lateral 
aspects  of  the  cricoid,  and  pass 
backwards  to  be  inserted  into 
the  muscular  processes  of  the 
arytenoids.  They  pull  these 
forwards,  and  so  swing  inwards 
their  anterior  processes  and 
approximate  the  vocal  cords  (fig.  224). 

A  set.  of  muscular  fibres  runs  between  the  arytenoids— the 
arytenoideus  transversus — while  other  fibres  run  from  the 
arytenoids  up  to  the  side  of  the  epiglottis.  These  help  to  close 
the  upper  orifice  of  the  larynx. 

The  thyreo-arytenoidei  bands  are  of  muscular  fibres  lying 
in  the  vocal  cords,  and  running  from  the  thyreoid  to  the 
arytenoids.     Their  mode  of  action  is  not  fully  understood. 

4.  Mucous  Membrane.— The  mucous  membrane  of  the  larynx 
is  raised  on  each  side  into  a  well-marked  fold  above  each  true 
vocal  cord— the  false  vocal  cord.  Between  this  and  the  true 
cord  on  each  side  is  a  cavity— the  ventricle  of  the  larynx. 
The  other  folds  of  mucous  membrane,  although  of  importance 
in  medicine,  have  no  special  physiological  significance. 


Fig.  22.3.— Side  View  of  the  Carti- 
lages of  the  Human  Larynx.  CV., 
cricoid  cartilage;  Ar.,  left  ary- 
tenoid cartilage;  Th.,  thyreoid 
cartilage.  The  dotted  line  shows 
the  change  in  the  position  of 
the  thyreoid  by  the  action  of 
the  crico-thyreoid  muscle,  and 
the  stretching  of  the  vocal  cords 
which  results. 


552 


VETERINARY   PHYSIOLOGY 


The  interior  of  the  larynx  may  be  examined  during  life  by 
the  laryngoscope  {Practical  Physiology). 

5.  Nerves. — The  muscles  of  the  larynx  are  supplied  chiefly 
by  the  recurrent  laryngeal  branch  of  the  vagus,  which  comes  oflF 

in  the  thorax,  and  arches 
upwards  to  the  larynx. 
On  the  left  side,  where  it 
curves  round  the  aorta,  it 
is  apt  to  be  pressed  upon 
in  aneurismal  swellings. 
Paralysis  of  this  nerve 
causes  the  vocal  cord  on 
that  side  to  assume  the 
cadaveric  position,  midway 
between  adduction  and  ab- 
duction, and  makes  the 
voice  hoarse  or  abolishes  it 
altogether. 

The  superior  laryngeal  is 
the  great  ingoing  nerve,  but 
it  also  supplies  motor  fibres 
to  the  crico  -  thyreoid 
muscle.  Paralysis  prevents 
the  stretching  of  the  vocal 
cords,  makes  the  voice 
hoarse,  and  renders  it  im- 
possible to  produce  a  high 
note. 
Centre. — These  nerves  are  presided  over  by  (a)  a  centre  in 
the  medulla.  When  this  is  stimulated  abduction  of  the  vocal 
cords  is  brought  about.  (6)  This  centre  is  controlled  by  a 
cortical  centre  situated  in  the  inferior  frontal  convolution. 
Stimulation  of  this  causes  adduction  of  the  cords  as  in 
phonation,  while  destruction  leads  to  no  marked  change. 


Fig.  224. — Cross  Section  of  the  Larynx, 
to  show  the  cricoid,  Cr.  ;  thyreoid, 
Th.  ;  arytenoid  cartilages,  Ar.  The 
continuous  line  shows  the  parts  at 
rest,  the  dotted  line  under  the  action 
of  the  lateral  crico-arytenoid  muscle, 
and  the  dot-dash  line  under  the  action 
of  the  posterior  crico-arytenoid. 


B.  Physiology  of  the  Voice. 

When  a  blast  of  air  is  forced  between  the  vocal  cords  when 
they  are  approximated  by  the  lateral  crico-arytenoids,  they  are 
set  in  vibration  both  wholly  and  in  segments  like  other  vibrating 


VOICE  553 

reeds  aud  sounds  are  thus  produced.  These  sounds  may  be 
varied  in  loudness,  pitch,  and  quality. 

The  loudness,  or  amplitude  of  vibration,  depends  upon  the 
size  of  tiie  larynx  and  of  the  resonating  chambers — the  pharynx, 
uaso-pharynx,  and  mouth — and  upon  the  force  of  the  blast  of  air 
acting  upon  the  cords. 

The  pitch,  or  number  of  vibrations  per  second,  depends  upon 
the  length  and  tension  of  the  vocal  cords.  The  greater  length 
of  the  vocal  cords  in  the  male,  as  compared  with  the  female, 
makes  the  voice  deeper.  The  tension  of  the  cords  is  varied 
by  the  action  of  the  crico-thyreoid  muscle. 

The  power  of  varying  the  pitch  of  the  voice  differs  greatly 
in  different  animals.  The  average  difference  between  the  lowest 
and  the  highest  note  which  the  ordinary  human  individual  can 
produce  is  about  two  octaves. 

The  quality  of  the  voice,  upon  which  the  characteristic  sound 
produced  by  each  species  of  animal  is  largely  due,  depends  upon 
the  overtones  which  are  made  prominent  by  resonance  in  the 
pharynx,  nose,  and  mouth.  By  varying  the  shape  and  size  of 
these  cavities,  and  more  especially  of  the  mouth,  the  quality 
of  sound  may  be  considerably  altered. 

Roaring  is  the  peculiar  sound  made  by  some  horses  when 
exercised.  It  is  due  to  paralysis  of  the  posterior  crico-arytenoid 
muscle  on  the  left  side.  This  when  in  action  abducts  the  vocal 
cord  of  that  side,  but  when  not  in  action  it  allows  it  to  be 
drawn  inward.  This  partially  occludes  the  rima  glottidis,  and 
thus  not  only  causes  the  characteristic  sound,  but  also 
limits  the  entrance  of  air  and  oxygen  to  the  lungs.  The  con- 
dition appears  to  be  due  to  disease  of  the  recurrent  laryngeal 
nerve.  It  is  more  common  in  stallions  than  in  mares,  and 
most  frequent  in  thoroughbreds.  By  some  it  is  considered  to 
be  hereditary.  Tracheotomy  with  the  insertion  of  a  tube  into 
the  trachea  relieves  the  evil  effects  of  the  condition. 


SECTION    VII. 
EXCRETION   OF   MATTER   FROM   THE   BODY. 


I.  EXCRETION  BY  THE  LUNGS  (see  Respiration,  p.  olSet  seq.). 

II.  EXCRETION  BY  THE  BOWEL  (see  p.  361). 

Ill  EXCRETION  BY  THE  KIDNEYS. 


I.  URINE. 

I.  General  Consideration. 

The    urine  is  a  fluid    formed    in   the  kidneys,   and  in   it  are 
excreted  from  the  body — 

1.  The  nitrogen,  sulphur,  and  phosphorus  containing  pro- 
ducts of  the  catabolism  of  proteins  and  of  nucleo-proteins  of  the 
body  and  of  the  blood. 

2.  Any  excess  of  H  or  OH  ions  in  the  blood. 

3.  Any  excess  of  certain  substances  taken  with  food  or 
produced  in  the  body,  e.g.  sugar  and  sodium  chloride. 

4.  Various  drugs. 

Its  composition  should  be  studied  in  the  light  of  the 
■  catabolism  of  proteins  and  nucleo-proteins. 

(In  reading  this  part,  the  Table  on  pj).  568  and  569 
should  be  constantly  referred  to.) 

1.  Catabolism  of  Proteins. — As  already  indicated,  the  pro- 
tein molecule  breaks  down  into  its  constituent  amino-acids 
(p.  16),  and  these  are  de-aminised  chiefly  in  the  liver  (p.  359). 

The  ammonia  liberated,  probably  as  ammonium  carbonate,  is 
dehydrated  chiefly  in  the  liver  (p.  359)  to  form  urea — 

554 


URINE  555 

0  0 

II  II 

H,— N  C  N— H.,  ^H.,N— C— NH. 

"I 


H„    O   i        i  0    H. 


+  2H.0 


When  a  condition  of  acidosis  (p.  482)  develops,  a  certain 
proportion  of  the  ammonia  is  not  dehydrated,  but  is  used  to 
neutralise  the  acid  and  to  decrease  the  Ch  of  the  blood 
(p.  482),  and  the  proportion  of  NH3  to  CO(NH,),  is  thus 
raised.  When  alkalies  are  given  the  proportion  of  ammonia  is 
decreased. 

In  the  diamino  acids,  such  as  lysin,  arginin,  histidin  (p.  17), 
the  amidogen  is  dealt  with  as  in  the  mon-amino  acids.  But  in 
the  case  of  arginin,  where  the  guanidin  nucleus — 

NH 

II 
H.,N-C-NH., 

is  present,  this  may  in  part,  at  least,  escape  complete  oxidation 
to  urea,  may  be  methylated  and  then  linked  to  acetic  acid  to 
form  creatin  (p.  209),  from  which  creatinin  may  be  formed  by 
dehydration. 

The  amino  acids  linked  to  the  benzene  ring,  e.g.  tyrosin, 
are  deaminised  and  the  amidogen  changed  to  urea,  while  the 
propionic  acid  chain  is  oxidised  from  its  free  end  with  the 
formation  of  homogentisic  acid  (alkapton)  then  hydroquinone, 
which  is  finally  oxidised  to  CO.,  and  H.O,  the  former  of  which 
is  largely  excreted  by  the  lungs. 

In  one  abnormality  of  metabolism  the  oxidation  stops  at 
alkapton,  and  this  is  excreted  in  the  urine.  It  oxidises  to  a 
black  pigment. 

In  tryptophan  not  only  is  the  propionic  acid  chain  oxidised, 
but  the  pyrrol  ring  is  split  off  and  the  molecule  then  undergoes 
the  same  changes  as  tyrosin. 

When,  as  a  result  of  bacterial  putrefaction  in  the  intestine 
(p.  329),  the  tryptophan  molecule  has  had  the  propionic  acid 
oxidised  without  the  removal  of  the  pyrrol  ring,  skatol  and 
indol  are  formed.     These  are  hydrated  to  skatoxyl  or  indoxyl. 


556  VETERINARY   PHYSIOLOGY 

and,  in  the  liver,  linked  to  sulphuric  acid  or  potassium 
sulphate  derived  from  the  sulphur  of  the  protein  molecule,  and 
thus  excreted  as  ethereal  sulphates,  the  amount  of  which  in 
the  urine  is  a  fair  measure  of  the  putrefaction  changes  in  the 
intestine. 

The  remaining  sulphur  of  the  protein  is  chiefly  oxidised  to 
sulphuric  acid,  linked  to  bases  and  excreted  as  the  inorganic 
sulphates. 

A  small  part  of  the  sulphur  which  in  the  protein  exists  as 
cystin  may  escape  oxidation  and  appear  in  the  urine.  In 
some  people  this  excretion  of  cystin  is  large,  and  cystin  crystals 
appear  in  the  urine  (neutral  sulphur). 

The  sulphates  and  the  neutral  sulphur  are  derived  from  the 
sulphur  of  the  proteins,  and  the  amount  excreted  in  the  urine  is 
a  measure  of  the  amount  of  protein  catabolised. 

Tbe  nucleo-proteins  are  first  split  into  the  protein  moiety, 
which  is  broken  down  as  described  above,  and  the  nucleic  acid 
moiety,  which  is  broken  down,  possibly  by  an  enzyme,  a 
nuclease,  and  the  phosphorus  and  purin  parts  then  undergo 
ciianges  and  are  excreted  in  the  urine. 

(a)  The  purins  of  nucleic  acid,  e.g.  adenin,  are  amino-purins 
i.e.  they  have  an  amidogen  molecule  attached.  This  is  split 
off,  possibly  by  the  action  of  an  enzyme,  and  excreted  as  urea. 

The  deaminised  purins,  such  as  hypoxanthin,  are  then 
oxidised  to  uric  acid  and  partly  excreted  in  this  form.  In  this 
process  an  enzyme  may  play  a  part.  About  one-half  of  the  uric 
acid  is  split  into  two  molecules  of  urea,  probably  by  the  action 
of  another  enzyme  (uricoclastase)  in  the  liver,  while  the 
connecting  chain  is  oxidised  to  C0„. 

(6)  The  phosphorus  is  oxidised  to  P.^Og  and  linked  with 
monobasic  sodium  and  potassium  and  dibasic  calcium  and 
magnesium. 

2.  Regulation  of  the  H— OH  ions  of  the  Blood. — The  phos- 
phates play  an  important  part  in  regulating  the  Ch  of  the  tissues 
and  in  determining  the  reaction  of  the  urine  (Appendix  III.). 

If  the  Ch  of  the  blood  is  increased  in  acidosis  (p.  481),  the 
Na„HPO,  of  the  cells  is  changed  to  (NaH.PO  J  (p.  482)  and 
is  turned  out  into  the  plasma  to  be  excreted  in  the  urine  and 
thus  to  carry  off  H  ions. 


URINE  557 

If,  on  the  other  hand,  a  condition  of  alkalosis  (p.  531) 
is  produced,  Na.^HPO^  is  excreted — OH  ions  being  elimin- 
ated. 

Under  normal  conditions  the  reaction  of  the  urine  varies 
between  Na„HPO^  and  NaH,PO,. 

The  administration  of  acids  does  not  materially  increase  the 
hydrogen  ion  concentration.  They  combine  with  the  sodium 
of  the  bicarbonate  of  the  plasma,  turn  out  the  CO2,  increase 
the  amount  of  dissolved  COo,  and  thus  stimulate  the  respiratory 
centre  to  increase  the  ventilation  of  the  lungs  and  then  to 
eliminate  the  COo  and  adjust  the  proportion  of — 

HXO3    ^  2 
NaHCOg       20" 

But  when  alkalies  are  given,  or  the  citrates,  malates,  and  tartrates 
of  sodium  or  potassium  which  are  oxidised  to  carbonates  in  the 
body,  the  kidneys  then  act  in  eliminating  the  increased  OH 
ions  and  thus  readjusting  the  balance  of  ions  in  the  blood 
plasma. 

II.  Physical  Characters. 

The  characters  of  the  urine  depend  largely  on  the  relative 
proportion  of  water  and  of  solids  which  are  excreted  in  it :  at 
one  time  it  may  be  very  concentrated,  while  at  another  time  it 
may  be  very  dilute  indeed.  For  this  reason  its  specific  gravity, 
which  depends  upon  the  percentage  of  solids  in  solution,  varies 
within  wide  limits.  But  the  average  specific  gravity  in  the 
horse  is  about  1036.  It  is  possible  from  the  specific  gravity 
to  form  a  rough  idea  of  the  amount  of  solids  present,  for  by 
multiplying  the  last  two  figures  by  2'22  the  amount  of  solids 
per  1000  parts  is  given. 

Since  the  percentage  of  pigments  in  the  urine  varies  like 
that  of  the  other  constituents,  the  colour  of  the  urine  shows 
wide  divergence  in  the  normal  condition.  A  concentrated 
urine  has  a  dark  amber  colour,  while  a  dilute  urine  may  in 
some  animals  be  almost  colourless.  Under  average  conditions 
the  urine  has  a  straw-yellow  colour. 

The  reaction  of  urine  is  normally  acid  in  dog  and  other 
carnivora. 


558  VETERINARY   PHYSIOLOGY 

In  herbivora,  when  suckling  or  when  fasting,  the  urine  is 
acid,  but  when  on  their  normal  diet  it  is  alkaline. 

The  alkalinity  is  due  to  the  presence  of  alkaline  carbonates 
formed  from  the  citrates,  malates,  and  tartrates  of  the  vegetable 
foods,  and  also  from  the  acetates,  etc.,  produced  by  the  decom- 
position of  cellulose  in  the  rumen  and  intestine. 

Urine  in  carnivora  is  normally  transparent;  but  when  it 
has  stood  for  a  few  hours,  a  cloud  of  a  mucin-like  substance  is 
seen  floating  in  it.  In  herbivora,  as  the  urine  cools,  it  rapidly 
becomes  turbid  and  throws  down  a  white  precipitate  composed 
chieriy  of  carbonate  of  lime. 

The  smell  of  urine  is  characteristic,  and  it  may  be  modified 
by  the  ingestion  of  many  different  substances. 

III.  Composition. 

The  tests  for  the  various  constituents  of  the  urine  must  he 
studied  ijractically  {Chemical  Physiology). 

Since  the  relative  amounts  of  water  and  solids  vary  within 
such  wide  limits,  the  percentage  amount  of  the  later  is  of  little 
moment.  Under  average  conditions,  the  water  constitutes 
about  96  per  cent.,  and  the  solids  about  4  per  cent.  Of  these 
solids,  rather  more  than  half  is  organic,  rather  less  than  half 
is  inorganic.  Since  water  and  solids  are  derived  from  the 
water  and  solids  taken  by  the  animal,  the  amounts  excreted 
depend  upon  the  amounts  taken,  and  must  be  considered  in 
connection  with  them.  Thus,  if  a  horse  takes  little  fluid,  it  will 
pass  little  water  in  the  urine.  If  it  takes  little  food,  a  small 
quantity  of  solids  will  be  excreted  by  the  kidneys. 

Since  excretion  and  ingestion  must  be  studied  in  relation- 
ship to  one  another,  it  is  convenient  to  compare  them  during  a 
definite  period  of  time,  and  the  natural  division  into  days  of 
twenty-four  hours  is  generally  adopted. 

Under  ordinary  conditions,  the  amount  of  solid  food  taken 
per  day  does  not  vary  very  greatly,  but  the  amount  of  fluid 
imbibed  varies  within  much  wider  limits.  For  this  reason, 
Avhile  the  amount  of  water  excreted  in  the  urine  per  diem  varies 
enormously,  the  amount  of  solids  is  more  fixed. 

In  the  horse,  on  an  averagfe  diet,  about  5  to  8  litres'  of  water 


URINE  559 

are  daily  eliminated,  while  in  the  ox  as  much  as  20  litres  may 
be  passed. 

1.  Nitrogenous  Constituents. 

The  waste  nitrogen  of  the  body  occurs  in  the  urine  in 
different  substances,  the  origin  of  each  of  which  has  been 
considered. 

A.  Urea. — Urea  is  the  most  abundant  constituent  of  the 
urine.  Its  chemistry  and  mode  of  formation  have  been  discussed 
on  p.  359.  The  amount  excreted  depends  upon  the  amount  of 
iwotein  taken  in  the  food,  and  for  this  reason,  in  man  during 
fasting,  the  excretion  may  fall  as  low  as  10  grms.  per  diem,  while 
on  a  diet  containing  an  average  amount  of  proteins,  about  86  grms. 
of  urea — 16 "7  grms.  of  nitrogen — are  excreted.  On  a  normal 
diet  about  90  per  cent,  in  the  dog  and  80  per  cent,  in  the  horse 
of  the  waste  nitrogen  is  excreted  as  urea,  but,  when  the  protein 
intake  is  decreased  and  non-nitrogenous  food  is  substituted, 
the  proportion  of  urea-nitrogen  may  fall  as  low  as  60  per  cent, 
of  the  whole  (p.  562). 

When  urine  is  allowed  to  stand,  micro-organisms  are  apt  to 
gain  access,  and  to  cause  a  hydration  of  the  urea,  whereby  it  is 
changed  into  ammonium  carbonate — 

0  O 

II  il 

H,N— C— NH,-F2H,0  =  H,N— O— C— 0— NH, 

The  urine  is  thus  made  alkaline,  and  the  earthy  jDhos- 
phates  are  precipitated.  The  magnesium  phosphate  combines 
with  the  ammonia  to  form  ammonium-magnesium-phosphate, 
XH^MgPO^ +6H^0  (triple  phosphate),  which  crystallises  in 
characteristic  prism-like  crystals  (fig.  225). 

B.  Non-Urea  Nitrogen. — Some  20  per  cent,  of  nitrogen  which, 
on  an  ordinary  diet,  is  not  excreted  as  urea  is  distributed  in — 

1.  Ammonium  Salts. — In  herbivora  a  very  small  proportion 
of  nitrogen  is  normally  excreted  as  ammonium  salts.  But, 
under  certain  conditions,  the  proportion  is  increased  (p.  554). 
Anything  which  tends  to  raise  the  Ch  of  the  blood,  e.g.  the 
formation  of  /3-oxybutyric  acid  (p.  358),  causes  an  in- 
creased   excretion    of    ammonia — the    ammonia    being    formed 


560  VETERINARY  PHYSIOLOGY 

from  the  proteins  to  neutralise  the  acids.  In  carnivorous  and 
omnivorous  animals  the  production  of  ammonia  is  a  protective 
mechanism  against  acid  intoxication,  Herbivora  have  not  the 
same  power  of  forming  ammonia  to  neutralise  acids. 

2.  Creatinin. — Creatinin,  like  the  creatin  in  muscle  (p.  209), 
is  characterised  by  containing  the  guanidin  nucleus. 

It  may  be  readily  formed  from  creatin  by  treatment 
with  acids  which  remove  a  molecule  of  water. 

NH 

II 

C 
/^N— H 
CH.,— N  \ 

I  NH 

^\C— C—    iO— H; 


0 

But,  when  creatin  is  administered  by  the  mouth  or  injected 
subcutaneously,  the  creatinin  of  the  urine  is  not  proportionately 
increased ;  some  of  the  creatin  may  appear  in  the  urine,  but 
much  of  it  may  not  be  recoverable,  especially  on  a  protein- 
free  diet  with  abundance  of  carbohydrates.  It  seems  to  be 
retained  or  changed  in  the  body,  possibly  being  used  in 
the  resynthesis  of  the  protein  molecule.  On  the  other  hand, 
there  is  evidence  that  the  creatinin  in  the  urine  may  be 
taken  as  a  rough  measure  of  the  muscular  development  and 
tone  of  the  individual,  and  it  is  difficult  to  avoid  the  conclusion 
that  it  is  derived  either  from  the  creatin  of  the  muscle,  or  from 
some  guanidin-containing  precursor  common  to  both.  It  is 
possible  that  every  animal  has  a  limited  power  of  changing 
creatin  to  creatinin,  and  that  this  limit  is  readily  overstepped — 
as  the  limit  of  sugar  tolerance  may  be  overstepped — and  that 
under  these  conditions  creatin  is  not  converted  to  creatinin, 
but  is  excreted  as  creatin  or  changed  in  the  body  to  some  other 
substance  not  yet  identified.  In  young  children  creatin  is  a 
normal  constituent  of  the  urine. 

In  the  wasting  of  muscles  which  occurs  in  fasting,  creatin 
appears  in  the  urine  along  with  creatinin.  In  birds  it  takes  the 
place  of  creatinin  in  the  urine  and  the  excretion  is  increased 
when  the  muscles  waste. 


URINE  561 

3.  Purin  Bodies. — A  very  small  proportion  of  the  nitrogen  is 
found  in  these  bodies.  They  consist  of  two  unmodified  or 
modified  urea  molecules,  linked  together  by  a  nucleus  of  an 
acid  radicle.  In  birds  and  reptiles  the  most  important  have 
as  the  linking  part  an  oxy-acid  with  three  carbons  in 
series  —  Uric  Acid— Tri-oxy-purin  —  an  exceedingly  insoluble 
substance  which  tends  to  crystallise  in  large  polymorphic 
crystals. 

O 

II 
H— X— C 

I       I 
0  =  C     C— N— H 

I    II      >c=o 

H— N— C— N— H 

Uric  Acid. 

In  these  animals  uric  acid  largely  replaces  urea  as  the  sub- 
stances in  which  nitrogen  is  eliminated,  and  they  are  formed 
in  the  liver  from  the  various  products  of  the  decomposition 
of  protein  molecules.  But  in  mammals  the  purins  appear 
to  be  very  largely  derived  from  the  decomposition  of  nucleic 
acid  (p.  556).  Even  when  all  supplies  of  nucleins  and  purin 
bodies  from  without  are  cut  off,  a  certain  amount  of  these 
purins  is  daily  eliminated.  These  have  been  called  the  "  endo- 
genous "  purins,  while  those  derived  from  the  constituents  of 
the  food  are  termed  the  "  exogenous "  purins.  A  small 
amount  is  undoubtedly  formed  from  the  purins  of  muscle 
(p.  210). 

In  most  mammals  the  chief  purin  is  Allantoin.  In  this 
two  urea  molecules  are  linked  by  the  radicle  of  glyoxylic 
acid. 

H 

I 
H— N— C— N— H 


I 

C  =  0 


I 

o  =  c 

I 

H— N     C— N— H 


I       II 
H     0 
36 


562  VETERINARY   PHYSIOLOGY 

4.  Hippuric  Acid. — This  is  benzoyl-amino-acetic  acid — 

O 

II 
-C— 


CeHJ 


H    H    O 

i     I        II 
.N—C-C— O— H 

I 
H 


It  is  fornied  from  benzoic  acid  taken  in  the  food  by  linking 
it  to  g'lycin — amino-acetic  acid.  This  synthesis  appears  to 
take  place  in  the  kidneys,  for  it  has  been  found  that  hippuric 
acid  is  not  formed  when  these  organs  are  excised,  and  that,  when 
blood  containing  benzoates  is  circulated  through  them,  hippuric 
acid  is  produced.  Its  chief  interest  is  in  the  fact  that  it  is  one 
of  the  first  organic  compounds  which  were  demonstrated  to  be 
formed  synthetically  in  the  animal  body.  Normally  it  is  present 
in  human  urine  in  very  small  quantities,  but  in  the  urine  of 
herbivora  the  amount  is  considerable,  from  the  presence  of 
benzoic  acid  in  the  fodder,  and  is  most  abundant  on  a  diet  of 
grass  or  hay,  less  so  upon  one  of  oats.  The  acid  itself  is 
insoluble,  and  it  occurs  as  the  soluble  sodium  salt. 

5.  Mon-amino  Acids  are  always  present  in  traces  in  the  urine, 
and  any  interference  with  the  activity  of  the  liver,  the  organ 
chiefly  concerned  in  changing  them  to  urea,  leads  to  their 
appearance  in  increased  amounts  (p.  860). 

6.  Undetermined  Nitrogen. — A  small  quantity  of  the  nitrogen 
in  the  urine  exists  in  substances  the  chemical  nature  of  which 
has  not  been  determined.  The  amount  of  these  varies  con- 
siderably. 

The  proportions  of  the  total  nitrogen  of  the  urine  in  these 
various  compounds  varies  with  the  kind  of  food  taken.  Folin 
gives  the  following  table  for  the  human  subject,  which  shows 
this  very  clearly : — 


Per  cent,  of  Total  Nitrogen. 
Protein-rich  Diet.  Protein-poor  Diet. 


Total  Nitrogen 

Urea         , , 

NH3 

Creatinin  Nitrogen 

Uric  Acid  Nitrogen 

Undetermined  Nitrogen 


14-8  to  18-2  grms. 
86-3  ,,  89-2  per  cent. 

3-3  ,,     5-1 

3'2  „    4-5        ,, 

0-5  „     10 

2-7  ,,    5-3        „ 


4-8  to    8-0  grni.'?. 
62-0  ,,  80-4  per  cent. 
4-2  ,,   11-7 
5-5  „   111 
1-2  „     2-4 
4-8  „   14-6 


URINE  563 

From  these  results  he  draws  the  conclusion  that  urea  is 
largely  the  result  of  the  metabolism  of  the  proteins  of  the 
food,  of  "  exogenous  "  protein  metabolism,  and  that  the  creatinin 
and  uric  acid  nitrogen  and  neutral  sulphur  (p.  564)  are  chiefly 
the  result  of  "  endogenous  "  protein  metabolism. 

2.  Sulphur-containing  Bodies. 

The  sulphur  excreted  in  the  urine  is  derived  from  the 
sulphur  of  the  protein  molecule,  and  the  amount  of  sulphur 
excreted  may  be  taken  as  a  measure  of  the  amount  of  protein 
decomposed. 

A.  Acid  Sulphur. — The  greater  part  of  the  sulphur  is  fully 
oxidised  to  sulphuric  acid. 

(a)  Preformed  Sulphates. — The  greater  part  of  this  is 
linked  with  bases  to  form  ordinary  inorganic  sulphates. 

(h)  Ethereal  Sulphates. — Nearly  all  of  the  remaining  sulphur 
is  in  organic  combination,  linked  to  benzene  compounds,  formed 
by  oxidation  of  the  indol,  skatol,  and  phenol  (see  p.  330),  which 
in  carnivora  are  produced  by  the  putrefaction  of  proteins  in  the 
bowel,  and  in  herbivora  from  the  aromatic  compounds  which 
occur  chiefly  in  the  roughage  of  the  food. 

Indol  is  oxidised  into  indoxyl  thus — 


f^    \ ,0-H 

NH 

This,  when  linked  to  potassium  sulphate,  forms  potassium 
indoxyl-sulphate  or  indican. 

,O.SO.,OK 


NH 

From  skatol,  which  is  methyl-indol,  potassium  skatoxyl- 
sulphate  is  formed  in  the  same  way. 

These  bodies  are  colourless,  but  when  oxidised  they  yield 
pigments — indican  yielding  indigo  blue,  skatoxyl-sulphate  of 
potassium  yielding  a  rose  colour. 


564  VETERINARY  PHYSIOLOGY 

Phenol- 


IC.H  J -''-''  kHl-°-'°  =  °^ 


is  also  linked  with  potassium  sulphate,  and  excreted  in  the 


unne. 


The  amount  of  these  ethereal  sulphates  depends  upon  the 
activity  of  putrefaction  in  the  intestine,  and  is  a  good  index  of 
its  extent. 

Dioxybenzene  or  Pyrocatechin — 


— 0-H 


l^eH,,  _o_H 


is  also  linked  to  potassium  sulphate  and  excreted.  This  com- 
pound is  always  present  in  the  urine  of  the  horse.  When 
oxidised  it  yields  a  greenish-brown  pigment  when  urine 
containing  it  is  allowed  to  stand. 

B.  Neutral  Sulphur. — A  small  quantity  of  sulphur  is  excreted 
in  a  less  oxidised  state,  in  the  form  of  neutral  suljjJiur.  The 
most  important  compound  of  this  kind  is  cystin,  the  disulphide 
of  amino-propionic  acid — two  molecules  of  amino-propionic  acid 
linked  by  sulphur — 

Amino-propionic  acid. 

I 
Sulphur. 

I 
Sulphur. 

Amino-propionic  acid. 

H  H 

I  I 

H_C-S     —     S-C-H 

I  I 

H— C— NH..     H..NC— H 

I  "         ■     I 
C_OH      HO— C 

II  li 

o  o 


URINE  565 

The  condition  of  cystinuria  has  been  already  explained 
(p.  556),  as  has  also  the  occurrence  of  alkaptonuria  (p.  555). 

3.  Phosphorus-containing  Bodies. 

In  herbivorous  animals  phosphates  are  practically  absent 
from  the  urine.  They  are  excreted  from  the  mucous  membrane 
of  the  bowel.  Hence,  in  the  horse,  crystals  of  triple  phosphates 
are  found  in  the  faeces,  not  in  the  urine. 

In  carnivores  the  phosphorus  in  the  urine  is  derived  partly 
from  phosphates  taken  in  the  food,  and  partly  from  the  nucleins 
of  the  food  and  tissues  and  from  the  bones. 

(a)  Normally  the  phosphorus  is  fully  oxidised  to  PoO,, 
which  is  linked  to  alkalies  and  earths,  and  excreted  in  the 
urine.  The  most  important  phosphate  is  the  phosphate  of  soda, 
NaH.PO^,  which  is  the  chief  factor  in  causing  the  acidity 
of  the  urine.  When  the  urine  becomes  ammoniacal,  triple 
phosphate  is  formed  (p.  559). 

(b)  It  is  probable  that  a  small  quantity  of  the  phosphorus 
is  excreted  in  organic  compounds,  such  as  glycero-phosphates ; 
but  so  far  these  have  not  been  fully  investigated. 

4.  Chlorine  Compounds. 

Sodium  chloride  is  the  chief  salt  of  the  urine.  It  is  entirely 
derived  from  the  salt  taken  in  the  food,  and  its  amount  varies 
with  the  amount  ingested.  From  10  to  15  grms.  are  usually 
excreted  per  diem  in  a  person  on  a  normal  diet. 

In  starvation,  to  a  certain  extent,  and  very  markedly  in 
fever,  the  tissues  of  the  body  have  a  great  power  of  holding  on 
to  the  chlorine,  and  the  chlorides  may  almost  disappear  from 
the  urine. 

5.  Inorganic  Bases  of  the  Urine. 

Sodium,  potassium,  calcium,  and  magnesium  occur  in  the 
urine  in  amounts  varying  with  the  amounts  taken  in  the  food. 
On  a  flesh  diet  and  in  starvation  potassium  is  in  excess  of  the 
others.  Calcium  and  magnesium  are  present  in  much  smaller 
quantities.  In  herbivora  potassium  is  the  chief  salt,  and  in  the 
horse  calcium  is  also  abundant. 


566 


VETERINARY  PHYSIOLOGY 


6.  Pigments. 

A  brown  hygroscopic  substance,  Avhich  gives  no  bands  in 
the  spectrum,  may  be  extracted  from  urine.  This  has  been 
termed  urochrome.  By  reducing  it,  another  pigment,  urobilin, 
is  produced,  which  gives  definite  bands,  and  which  is  frequently 
present  in  the  urine.  It  is  probably  identical  with  the  hydro- 
bilirubin  which  has  been  prepared  from  the  bile  pigments,  and 
it  contains  C,  H,  0,  and  N. 

The  pigment  that  gives  the  pink  colour  to  urates  has  been 
called  uroerythyrin,  and  its  chemical  nature  is  unknown. 

Hsematoporphyrin  (see  p.  491)  is  normally  present  in  traces 
in  the  urine,  but  in  certain  pathological  states  it  is  increased  in 
amount  and  gives  a  brown  colour  to  the  urine. 

7.  Nucleo-Protein. 

A  mucin-like  nucleo-protein,  derived  from  the  urinary 
passages,  is  always  present  in  small  amounts,  and  forms  a  cloud 
when  the  urine  stands. 


8.  Carbonic  Acids. 
1.  Carbonic  Acid. — Small    amounts    of   this    are  present  in 
urine  of  carnivora. 

In  herbivora  it  is  present  in  large  amounts,  combined  with 


Fig.  225. — The  Three  most  Common  Urinary  Crystals  :   1,  Triple  phosphate; 
2,  uric  acid  ;  3,  calcium  oxalate. 

potassium,  lime,  and  magnesia,  and  also  free.  The  carbonate  of 
lime  readily  crystallises  out  in  large  dumb-bell-like  crystals 
which  may  be  confused  with  crystals  of  oxalate  of  lime,  but 
which  are  quickly  soluble,  with  effervescence,  on  the  addition 
of  an  acid. 


FORMATION  OF   URINE  567 

9.  Oxalic  Acid 
O    O 

II     II 
H_0— C— C-0— H 

is  a  substance  in  a  stage  of  oxidation  just  above  that  of  carbonic 
acid.  It  is  frequently  present  in  the  urine  linked  with  lime, 
and  the  lime  salt  tends  to  crystallise  out  in  characteristic 
octohedra,  looking  like  small  square  envelopes  under  the  micro- 
scope. Under  certain  conditions  these  crystals  assume  other 
shapes.  The  oxalic  acid  of  the  urine  is  chiefly  derived  from 
oxalates  in  vegetable  foods,  but  it  has  been  detected  in  the 
urine  of  animals  on  a  purely  flesh  diet. 

The  differences  between  the  urines  of  different  herbivora 
are  not  important.  The  urine  of  the  ox  and  cow  is  more 
abundant  and  more  dilute  than  the  urine  of  the  horse,  while 
the  urine  of  the  sheep  is  considerably  more  concentrated  and 
contains  a  very  high  proportion  of  hippuric  acid. 


II.  FORMATION    OF   URINE. 

No  problem  of  physiology  has  proved  more  difficult  than 
that  of  the  mode  of  formation  of  urine  in  the  kidneys. 

This  is  largely  due  to  the  fact  that  theories  were  made  by 
famous  physiologists  upon  imperfect  data,  and  that  subsequent 
workers  have  tended  to  view  their  results  in  the  light  of  one  or 
other  of  these  theories. 

The  purpose  of  the  formation  of  urine,  as  already  explained, 
is  twofold — 

1 .  To  get  rid  of  waste  matter  from  the  body. 

2.  To  help  to  maintain  the  Oh  of  the  blood  plasma  (p.  554). 
The  problem  of  how  it  is  produced  may  best  be  approached 

by  considering — 

1st.  The  differences  between  the  urine  formed  and  the  blood 
from  which  it  is  formed. 

2nd.  The  apparatus  which  has  to  bring  about  these  changes 
— the  kidney. 


RELATIONS   OF   THE   DECOMPOSITION    PRODUCfS 


NUCLEO-PROTEIXS 

Xiicleases 

I 
NL^CLEIC  ACID 


P,05 


PURIXS 


e.g.  Adenin 


PCRINS 
OF 

Flesh 


Mox-Amixo 
Acids 


N=C    H 

I  i       I 
-C     C— N. 

II  II         \C-H 
N— C— N — 

Deamidase 


e.g. 


I 
Hypoxanthine 
O 


-N— C    H 


I       I       I 
-C     C-N 


X— c- 


-n/ 


C— H 


Oxydase 

I 

Uric  Acid 


H  0 

i       :i 

N  — C 

I     :     I 
=C     ;    C 

I 
N 

I 
H 


H    0 

-C— C- 

l 
H 


-OH 


Deamidase 


a.mmonium 
Carbonate 


Di-Amino  Acid3 
e.g.  Argiiiin 

I       .  \    ? 

NHo  H    H    H  i  H    N 

i      i      I      M  I     II 
HO-C-C— C-C-C-N-C-^] 

1        li    I    I    I     '  :       ! 

I  0    H    H    H    H 

I  I 

I     Ornitliin  Guanija 

I  I 


H,N-0-C-0— NH, 

Dehydrating 
Enzymes 


H„N-C- 


Creatiii 


XHCH3H    0 

I!        i        I        , 


N— C— C— OH 

I 
H 


I 

Creatiniii 

NH 


HX'- 


C 

A 

-N       N-H 


Trea 
U 


H^C 


Phos- 
phates 


Uricodastase 7-  H.X — C — XH, 

Uric  Acid  Urea  'or  Ammoxia) 

The  NH2  groups  marked  with  ♦  are  chaii^ 


C 


Creatini>- 
to  urea. 


)F  PROTEINS   TO   THE   CONSTITUENTS   OF   URINE 

V    PROTEINS 
'OLYPEPTIDES 


'  1                                1 

jiiDES                          Benzene  Compodnds, 
e.g.  Tryptophan 

1 

Sulphur,  Organic 

Compounds,  e.g.  Cystin 

0 

II    ♦ 
t-C-NH,               /\ 

H 

1 
-C- 

H    0 

1      II 
-C— C— OH 

0 

H,N- 

H                 H 

1           1           1 

\/\/ 
NH 

1 
H 

1 
NH, 

Jc           fo 

-C— H     H-C— NH. 

1                    1 

(a)  Digestion  and 
Tissue  Enzymes 

Tyrosin 
/x^OH 
1           1     H    H    0 

(6)  Bacteria 

Skatol 

/^              CH3 

1            ' 

H- 

-C-S- 

1 

H 

_  S-C-H 

1 
H 

\/_(^_C-C- 

1    1  ♦ 

H    NHj 

Homogentisic  Acid 

-0- 

-h\/\/^ 

NH 
Indol 

(Alkapton) 

\         '            ' 

,A0H 

NH 

\^  _C-C-0- 

1 

H 

Hydroquinone 

-H 

Indoxyl 

/\ 

1   1   r 

NH 

"  Indican  " 

r 

0 

HOI^ 

/\              -0 

II 

— S- 

11 

0-K 

III 

NH 

Ethereal  Sulphates 

K,& 

iNORGi 

Sulph 

O4 

vNic    Neui 

ATES     SuLP 

e.g.C 

^RAL 

hur, 
ystin 

570  VETERINARY   PHYSIOLOGY 

1st.    The  chief  differences  between  the   urine  and  the   blood 

plasma. 1.  The  blood  plasma  contains  some  7  to  8  per  cent,  of 

native  proteins  ;  in  normal  urine  these  are  absent. 

2.  The  blood  plasma  is  almost  neutral,  i.e.  it  has  a  Ch 
of  10"'^  or  pH  7'4.  The  urine  is  generally  markedly  acid 
with  a  pH  of  about  6  (Appendix  III.). 

3.  The  molecular  concentration  of  the  plasma,  as  determined 
bv  the  reduction  of  the  freezing-point,  corresponds  to  about 
A  0-56''  C,  corresponding  with  about  0-9  per  cent,  of  NaCl. 
The  urine  has  a  molecular  concentration  of  from  Al  to  2-5  or 
even  higher. 

4-.  The  urea  in  the  plasma  amounts  to  about  O'OS  per  cent. 
In  the  urine  it  is  about  2  per  cent.,  i.e.  it  is  concentrated  60 
times. 

5.  Uric  acid  is  concentrated  25  times. 
Ammonia  „  40      ,, 

PO,  „  30      „ 

SO,  „  60      „ 

2nd.  The  apparatus  which  brings  about  these  changes. — 
The  kidnev,  the  apparatus  which  has  to  effect  this  change,  is  a 
structure  evolved  from  the  nephridium  of  the  annelid. 

This  nephridium  consists  of  a  tube,  opening  by  a  funnel- 
shaped  end  into  the  coelomic  cavity  and  by  a  narrower  orifice 
on  the  surface.  The  tube  is  lined  by  a  syncytial  secreting 
epithelium,  in  which  may  be  seen  the  waste  particles  which 
occur  in  the  coelomic  fluid,  or  which  have  been  injected  into  the 
ccelome,  e.g.  Indian  ink.  It  thus  allows  an  escape  of  fluid 
from  the  cffilome  and  passes  out  waste  particles  through  the 
epithelium.  In  the  vertebrate  the  coelomic  orifice  of  each 
tubule  is  invaded  by  a  tuft  of  capillary  vessels,  forming  the 
glomerular  tuft  round  which  the  remains  of  the  funnel-shaped 
opening  persists  as  the  capsule  of  Bowman.  The  whole 
structure  is  often  called  a  Malpighian  body.  The  tubule  in  the 
vertebrate  becomes  enormously  lengthened  and  differentiated 
into  distinct  segments.  Myriads  of  these  Malpighian  bodies 
and  tubules  are  massed  together  to  form  the  kidney. 

This  may  be  briefly  described  as  follows  : — 


FORMATION   OF   URINE 


571 


I.  Structure  of  the  Kidney. 

{TJiis  must  he  studied  practically.) 
Each  kidney  presents  a  depression  or  hilus  on  its  inner  aspect 
from  which  the  ureter,  the  duct  of  the  kidney  passes,  and  by 
which  the  renal  artery  enters  and  the  renal  vein  emerges.  The 
nerves  and  lymphatics  of  the  organ  pass  along  with  these.  The 
whole  organ  is  enclosed  in  a  fibrous  capsule,  from  which  processes 
of  fibrous  tissue  carrying  small  blood-vessels  enter  the  organ. 


Fk;.  226. —Diagram  of  the  Structure  of  the  Kidney.  M.P.,  Malpighian 
pyramid  of  the  medulla ;  M.R.,  medullary  ray  extending  into  cortex  ; 
L.,  labyrinth  of  cortex;  M.B.,  a  Malpighian  body  consisting  of  the 
glomerular  tufi  and  Bowman's  capsule;  P.C.T.,  a  proximal  convoluted 
tubule;  H.L.,  Henle's  loop  on  the  tubule;  D.C.T.,  distal  convoluted 
tubule;  C.T.,  collecting  tubule;  R.A.,  branch  of  renal  artery,  giving 
off  I. LA,  interlobular  artery,  to  supply  the  glomeruli  and  the  con- 
voluted tubule;  ZL.Z?.,  interlobular  artery  bringing  blood  back  from 
the  cortex. 

The  ureter  opens  from  the  basin  of  the  kidney,  and  into  this 
the  renal  tissue  projects  as  pyramidal  processes. 

This  renal  tissue  is  clearly  divided  into  a  thin  outer  cortex 
and  an  internal  medulla.  This  latter  is  again  subdivided  into 
a  paler  pyramidal  part,  and  a  redder  part  between  this  and  the 
cortex— the  boundary  zone.  The  medulla  extends  out  into  the 
cortex  in  a  series  of  long  medullary  rays  (fig.  226),  so  that  the 


572  V^ETERINARY   PHYSIOLOGY 

cortex  may  be  subdivided  into  these  rays  and  the  parts  between 
the  rays — the  labyrinth  of  the  cortex. 

It  is  in  this  that  the  Malpighian  bodies  already  described 
are  situated. 

Extending  away  from  each  of  them  is  a  proximal  convoluted 
tubule,  also  in  the  labyrinth  {P.C.T.),  lined  by  pyramidal  and 
granular  epithelial  cells.  This  dives  into  the  boundary  zone  of 
the  medulla,  becomes  constricted  and  lined  by  a  transparent 
flattened  epithelium,  and  is  known  as  the  descending  limb  of 
the  looped  tubule  of  Henle.  Turning  suddenly  upwards  and 
becoming  lined  by  a  cubical  granular  epithelium,  it  forms  the 
ascending  limb,  and,  reaching  the  labyrinth  of  the  cortex,  it  ex- 
pands into  the  distal  convoluted  tubule  (D.C.T.),  which  resembles 
the  proximal.  It  opens  into  a  collecting  tubule,  running  in 
a  medullary  ray,  and  {G.T.),  lined  by  a  low  transparent  epi- 
thelium, which  conducts  the  urine  to  the  pelvis  of  the  kidney. 

The  renal  artery  breaks  up  and  gives  off  a  series  of  straight 
branches — the  interlobular  arteries  (IL.A.) — which,  as  they  run 
towards  the  surface,  give  off  short  side  branches  which  terminate 
in  the  glomeruli.  The  efferent  vein  passing  from  each  glomerulus 
breaks  up  again  into  a  series  of  capillaries  between  the  con- 
voluted tubules,  and  these  pour  their  blood  into  the  interlobular 
veins  (IL.V.).  This  arrangement  helps  to  maintain  a  high 
pressure  in  the  capillary  loops  of  the  glomerular  tuft. 

Nerves  to  the  kidney  in  the  dog  pass  in  the  splanchnic 
nerves  from  the  anterior  roots  of  the  sixth  to  the  thirteenth 
dorsal  nerves,  and  from  the  vagus,  chiefly  through  the  semilunar 
ganglion,  and  the  renal  plexus  upon  the  renal  blood-vessels. 
The  terminal  fibres  not  only  supply  the  arterioles,  but  may  be 
traced  into  the  secreting  cells  of  the  tubules. 

Apparently  the  old  differentiation  of  the  nephridium  with 
the  arrangement  for  allowing  the  escape  of  coelomic  fluid  and 
for  the  excretion  of  waste  material  is  preserved  in  the  vertebrate 
kidney,  but  now  the  coelomic  fluid  is  confined  to  blood-vessels 
which  are  related  to  both  the  coelomic  expansions  and  the 
tubules. 

II.  Physiology  of  the  Formation  of  Urine. 

Bowman,  from  his  investigations  of  the  structure  of  the 
kidney,  but  without  giving  consideration   to   its   phylogenetic 


FORMATION   OF   URINE  573 

development,  pointed  out  that  two  distinct  mechanisms 
exist — 

1st.  In  the  Malpighian  bodies,  an  arrangement  manifestly- 
suited  to  allow  of  filtration  from  the  blood. 

2nd.  In  the  tubules,  a  series  of  secreting  structures. 

A.  Malpighian  Bodies. 

1.  It  has  been  shown  by  injecting  acid  fuchsin,  which  is 
colourless  in  alkaline  solution  and  red  in  acid  solution,  into  the 
blood-vessels  that  the  urine  formed  in  these  bodies  is  alkaline 
in  reaction.  It  becomes  acid  as  it  passes  down  the  convoluted 
tubules. 

2.  It  is  also  known  that  these  bodies  are  thrown  out  of 
action  by  lowering  the  pressure  in  the  renal  arterioles  and  by 
decreasing  the  flow  of  blood  through  the  kidney. 

The  amount  of  blood  in  the  kidneys  may  be  measured  by 
enclosing  the  organ  in  a  closed  vessel  with  rigid  walls  connected 
with  a  piston  recorder — an  oncometer — so  that  changes  in  the 
volume  of  the  organ  may  be  recorded,  while  the  rate  of  flow  may 
be  estimated  by  measuring  the  amount  of  blood  coming  from 
the  renal  vein  (p.  473).  These  two  methods  are  frequently  used 
in  combination. 

(a)  Section  of  the  splanchnic  nerves  to  the  kidney  causes  a 
dilatation  of  the  renal  arterioles,  an  expansion  of  the  kidney,  and 
an  increased  flow  of  urine.  (6)  Stimulation  of  these  nerves  has 
the  opposite  eff"ect.  Sometimes  stimulation  with  slow  induction 
shocks  may  cause  a  dilatation,  but  the  action  of  these  dilator  fibres 
is  generally  masked  by  that  of  the  constrictors,  (c)  A  fall  in  the 
general  arterial  pressure,  to  about  50  mm.  Hg  in  the  dog,  causes 
a  decreased  flow  of  blood  through  the  kidney  and  practically 
stops  the  flow  of  urine,  although  the  tubules,  as  will  presently 
be  shown,  still  act.  (d)  Conversely,  a  stoppage  of  the  formation 
of  urine  may  be  brought  about  by  raising  the  pressure  in  the 
ureter  to  about  50  mm.  Hg. 

3.  In  the  frog  the  renal  arteries  supply  the  Malpighian 
bodies,  while  portal  veins,  from  the  posterior  end  of  the  animal, 
supply  the  convoluted  tubules.  Ligature  of  the  renal  arteries 
stops  the  flow  of  urine  ;  but  the  flow  may  be  again  induced  by 
injecting  urea  and  other  substances. 

4.  Even  when  this  flow  is  induced    dextrose,  egg  albumin 


574 


VETERINARY   PHYSIOLOGY 


or  peptone  when  injected  into  the  blood  are  not  excreted, 
although  in  the  frog  with  the  vessels  unligatured  they 
appear  in  the  urine. 

These  observations  seem  to  show  that  the  Malpighian  bodies 
have  to  do  chiefly  with  the  filtering  off  from  the  blood  plasma 
of  water  and  of  solids  held  in  true  solution,  and  that  their 
activity  depends  upon  the  rate  of  blood-flow  through  them,  and 
upon  the  pressure  in  the  glomerular  capillaries.  That  such  a 
purely  physical  process  is  involved  seems  to  be  indicated  by  the 
fact  that  a  free  flow  of  urine  (diiwesis)  can  be  induced,  without 
increasing  the  chemical  changes  in  the  kidneys,  as  indicated  by 


Fk;.  227.— To  show  the  reUitionship  between  the  i^roduction  of  urine  and  the 
consumption  of  oxygen  by  the  kidney  under  the  influence  of  Ringer- 
Solution  and  of  Sodium  Sulphate.  The  black  area  indicates  the  amount 
of  urine  secreted,  the  thin  line  the  consumption  of  oxygen.    (Barcroft.) 

the  oxygen  consumption,  by  injecting  into  the  blood  hypertonic 
solutions  of  various  salines,  such  as  NaCl,  which  dilute  the  blood 
and  increase  its  volume  (fig.  227). 

The  reason  why  the  formation  of  urine  stops  when  the 
arterial  pressure  falls  to  about  50  mm.  Hg  seems  to  be  due  to 
the  fact  that  the  filtration  pressure  must  be  well  above  the 
osmotic  pressure  of  the  colloids  of  the  blood  which  are  not  filtered 
off.  By  determining  the  difference  between  the  osmotic  pressure 
of  blood  serum  and  of  the  filtrate  from  it  through  a  semi-perme- 
able membrane  Starling  concluded  that  the  osmotic  pressure 
of  the  blood  proteins  is  about  30  mm.  Hg,  and  that  therefore  the- 
filtration  pressure    must  be   above  this.     When   this   osmotic 


FORMATION    OF   URINE  575 

pressure,  due  to  the  colloids,  is  decreased  formation  of  urine  goes 
on  at  a  lower  pressure  than  50  mm.  Hg.  He  showed  that  the 
formation  of  urine  is  increased  by  injecting  hypertonic  solutions 
of  glucose  which  caused  a  hydrsemic  plethora  by  producing  an 
endosmosis  from  the  tissues  to  the  blood  (p.  450),  and  he  further 
proved  that  this  is  not  simply  the  result  of  an  increased  pressure 
due  to  the  increased  volume  of  the  blood  by  demonstrating  that 
the  injection  of  fluids  of  high  colloidal  content  did  not  increase 
the  flow  of  urine. 

It  is  thus  clear  that  filtration  is  the  main  factor  in  the 
formation  of  urine  in  the  Malpighian  bodies,  and  the  membrane 
of  cells  through  which  this  occurs  must  form  a  semi-permeable 
membrane  which  prevents  the  passage  of  the  colloidal  proteins 
of  the  blood. 

On  the  other  hand,  that  a  selective  action  of  the  epithelium 
is  involved  seems  to  be  suggested  by  the  passage  into  the  urine 
in  Bowman's  capsule  of  such  large  molecules  as  those  of  egg 
albumin  and  haemoglobin  and  of  various  pigments  such  as 
carmine. 

The  point  of  practical  importance  is  that  the  secretion  of 
water  takes  place  cliiejiy  through  the  Malpighian  bodies,  and 
that  this  is  reduced  or  stopped  by  a  fall  in  the  general 
arterial  pressure,  such  as  occurs  in  failure  of  the  heart.  The 
decreased  excretion  of  water  may  lead  to  the  development  of 
dropsy. 

B.  The  Tubules. 

In  the  filtrate,  the  percentage  of  the  various  substances  in 
solution  cannot  be  higher  than  it  is  in  the  blood  plasma. 

But,  as  already  stated,  the  concentration  of  most  of  the  con- 
stituents of  the  plasma  is  generally  enormously  increased  in 
the  urine  while  the  reaction  is  acid. 

These  changes  must  be  effected  in  the  tubules. 

There  are  manifestly  two  ways  in  which  they  might  be 
brought  about. 

1.  By  the  secretion  of  solids  by  the  epithelium  from  the 
blood  to  the  urine  (Bowman  and  Heidenhain). 

2.  By  the  absorption  of  some  of  the  water  filtered  through 
the  crlomeruli  back  into  the  blood  (Ludwig).     Since  the  degree 


576 


VETERINARY  PHYSIOLOGY 


of  concentration  of  the  various  urinary  constituents,  compared 
with  their  amount  in  the  blood,  varies  very  greatly  (p.  570), 
some  of  these  substances  would  also  have  to  be  absorbed  along 
with  the  water. 

It  is  of  course  possible  that  both  these  processes  are  in 
operation. 

In  carrying  out  either  process  the  epithelium  has  to  do  an 
equal  amount  of  ivork. 

This  may  be  rendered  clearer  by  the  figure  228 — 


URINE 
IN  TUBULE 


CELLS 


BLOOD 


■^l||H,.0 

|,|li'i         reabsorbed. 


Ions   secreted. 


Fig.  228. — To  illustrate  the  two  views  of  the  mode  of  action  of  the  renal 
tubules.  The  figures  16-2  and  6"7  give  the  osmotic  pressures  of  the 
urine  and  of  the  blood  plasma  respectively. 


On  the  secretion  theory,  electrolytes  have  to  be  piled  up 
from  the  point  of  low  concentration  in  the  blood  to  the  point  of 
high  concentration  in  the  urine ;  while  on  the  reabsorption 
theory  water  has  to  be  taken  from  a  point  of  high  osmotic 
pressure  and  passed  to  a  point  of  low  osmotic  pressure. 

It  is  well  to  understand  what  the  reabsorption  theory 
implies. 

To  produce  the  average  30  grms.  of  urea  excreted  per  diem 
by  a  man  from  the  plasma  containing  0'03  per  cent,  would  mean 
the  filtration  through  the  glomeruli  of  some  30,000  cc.  But 
since  only  1500  cc.  of  urine  are  generally  secreted,  this  would 
mean  the  reabsorption  of  no  less  than  28,500  cc. — about  95 
per  cent,  of  what  was  filtered  off!  The  work  of  filtering  off 
some  30,000  cm.  has  superimposed  upon  it  the  work  of  reabsorb- 


FORMATION   OF   URINE  577 

ing  28,500  cc. !     The  process,  on  the  face  of  it,  seems  somewhat 
wasteful. 

But  such  evidence  as  has  been  procured  must  be  examined 
on  its  merits,  and  the  reabsorption  theory  cannot  be  rejected 
simply  because  it  seems  improbable. 

1.  Uric  acid  crystals  are  frequently  found  in  the  cells  of  the 
convoluted  tubules  of  the  kidney  of  birds.  Further,  uric  acid  is 
very  soluble  in  piperazine,  and  when  injected  in  solution  of  this 
substance  into  the  veins  of  a  mammal,  the  uric  acid  appears  in 
the  tubules  and  in  the  cells  of  the  convoluted  tubules,  but  not 
in  the  glomeruli  or  in  the  medulla. 

2.  Heideuhain,  by  injecting  into  the  circulation  of  the  rabbit 
a  blue  pigment — sulph-indigotate  of  soda — found  that  the  cells 
of  the  convoluted  tubules  take  it  up  and  seem  to  pass  it  into 
the  urine.  In  the  normal  rabbit  the  whole  of  the  kidney  and 
the  urine  became  blue.  But,  if  the  formation  of  urine  in  the 
Malpighian  bodies  be  stopped  by  cutting  the  spinal  cord  in  the 
neck  so  as  to  lower  the  blood  pressure,  then  the  blue  pigment 
is  found  in  the  cells  of  the  convoluted  tubules  and  of  the 
ascending  limb  of  Henle's  tubule,  since  it  is  not  washed  out 
of  these.  The  fact  that  later  investigators  have  found  after 
injection  of  aniline  blue  and  Congo  red,  that  these  pigments 
appear  first  in  the  part  of  the  cells  next  the  lumen  of  the  tubules 
seems  of  small  significance.  They  might  well  accumulate  there 
before  being  excreted. 

The  subcutaneous  injection  into  rats  of  pyrrol  blue  leads  to  the 
appearance  of  the  pigment  in  the  cells  of  the  convoluted  tubules 
but  not  in  the  Malpighian  bodies  nor  in  the  collecting  tubules. 

3.  When  the  Malpighian  bodies  of  the  frog  have  been 
thrown  out  of  action  by  ligaturing  the  renal  arteries,  the 
injection  of  urea  still  causes  a  flow  of  urine  and  the  excretion 
of  urea  by  the  tubules.  When  the  portal  veins,  which  supply 
the  tubules,  are  ligatured  on  one  side,  it  is  found  that  less 
urine  is  formed  on  the  ligatured  than  on  the  uniigatured 
side. 

4.  If  the  formation  of  urine  in  the  Malpighian  bodies  of  a 
dog  be  stopped  by  cutting  the  spinal  cord  in  the  neck,  the 
administration  of  certain  substances  such  as  of  caffeine, 
or  Na„SO^,  causes  an  increased  flow  of  urine,  although  the  blood 
pressure  in  the  kidneys  is  not  raised.     In   this   diuresis   the 

37 


578  VETERINARY   PHYSIOLOGY 

consumption  of  oxygen  by  the  kidney  is  increased,  indicating 
an  increased  metabolism  (fig.  227).  The  renal  cells  are  in  fact 
doing  work. 

These  last  experiments  seem  to  indicate  that  the  cells  of  the 
convoluted  tubules  are  capable  of  secreting  water  as  well  as 
solids. 

Large  doses  of  caffeine  poison  the  cells,  and  in  this  condition 
a  flow  of  urine  without  increased  consumption  of  oxygen  is 
produced.  This  supports  the  view  that  filtration  from  the 
glomeruli  plays  a  part. 

The  stimulating  action  of  such  drugs  as  caffeine  is  taken 
advantage  of  in  cases  of  heart-disease  when  the  secretion  of 
urine  is  almost  arrested  from  low  arterial  pressure  and  when 
dropsy  is  rapidly  advancing.  The  kidneys  may  be  stimulated 
to  get  rid  of  water  by  means  of  such  diuretics  until  compensa- 
tion of  the  heart  is  established. 

5.  The  fact  that  after  drinking  copious  amounts  of  water  a 
urine  of  a  lower  osmotic  pressure  than  the  plasma  may  be 
produced,  can  be  equally  well  explained  by  an  increased 
secretion  of  water  by  the  tubules  as  by  the  reabsorption  of 
solids  by  them. 

6.  The  action  of  extracts  of  the  hypophysis  cerebri  in  increas- 
ing the  flow  of  urine  while  actually  lowering  the  arterial 
pressure  (p.  594)  seems  to  indicate  a  direct  stimulation  of  the 
cells  of  the  tubules. 

7.  When  a  mixture  of  NaCl  and  of  Na.SO^  are  injected,  the 
proportion  of  the  latter  in  the  urine  increases  after  some  time, 
and  the  conclusion  has  been  drawn  that  the  NaCl  is  being 
reabsorbed  to  be  returned  to  the  blood.  If  this  were  the  case 
the  cells  would  do  work  and  the  0„  consumption  should  be 
increased.  But  the  injection  of  NaCl  leads  to  a  diuresis,  as 
explained  above,  which  is  not  accompanied  by  increased 
consumption  of  0„,  while,  on  the  other  hand,  the  diuresis 
caused  by  Na„S0^  is  accompanied  by  an  increased  metabolism 
of  the  kidney  (fig.  227). 

An  increased  secretion  of  Na^SO^  seems  to  explain  the  facts 
of  the  case  just  as  well  as  a  supposed  increased  absorption  of 
NaCl.  Similarly,  the  fact  that  when  NaCl  is  withheld,  it 
practically  disappears  from  the  urine,  although  it  persists  in  the 
blood    may  just    as    well    be    explained    on    the    theory   of  a 


FORMATION   OF   URINE  579 

decreased  elimination,  possibly  as  the  result  of  its  more  intimate 
association  with  the  blood  proteins  retarding  its  filtration,  as 
on  the  theory  of  an  increased  absorption. 

At  present  it  seems  that  no  conclusive  evidence  of  the 
theory  that  the  concentration  of  the  urine  depends  upon 
reabsorption  has  been  adduced.  Much  of  the  evidence  points 
to  an  active  secretion,  and  the  facts  which  do  not  directly  point 
to  this  may  be  explained  as  well  on  the  theory  of  secretion  as 
on  that  of  reabsorption.  A  verdict  of  not  proven  must  be  given, 
but  since  in  the  nephridial  tubules  of  the  annelid  the  epithelium 
is  secretory,  the  onus  of  proving  that  the  changes  in  the  con- 
centration and  reaction  of  the  urine  are  due  to  absorption  lies 
upon  the  supporters  of  this  theory. 

The  secretion  theory  does  not  raise  the  difficulty  of  explaining 
why  a  mechanism  involving  the  filtration  under  pressure  of  such 
an  enormous  quantity  of  water  with  the  sole  purpose  of  having 
it  again  reabsorbed  has  been  evolved,  or  of  attempting  to 
say  at  v/hat  stage  of  evolution  the  epithelium  of  the  nephridial 
tubules  reversed  their  function.  It  has  been  suggested  that, 
when  animals  became  terrestrial,  the  need  of  conserving  water 
arose  and  the  tubules  took  upon  themselves  this  function.  But 
the  tubules  of  aquatic  animals  are  as  well  developed  as  those  of 
terrestrial  animals. 

Those  who  accept  the  reabsorption  hypothesis  claim  that 
while  such  substances  as  urea  are  eliminated  as  fully  as  possible, 
other  substances  which  are  noi'mally  present  in  the  blood  in 
appreciable  amounts  are  reabsorbed  to  the  extent  of  maintain- 
ing that  amount.  The  first  set  of  substances  they  call  "  non- 
threshold  substances,"  the  second  "  threshold  substances." 
But  the  differentiation  between  these  is  just  as  readily 
explained  on  a  theory  of  secretion  as  on  a  theory  of  reab- 
sorption. 

While  the  evidence  at  present  forthcoming  does  not  directly 
point  to  the  changes  in  the  urine  as  it  passes  down  the  tubules 
being  due  to  reabsorption,  it  by  no  means  excludes  the 
possibility  that  some  reabsorption  may  take  place. 

The  extraordinary  differences  in  the  structure  of  the 
epithelium  in  the  convoluted  tubules  on  the  one  hand,  and  of 
the  looped  tubules  of  Henle  on  the  other,  suggests  the  possibility 
that  different  processes  may  be  carried  on  in  these  parts,  that 


580  VETERINARY    PHYSIOLOGY 

possibly  secretion  may  occur  in  the  former  and  absorption  in 
the  latter. 


The  kidney  responds  readily  to  very  small  changes  in  the 
concentration  or  composition  of  the  blood.  Dilution,  even  to  an 
amount  insufficient  to  disturb  the  blood  pressure,  may  lead  to 
increased  secretion  of  water,  and  the  increase  of  various  salts  in 
the  blood,  and  especially  of  anions,  may  bring  about  an  increase 
of  secretion.  Thus  an  arrangement  is  secured  by  which  the 
composition  of  the  blood  plasma  is  kept  constant,  and  its  carry- 
ing capacity  for  carbon  dioxide  is  regulated.  Hence  renal 
disease  mav  induce  disturbance  in  the  respirations  (p.  527). 


The  Influence  of  the  Nervous  System  on  the  Kidneys. 

That  renal  secretion  is  fundamentally  independent  of  the 
control  of  the  central  nervous  system  is  shown  by  the  facts 
(1)  that  it  goes  on  after  the  nerves  to  the  kidneys  have  been 
cut;  (2)  that  it  proceeds  normally  in  a  kidney  which  has 
been  excised  and  transplanted. 

As  already  indicated  (p.  573),  stimulation  of  the  splanchnic 
nerves  causes  a  constriction  of  the  renal  vessels  and  a  stoppage 
of  the  formation  of  urine.  Stimulation  of  the  vagus  by 
inhibiting  the  heart  and  lowering  the  arterial  pressure  causes 
a  fall  in  the  secretion  of  urine.  Stimulation  below  the  cardiac 
branch,  or  after  its  cardiac  endings  have  been  poisoned  with 
atropine,  seems  to  produce  no  definite  result. 

The  action  of  the  nervous  system  is  therefore  probably 
entirely  through  the  vaso-motor  mechanism.  Vaso-constriction 
may  be  brought  about — 

1st.  By  direct  stimulation  of  the  vaso-constrictor  centre  as 
in  asphyxia. 

2ncl.  Reflexly  by  stimulation  of  many  ingoing  nerves, 
e.g.  (a)  by  the  application  of  cold  to  the  skin  ;  (6)  by 
irritation  of  the  bladder  or  urethra  after  the  use  of  the 
catheter. 

Vaso-dilator  effects  on  the  kidney  are  produced  by  stimu- 
lating the  posterior  roots  of  the  lower  dorsal  nerves,  which  may 
explain   the  beneficial   action  of  warm   applications  over  the 


EXCRETION    OF  URINE  581 

loins  in  suppression  of  urine.  There  is  some  evidence  that 
slight  obstruction  of  a  ureter  may  also  cause  a  reflex  vaso- 
dilatation with  increased  secretion  of  urine. 


III.  EXCRETION   OF  URINE. 

1.  Passage  from  Kidney  to  Bladder. — The  pressure  under 
which  the  urine  is  secreted  is  sufficient  to  drive  it  along  the 
ureters  to  the  bladder.  If  these  are  obstructed,  the  pressure 
behind  the  obstruction  rises,  and  may  distend  the  ureters  and 
the  pelvis  of  the  kidney,  and  when  it  reaches  about  50  mm.  Hg 
in  the  dog,  the  secretion  of  urine  is  stopped.  The  muscular 
walls  of  the  ureters  show  a  rhythmic  peristaltic  contraction, 
which  must  also  help  the  onward  passage  of  the  urine  to  the 
bladder. 

2.  Micturition — As  the  urine  accumulates  in  the  urinary 
bladder,  the  viscus  expands  to  accommodate  it,  the  tone  of 
the  visceral  muscular  fibres  being  adapted  to  the  degree  of 
distension.  The  backward  passage  of  the  urine  into  the  ureters 
is  prevented  by  the  way  in  which  these  tubes  pass  obliquely 
through  the  muscular  coat  of  the  bladder.  When  a  certain 
distension  is  reached,  rhythmic  contractions  are  produced, 
which  become  more  and  more  powerful.  These  are  primarily 
dependent  on  the  muscular  fibres ;  but  the  wall  of  the  bladder 
is  richly  supplied  with  peripherally  placed  neurons,  and  the 
possible  action  of  these  in  controlling  the  contractions  has  not 
been  excluded.  Even  after  section  of  the  nerves  to  the  bladder, 
this  peripheral  mechanism  is  capable  of  controlling  the  act  of 
micturition. 

This  involves  not  merely  the  contraction  of  the  wall  of  the 
bladder,  but  also  the  relaxation  of  the  visceral  muscular  fibres 
(the  sphincter  trigonalis)  which  surround  the  neck  of  the 
bladder,  and  of  the  striped  fibres  which  surround  the  upper 
part  of  the  urethra. 

It  is  therefore  an  act  requiring  the  co-ordinated  contraction 
and  relaxation  of  muscles,  and  it  is  presumably  presided  over 
by  the  nervous  mechanism  in  the  wall  of  the  bladder. 


582  VETERINARY  PHYSIOLOGY 

Normally  this  is  controlled  by  the  central  nervous  system. 
The  bladder  is  supplied  hy  the  ijelvic  nerve,  in  which  white  fibres 
run  to  the  peripheral  plexus,  and  by  si/mpathetic  fibres  which 
have  their  cell  stations  in  the  inferior  mesenteric  ganglion,  from 
which  post-ganglionic  fibres  run  to  the  bladder.  In  most 
animals  the  former  are  chiefly  augmentor,  and  the  latter 
inhibitory,  but  in  some  animals,  e.g.  the  ferret,  the  reverse  is 
the  case.  Adrenalin  causes  relaxation  or  contraction,  according 
to  whether  the  inhibitory  or  augmentor  fibres  run  in  the 
sympathetics. 

The  nerves  are  derived  from  a  centre  in  the  lumbar  region 
of  the  spinal  cord  which  normally  controls  the  peripheral 
mechanism,  and  which  may  be  reflexly  excited  by  the  passage 
of  some  urine  from  the  bladder  into  the  urethra  or  in  other 
wa3'S,  e.g.  in  the  dog  by  sponging  the  anus  with  warm  water. 

In  some  cases  of  inflammation  of  the  spinal  cord  (myelitis), 
the  increased  activity  of  the  centre  may  prevent  the  expulsion 
of  urine,  while  later  in  the  disease,  when  the  nerve  structures 
have  been  destroyed,  the  urine  is  not  retained  and  dribbles 
away  on  account  of  the  absence  of  the  tonic  contraction  of  the 
sphincter  arrangement. 

The  expulsion  of  the  last  drops  of  urine  is  carried  out  by 
the  rhythmic  contraction  of  the  bulbo-cavernous  muscle ;  while 
the  peristaltic  contraction  of  the  bladder  wall  is  assisted  by  the 
various  muscles  which  press  upon  the  contents  of  the  abdomen 
and  the  bladder.  The  horse  micturates  standing,  but  the  ox 
can  do  so  while  walking. 

In  the  young,  micturition  is  a  purely  reflex  act,  and 
in  the  dog  it  is  perfectly  performed  when  the  spinal  cord  is  cut 
in  the  back.  As  age  advances,  the  reflex  mechanism  comes  to 
be  more  under  the  control  of  the  higher  centres,  and  the  activity 
of  the  sphincters  may  be  increased  or  abolished  as  circumstances 
indicate. 

IV.  Excretion  by  the  Skin. 

The  skin  is  really  a  group  of  organs,  and  some  of  these  have 
been  already  studied.  (The  structure  of  the  skin  and  its 
appendages  must  be  studied  practically.) 

(1)  The  Protective  functions  of  the  horny  layer  of  epidermis, 


SKIN  583 

with  its  development  in  hair,  and  of  the  layer  of  sub- 
cutaneous fat,  are  manifest. 

Hair. — The  hairy  coat  of  animals  maintains  a  layer  of  air 
next  the  skin  at  a  more  equable  temperature  than  that  of  the 
surrounding  air,  and  so  plays  an  important  part  in  the  regula- 
tion of  temperature  (p.  269). 

The  strong  hairs  developed  about  the  muzzle  and  in  the 
eyelashes  are  tactile  organs  (p.  102).  Attached  to  each  hair 
follicle  is  a  band  of  non-striped  muscle,  the  arrector  pili,  which 
can  erect  the  hair  by  contracting.  These  muscles  are  under  the 
control  of  the  central  nervous  system,  and  the  nerve  fibres  have 
been  demonstrated  in  the  cat  to  take  much  the  same  course  as 
the  vaso-constrictor  fibres  of  somatic  nerves.  They  belong  to 
the  true  sympathetic  set  of  nerves. 

A  hair  after  a  time  ceases  to  grow,  and  the  lower  part  in 
the  follicle  is  absorbed  and  the  hair  is  readily  detached.  From 
the  cells  in  the  upper  part  of  the  follicle,  a  new  down-growth 
occurs,  a  papilla  forms  and  the  hair  is  regenerated.  In  the 
horse  this  process  occurs  twice  a  year,  and  the  thickness  of  the 
coat  grown  depends  upon  the  degree  of  exposure  to  cold.  The 
hair  of  the  mane  and  tail  and  the  tactile  hairs  are  not  shed 
with  the  rest  of  the  coat. 

(2)  The  Sensory  functions  have  been  studied  under  the 
Receptors  (p.  99  et  seq.). 

(3)  The  Respiratory  action  of  the  skin  in  mammals  is  of 
little  importance. 

(4)  The  Excretory  Function  of  the  Skin. 

Two  sets  of  glands  develop  in  the  skin — sweat  glands  and 
sebaceous  glands. 

A.  Sweat  Glands. 

1.  Structure. — The  sweat  glands  are  simple  tubular  glands 
coiled  up  in  the  subcutaneous  tissue  with  ducts  opening  on  the 
surface  of  the  skin.  The  secreting  epithelium  somewhat 
resembles  that  of  the  convoluted  tubules  of  the  kidney.  Sweat 
glands  are  Avidely  distributed  over  the  skin  of  the  horse.  In 
oxen  and  sheep  they  are  less  abundant,  being  most  developed 
on  the  muzzle.  In  the  dog  and  cat  they  are  found  in  the  nose 
and  in  pads  of  the  feet. 


584  VETERINARY   PHYSIOLOGY 

2.  Functions. — From  these  glands,  a  considerable  amount  of 
sweat  is  poured  out ;  but  to  form  any  estimate  of  the  daily 
amount  is  no  easy  matter,  since  it  varies  so  greatly  under 
different  conditions  (p.  269).  When  poured  out,  sweat  evapor- 
ates, and  in  doing  so  causes  loss  of  heat.  When  large  quantities 
are  formed,  or  when,  from  coldness  of  the  surface,  or  of  the  air, 
or  from  the  large  quantity  of  watery  vapour  already  in  the  air, 
evaporation  is  prevented,  it  accumulates,  and  when  it  evaporates 
causes  loss  of  heat.  Hence  the  importance  of  grooming  after 
exercise.  In  the  horse  the  salts  of  evaporated  sweat  may 
accumulate  on  the  coat  if  evaporation  is  allowed. 

A  free  secretion  of  sweat  is  usually  accompanied  by  a 
dilatation  of  the  blood-vessels  of  the  skin,  but  this  may  be 
absent,  and  it  may  occur  without  any  sweat  secretion,  e.g. 
under  the  influence  of  atropine.  The  secretion  of  sweat  and 
the  condition  of  the  blood  vessels  play  an  important  part  in 
regulating  the  temperature  of  the  body  (p.  269). 

3.  Nervous  Mechanism  of  Sweat  Secretion. — That  the  sweat 
glands  are  under  the  control  of  the  central  nervous  system 
may  be  demonstrated  in  the  cat.  The  sweat  glands  are  chiefly 
in  the  pads  of  the  feet,  and,  if  a  cat  be  put  in  a  hot  chamber,  it 
sweats  on  the  pads  of  all  its  feet.  But  if  one  sciatic  nerve 
be  cut  the  foot  supplied  remains  dry.  If  the  cat  be  placed 
in  a  warm  place  and  the  lower  end  of  the  cut  sciatic  stimu- 
lated, a  secretion  of  sweat  is  produced.  The  secreting  fibres 
for  the  sweat  glands  run  in  the  true  sympathetic  system.  They 
leave  the  cord  by  the  anterior  roots  in  the  thoracico-abdominal 
region,  pass  to  the  sympathetic  ganglia,  where  they  have  their 
cell  stations.  From  these,  nou-medullated  fibres  pass  back  by 
the  grey  ramus  into  the  somatic  branch  of  the  nerve  and  so 
onwards  to  plexuses  round  the  sweat  glands. 

The  centres  presiding  over  these  nerves  are  distributed 
down  the  medulla  and  cord.  They  are  capable  (a)  of  reflex 
stimulation,  as  when  pepper  is  taken  into  the  mouth  ;  and 
(6)  of  direct  stimulation  (i.)  by  a  venous  condition  of  the  blood, 
as  in  the  impaired  oxygenation  of  the  blood  which  so  fre- 
quently precedes  death  as  the  respirations  fail,  and  (ii.)  by  a  rise 
in  the  temperature  of  the  blood  supplied  to  them. 

Even  after  the  nerves  of  the  sweat  glands  are  cut,  the 
glands  may  be  stimulated  by  certain  drugs,  e.g.  pilocarpine. 


SKIN  585 

Adrenaliu  causes  so  powerful  a  contraction  of  the  cutaneous 
vessels  that  any  stimulating  action  it  may  have  upon  the  sweat 
glands  is  masked.  The  action  of  heat  seems  also  to  be  chiefly 
peripheral,  setting  up  an  unstable  condition  of  the  gland  cells  so 
that  they  respond  more  readily  to  stimulation. 

4  Chemistry  of  Sweat. — Sweat  from  the  horse  is  a  sherry- 
coloured  fluid,  which,  when  pure,  has  a  neutral  or  faintly 
alkaline  reaction.  Its  specific  gravity  is  about  1020  in  the 
horse,  and  it  contains  about  5'5  per  cent,  of  solids,  of  which 
5  per  cent,  are  inorganic  and  about  0"5  organic.  When  the 
sweat  dries  on  the  coat  a  white  deposit  is  left.  Potassium  is  the 
most  abundant  base.  Chlorides  are  present  in  small  amounts. 
The  chief  organic  substances  present  are  proteins — some 
globulin  and  some  albumin.  Fat  is  also  present,  probably 
derived  from  the  sebaceous  secretions,  and  it  combines  with  the 
potassium  to  form  a  soap. 

B,  Sebaceous  Glands. 

The  sebaceous  glands  are  simple  racemose  glands  which 
open  into  the  hair  follicles,  and  their  function  is  to  supply 
an  oily  material  to  lubricate  the  hairs.  This  secretion  is  pro- 
duced by  the  shedding  and  breaking  down  of  the  cells  formed 
in  the  follicles  of  the  glands.  Those  lining  the  basement  mem- 
brane are  in  a  condition  of  active  division,  but  the  cells  thrown 
off  into  the  lumen  of  the  follicle  disintegrate  and  become  con- 
verted into  a  semi-solid  oily  mass,  which  consists  of  free  fatty 
acids  and  of  neutral  glycerol  and  cholesterol  fats.  These 
cholesterol  fats  are  the  lanolins,  which  differ  from  ordinary 
fats  in  being  partly  soluble  in  water.  Free  cholesterol  is  also 
present  in  the  sebum. 

Grooming.— This  is  of  great  importance  in  the  horse.  It 
removes  salts  of  the  sweat,  shed  epithelium,  and  loose  hairs  and 
dirt.  It  prevents  the  development  of  mange  and  of  lice,  and  it 
acts  as  a  form  of  massage  to  the  skin  and  subjacent  muscles. 


SECTION    VIII. 

THE   REGULATION   OF   GROWTH   AND   FUNCTION. 

In  all  the  members  of  a  species  the  course  of  the  chemical 
changes  in  the  various  tissues  and  organs  are  fairly  constant 
and  depart  but  little  from  a  normal  course.  Upon  these 
changes  depend  not  only  the  development  and  growth  of 
each  tissue  and  organ  and  of  the  animal  as  a  whole,  what  might 
be  called  the  static  adaptation  to  surrounding  conditions,  but 
also  the  various  responses  to  changes  in  external  conditions,  the 
functional  adaptation.  The  development  and  the  activities 
of  each  organ  are  co-related  and  co-ordinated  with  those  of  all 
the  other  organs,  and  in  this  co-relation  three  main  factors 
play  a  part. 

I.  Heredity. 

This  is  primarily  the  result  of  the  chemical  changes 
inherited  from  the  parents.  The  principle  of  Inertia,  that — 
"  Every  particle  of  matter  in  the  universe  remains  in  a  state 
of  rest  or  of  uniform  motion  in  a  straight  line,  unless  it  is 
acted  upon  by  external  force,"  is  applicable  to  living  as  w.ell 
as  to  dead  matter. 

Generation  after  generation  a  similar  piece  of  protoplasm 
the  ovum,  undergoing  the  same  molecular  movements,  is  placed 
in  the  same  external  conditions,  and  hence  must  undergo  the 
same  course  of  development,  under  the  influence  of  what  may 
be  called  Hereditary  Inertia.  A  proof  of  it  is  afforded  by  the 
development,  both  structural  and  functional,  of  embryonic 
tissues  removed  from  the  body  and  kept  in  the  plasma  of  the 
animal  blood.  A  fragment  of  the  cell  mass  from  which  the 
heart  develops  undergoes  the  change  into  the  muscular  fibres  of 


REGULATORS  587 

the  heart,  and  these  manifest  their  characteristic  regular 
rhythmic  pulsation. 

Scraps  of  some  organs  when  removed  and  transplanted  in 
other  parts  of  the  body  may  grow,  and  the  cells  may  multiply 
and  develop  into  those  characteristic  of  the  organ  from  which 
they  were  taken. 

Hereditary  inertia  seems  to  be  all-powerful  in  early  embry- 
onic life,  and  its  influence  extends  on  into  the  adult  condition. 
Hooker  found  that,  even  after  all  that  part  of  the  central  nervous 
system  from  which  the  nerves  to  the  heart  arise  has  been 
destroyed  in  the  tadpole,  the  heart  develops  and  beats  in  the 
usual  way. 

Its  influence  is  dominant  both  in  structural  develop- 
ment and  in  the  development  of  functional  activity,  not  only 
in  such  simple  actions  as  cardiac  contraction,  but  in  the  most 
complex  responses  of  the  central  nervous  system  upon  which 
the  conduct  of  the  individual  depends. 


II.  The  Nervous  System. 

As  development  advances,  the  nervous  system  comes  to 
play  a  part  in  the  regulation  of  metabolism,  and  thus  in  the 
development  of  the  static  adaptations.  Its  effect  is  seen  in 
the  failure  of  regeneration  after  removal  of  structures  in  some 
invertebrates  and  in  many  amphibia  if  the  nerves  to  the  part 
are  destroyed.  It  is  also  seen  in  infantile  paralysis,  a  disease 
which  follows  destruction  of  the  cells  of  the  anterior  horn  of 
grey  matter.  The  growth  of  the  limb  connected  with  that  part 
of  the  spinal  cord  becomes  arrested.  The  condition  of  herpes 
zoster,  or  shingles,  a  painful  eruption  of  vesicles  on  the  skin 
over  a  nerve,  has  already  been  considered  (p.  91)  as  an  example 
of  the  trophic  influence  of  the  nervous  system. 

The  part  played  by  the  nervous  system  in  regulating  and 
co-ordinating  the  functional  adaptations,  i.e.  the  activities  of  one 
structure  with  those  of  others  has  been  repeatedly  indicated  in 
the  previous  pages.  The  effect  of  a  heightened  arterial 
pressure  in  inhibiting  the  heart  through  the  inferior  cardiac 
branch  of  the  vagus  may  be  taken  as  an  example;  and  that  this 


588  VETERINARY   PHYSIOLOGY 

is  fundamentally  an  alteration  in  the  course  of  metabolism  is 
indicated  by  Gaskell's  observation  that  the  vagus  is  an  anabolic 
nerve  (p.  421). 


III.  Chemical  Regulation. 

Yet  another  factor  plays  an  important  part.  The  products 
of  the  metabolism  of  one  structure  have  an  effect  upon  other 
structures,  and  so  co-relate  and  regulate  their  reciprocal 
activity. 

It  has  already  been  shown  that  the  carbon  dioxide  pro- 
duced in  muscle  stimulates  the  respiratory  centre  (p.  527).  The 
slightest  increase  in  the  Ch  of  the  blood  increases  the  activity 
of  the  kidneys  (p.  580).  A  product  of  the  activity  of  the 
duodenum — secretin — has  been  shown  to  cause  secretion  by 
the  pancreas  (p.  822). 

Chemical  regulation  plays  a  very  important  part  in  verte- 
brate animals,  and  special  organs  have  been  evolved  which 
have  as  their  function  the  elaboration  of  products  which  are 
passed  into  the  blood,  each  to  produce  definite  and  specific 
actions  in  the  body. 

Such  structures  may  be  called  glands  with  internal  secre- 
tions, or  endocrinetes.  The  name  of  hormones,  from  op/xaco 
"  I  excite,"  has  been  suggested  for  the  internal  secretions,  but, 
since  they  do  not  always  increase  functional  activity,  but  may 
check  it,  the  name  is  unsuitable. 

Before  deciding  that  any  structure  is  a  true  endocrinete, 
it  is  necessary  to  prove  that  it  produces  a  specific  product  or 
products  with  definite  and  specific  actions. 

The  evidence  required  is  of  various  kinds. 

1.  The  effects  of  removal  may  be  studied,  and  if  these  are 
definite — 

2.  The  effects  of  transplanting  a  part  of  the  structure  in 
preventing  the  onset  of  the  changes  may  be  tried.  If  this  is 
successful — 

3.  The  effects  of  removing  the  graft  may  be  observed. 

4.  The   effects  of  administering  extracts  of  the  structure 


REGULATORS  5S9 

either  with   or  without   its  previous  removal  may  be   investi- 
gated. 

This  method  has  been  largely  used,  but  it  must  be  employed 
with  great  care  for  the  following  reasons  : — 

i.  The  method  of  extraction  may  fail  to  remove  the  active 
product  from  the  structure. 

ii.  The  method  of  extraction  may  remove  all  sorts  of  con- 
stituents of  the  structure,  and  any  result  produced  may  be  due 
to  the  combined  action  of  many  substances. 

iii.  Products  of  decomposition  may  be  removed  either  alone 
or  along  with  the  active  substance,  and  when  administered  may 
produce  symptoms  which  may  be  ascribed  to  an  active  con- 
stituent which  may  not  exist.  The  demonstration  of  the  rapid 
development  in  decomposition  of  amines  having  a  powerful 
physiological  action  shows  that  this  is  a  real  danger. 

iv.  When  massive  doses  of  the  extracts  produce  an  effect, 
there  is  a  danger  in  concluding  that  a  similar  action  is  produced 
by  the  amount  normally  poured  into  the  blood,  but  it  has  been 
shown  that  a  massive  dose  may  produce  a  totally  different  effect 
from  a  small  dose. 

V.  It  has  been  found  that  the  action  of  these  extracts  may 
be  materially  altered  by  the  functional  condition  of  the 
structure  to  which  they  are  applied;  e.g.  the  uterus  of  the 
virgin  guinea-pig  may  respond  quite  differently  from  that 
of  the  recently  pregnant  animal. 

vi.  The  method  of  administration  may  modify  the  action. 
Thus,  while  the  intravenous  injection  of  the  product  of  the 
medulla  suprarenalis  causes  very  marked  symptoms,  these  may 
be  entirely  absent  when  it  is  injected  under  the  skin  or  given 
by  the  mouth. 

vii.  Lastly,  the  danger  of  expectancy  on  the  part  of  the 
observer  must  not  be  overlooked.  This  is  of  no  small  import- 
ance in  the  therapeutic  administration  of  such  extracts. 

Classification  of  the  Endocrinetes — These  endocrinetes  are 
derived  from  different  parts  of  the  embryo,  and  they  may  be 
arranged  according  to  their  embryological  source.  Such  a 
classification  is  more  satisfactory  than  one  based  upon  their 
anatomical  position,  for,  in  several  cases,  two  of  these  organs, 
entirely  separate  in  origin,  structure,  and  function,  have  come 


590 


VETERINARY   PHYSIOLOGY 


to    lie  in   close  juxtaposition   and   so   to    constitute    a    sini 
organ  according  to  anatomical  nomenclature. 


I.  From  the  Nervous  System. 

1.  Chromaffin  Tissue  (Medulla  suprai'enalis).< — 

2.  Hypophysis  Cerebri. 

II.  From  the  Buccal  Cavity. 

3.  Thyreoid.< 

4.  Pituitaiv.^ 


III. 


IV. 


From  the  Intestine. 

5.  Pancreas. 

6.  Mucosa  of  Small  Intestine. 

From  the  Branchial  Arches. 

7.  Parathyreoids.< 

8.  Thymus. 
V.   From  the  Mesothelium  of  the  Genital  Ridge. 

9.  Gonads. 
10.   Inter-renal  Bodies  (Cortex  suprarenalis).  ■? 

The  pairs  which  occur  in  anatomical  juxtaposition  are  indicated 
by  joining  lines. 


I.  From  the  Nervous  System. 
1.  Chromaffin  Tissue. 
This  in  mammals  is  chiefly  disposed  as  the  medullary  part 
of  the  suprarenal  bodies.  But  smaller  masses  are  found  along 
the  aorta  and  some  of  the  large  arteries.  In  fishes  it  lies 
entirely  separate  from  the  equivalent  of  the  cortex  suprarenalis — 
the  inter-renal  tissue. 

1.  Development. — It  is  developed  from  the  emigrating  cells 
which  form  the  true  sympathetic  or  visceral  system  of  nerves 
(p.  54). 

2.  Structure. — It  consists  of  rather  large  irregular  cells 
containing  granules  which  stain  of  a  brown  colour  with  chrome 
salts — hence  the  names  of  the  tissue.  These  cells  occupy 
spaces  between  large  sinusoid  capillary  vessels. 

3.  Physiology. —  (I)  It  is  impossible  to  remove  the  chromaffin 
tissue  in  mammals  without  removing  the  inter-renal  tissue  of 


REGULATORS 


591 


the  cortex  with  it.  But  recently  Vincent  has  attempted  com- 
pletely to  destroy  the  medulla  in  dogs  by  thermocautery,  and 
he  has  found  that  the  animals  remain  apparently  normal. 

(2)  The  discovery  that  the  injection  of  extracts  of  the 
medulla  suprarenalis  gives  rise  to  marked  symptoms  was  the 
first  step  to  the  explanation  of  its  functions.  The  demonstra- 
tion that  the  active  constituent  is  a  definite  chemical  substance, 
Adrenalin,  has  enabled  physiologists  to  carry  out  investigation 
with  great  precision.     Adrenalin  is — 


HO- 
HO- 

Ortho-dioxy-phenyl. 


H     H 

I       I 
— C  — C- 

I       I 
OH   H 


ethanol. 


H     H 

I        I 
_N— C— H 

I 
H 


methyl-amine. 


It  has  been  prepared  synthetically. 

It  may  be  recognised  by  its  reaction  with  chrome  salts,  and 
by  the  ease  with  which  it  is  oxidised,  especially  in  alkaline 
solutions.  On  account  of  this  it  strikes  a  green  colour  with 
ferric  chloride.  With  phosphotungstates  along  with  phosphoric 
acid  it  gives  a  blue  colour,  and  by  this  test  1  part  in  3  million 
may  be  detected. 

That  it  is  a  product  of  the  chromaffin  tissue  is  shown  by  the 
fact  that  the  staining  of  the  medulla  is  proportionate  to  its 
physiological  activity,  and  by  the  great  amount  of  adrenalin 
in  the  blood  coming  from  the  suprarenals.  By  pressing  on  the 
suprarenals  its  amount  in  the  blood  may  be  increased.  Section 
of  the  branches  of  the  splanchnic  nerve  going  to  the  suprarenals 
checks  its  liberation,  while  stimulation  increases  it. 

It  acts — 

(1)  On  the  Blood-vessels.  When  injected  into  a  vein  or 
perfused  in  Ringer's  Solution  through  the  vessels,  it  causes 
constriction  of  the  peripheral  arterioles,  and  thus  raises  the 
arterial  blood  pressure  (Practical  Physiology).  Its  action  is 
most  powerful  on  the  abdominal  vessels,  and  hence  blood  is 
forced  to  the  muscles  of  the  limbs.  The  vessels  of  the  skin, 
however,  are  contracted.  On  the  pulmonary  vessels,  and  on 
the   intracranial    vessels   its   action  is  slioht.  while  it   causes 


592  VETERINARY   PHYSIOLOGY 

dilatation  of  the  coronary  arteries.  Hoskins  finds  that  in  very 
small  doses  it  causes  dilatation  of  all  the  arterioles.  That  it 
does  not  act  directly  on  the  muscle  fibres  is  shown  by  the  fact 
that,  after  the  administration  of  apocodeine  which  poisons  the 
endings  of  the  abdomino-thoracic  or  true  sympathetics,  it  no 
longer  acts,  although  barium  salts  still  cause  a  contraction, 
because  theyact  directly  on  the  muscle  (p.  455).  Ergotoxin  poisons 
the  endings  of  the  augmentor  fibres  of  the  true  sympathetic, 
and,  after  it  has  been  administered,  adrenalin  may  cause  a 
dilatation  of  vessels  because  its  action  on  the  endings  of  the 
inhibitory  fibres  is  thus  unmasked. 

In  fact,  the  action  of  adrenalin  is  absolutely  specific,  and 
consists  in  a  stimulation  of  the  endings  of  the  true  sympathetic 
nerves.  It  thus  produces  on  any  organ  the  effect  which  is 
produced  by  stimulating  these  nerves. 

(2)  On  the  Heart.  If  the  vagi  are  intact,  it  produces  a 
slowing  which  is  due  to  the  raised  arterial  pressure  (p.  420). 
When  the  vagi  are  cut  it  causes  an  acceleration  and  generally 
an  increased  amplitude  of  contraction  by  stimulating  of  the 
augmentor  endings. 

(3)  On  the  Alimentary  Canal  it  acts,  as  does  stimulation  of 
the  splanchnic  nerves,  by  inhibiting  peristalsis  and  stimulating 
the  sphincters  (p.  333). 

(4)  On  the  Bladder  its  action  varies  in  different  animals 
according  to  whether  motor  or  inhibitory  fibres  pass  to  the 
organs  through  the  splanchnics  by  way  of  the  inferior 
mesenteric  ganglion  and  hypogastric  nerves  (p.  582).  In  the 
former  case  it  causes  contraction  (ferret) ;  in  the  latter  case 
relaxation  (cat). 

(5)  On  the  Uterus  it  generally  causes  contraction,  but,  in 
the  virgin  uterus,  it  may  cause  relaxation,  showing  that  its 
action  may  be  modified  by  the  functional  condition  of  the 
tissue. 

(6)  On  the  Iris,  when  applied  to  the  excised  eye  of  the 
frog,  it  has  the  same  action  as  stimulation  of  the  cervical 
sympathetic — it  dilates  the  pupil.  But  in  mammals  this  occurs 
only  when  (a)  the  superior  cervical  ganglion  has  been  excised — a 
procedure  which  is  supposed  to  make  the  nerve  terminations 
more  sensitive  ;  and  (h)  in  some  cases  of  de-pancreatic  diabetes 
(p.  357).    Hence  it  has  been  concluded  that  an  internal  secretion 


REGULATORS  593 

of  the  pancreas  may  inhibit  the  action  of  adrenalin  on  these 
terminations. 

(7)  It  acts  on  the  sweat  glands,  which  are  supplied  by  the 
true  sympathetic  nerves,  but  its  action  is  masked  by  the 
constriction  of  the  vessels  which  supply  these  glands. 

(8)  On  the  kidney  its  constricting  action  on  the  arterioles  leads 
to  a  decreased  production  of  urine. 

(9)  As  already  indicated  (p.  356),  injections  of  extracts  of 
the  suprarenal  bodies  profoundly  modify  the  metabolism,  leading 
to  an  increase  of  sugar  in  the  blood  and  to  its  excretion  in  the 
urine.  This  is  best  marked  when  the  animal  is  well  fed  and 
has  a  store  of  glycogen  in  its  liver  ;  but,  since  it  occurs  in 
fasting  animals,  after  the  stored  carbohydrates  have  been 
markedly  reduced  by  the  administration  of  phloridzin  (p.  357), 
it  would  appear  to  be  due  in  part  to  an  increased  production  of 
sugar  from  proteins.  It  has  been  suggested  that  the  supra- 
renal secretion  acts  through  the  pancreas  by  preventing  the 
formation  of  the  internal  secretion  which  checks  carbohydrate 
metabolism  in  the  liver  (see  p.  356).  But  the  fact  that  it 
produces  glycosuria  in  the  bird  after  the  pancreas  has  been 
excised  negatives  this  view.  The  universality  of  the  law  that 
adrenalin  acts  on  the  terminations  of  true  sympathetic  nerves, 
and  the  fact  that  stimulation  of  these  nerves  to  the  liver  causes 
a  glycosuria,  indicates  that  it  probably  acts  on  these  nerve 
endings.  This  is  supported  by  the  fact  that  it  does  not  cause 
glycosuria  after  the  administration  of  ergotoxin. 

4.  Nervous  Control  of  the  Chromaffin  Tissue. — The  supply  of 
adrenalin  to  the  body  from  the  chromaffin  tissue  is  influenced 
by  the  nervous  system  through  the  splanchnic  nerves,  pregan- 
glionic medullated  fibres  of  which  go  to  the  gland.  Various 
injuries  to  the  central  nervous  system  may  stimulate  these 
nerves,  and  a  glycosuria  may  be  thus  produced,  as  in  Bernard's 
diabetic  puncture  (p.  356).  Since  this  does  not  occur  when  the 
suprarenals  are  removed,  it  has  been  supposed  that  it  is  caused 
by  the  excessive  supply  of  adrenalin  to  the  blood.  But  the 
amount  in  the  blood  after  Bernard's  puncture  is  insufficient  to 
cause  a  glycosuria.  The  adrenalin  probably  acts  merely  as  an 
adjuvant  to  nerves. 

5.  Significance  of  Adrenalin. — The  amount  of  adrenalin 
normally  present  in  the  blood  is  quite  insufficient  to  exercise 

38 


594  VETERINARY   PHYSIOLOGY 

any  marked  physiological  effect  or  to  account  for  the  tone  of  the 
arterioles.  An  amount  sufficient  to  act  upon  them  causes 
marked  paralysis  of  the  gut.  It  is  to  be  regarded  as  a  reserve 
stimulant  which  is  called  upon  when  the  true  sympathetic 
system  is  powerfully  stimulated,  as  it  is  in  various  disturbances 
of  the  central  nervous  system  which  are  accompanied  by  such 
emotional  conditions  as  fright  or  anger.  The  stimulation  of 
the  true  sympathetics  leads  to  the  increased  action  of  the 
heart  and  the  contraction  of  the  abdominal  vessels  with  the 
increased  flow  of  blood  to  the  muscles,  and  to  the  other 
physical  accompaniments  of  the  emotions  which  are  preparations 
for  meeting  the  conditions  producing  them.  If  the  stimulus  is 
sufficiently  powerful,  the  effect  is  augmented  and  sustained 
by  an  outpouring  of  adrenalin. 

5.  Detection  of  Adrenalin  in  the  Blood. — The  most  delicate 
method  for  testing  the  amount  of  adrenalin  is  the  inhibitory 
action  upon  a  strip  of  intestine  in  oxygenated  Ringer's  solution 
at  the  body  temperature.  This  action  is  manifested  by  as  little 
as  1  in  400  million. 

6.  Toxic  Action. — The  administration  of  large  doses  of 
adrenalin  may  cause  death  from  pulmonary  congestion.  Re- 
peated doses  result  in  degenerative  changes  in  the  liver,  and 
in  a  thickening  of  the  inner  coat  of  the  arteries. 


2.  Hypophysis  Cerebri. 

1.  Development. — Tliis  is  formed  as  a  hollow  downgrowth 
from  the  base  of  the  third  ventricle  of  the  brain.  In  some 
animals  the  stalk  remains  open,  but  in  man  it  is  closed 
(fig.  230). 

2.  Structure. — It  forms  what  is  anatomically  the  posterior 
lobe  of  the  pituitary  body  lying  in  the  sella  turcica.  It  is  com- 
posed of  neuroglia  cells,  but  in  it  are  frequently  found  little 
masses  of  colloid  and  cells  resembling  those  of  the  inter- 
mediate part  of  the  pituitary  (p.  599)  a  structure  which  closely 
embraces  the  hypophysis. 

3.  Physiology. — (a)  Removal  produces  no  marked  symptoms. 
(h)  Extracts,  when  injected  into  a  vein  cause — 

(1)  A  rise  of  blood  pressure  from  constriction  of  the 
arterioles.     If  the  dose  be  repeated  within  half  an  hour,  this 


REGULATORS  595 

may  be  replaced  by  a  dilatation  and  fall  of  pressure.  In  birds 
the  first  dose  produces  this  dilator  effect.  In  the  renal 
arterioles  it  causes  a  dilatation,  and  in  the  coronary  arteries  a 
constriction,  thus  differing  from  adrenalin. 

(2)  On  the  heart  it  acts  to  increase  the  force  of  contraction, 
whether  the  vagus  is  intact  or  is  cut.  After  section  of  the 
vagus  it  does  not  accelerate  the  heart  as  does  adrenalin. 

(3)  Upon  the  iris  it  acts  like  adrenalin. 

(4)  On  the  intestine,  uterus,  and  bladder  it  acts  as  an 
excitant,  and  it  also  increases  the  effect  of  stimulating  the 
hypogastric  nerves  on  the  last  two  organs. 

(5)  It  causes  a  great  outpouring  of  milk  from  the 
mammary  glands,  probably  by  its  action  on  the  walls  of  the 
ducts,  but  it  has  no  influence  on  milk  secretion. 

(6)  It  has  a  marked  diuretic  action,  and,  since  this  occurs 
even  after  a  second  dose  when  the  arterial  blood  pressure 
falls,  it  has  been  ascribed  to  a  specific  stimulating  action  on 
the  secreting  cells  of  the  kidney. 

(7)  Its  action  on  metabolism  requires  further  investigation. 
According  to  some  investigators,  it  causes  glycosuria. 

So  far  the  chemical  nature  of  the  active  principles,  which 
may  be  called  Hypophysin,  has  not  been  ascertained,  although 
active  crystalline  proilucts  have  been  prepared  Like  adrenalin, 
it  is  not  destroyed  by  heating.  Whether  it  is  a  normal  product, 
and  whether  it  has  any  physiological  significance,  has  yet  to  be 
proved.  Herring's  results  seem  to  show  that  the  passage  of  a 
colloidal  material  may  be  traced  into  the  ;erebro-spinal  fluid. 
It  is  entirely  formed  in  the  hypophysis  or  lu  the  pars  intermedia 
of  the  pituitary.  Some  observations  by  Herring  tend  to  show 
that  the  action  on  milk  flow  and  on  the  uterus  is  more  particu- 
larly due  to  a  product  of  the  latter. 

II.  From  the  Buccal  Cavity. 
3.  Thyreoid   Gland. 
1.  Development. — This  structure  is  formed  as  a  hollow  out- 
growth from  the  anterior  part  of  the  alimentary  canal,  which 
breaks  up  into  numerous  branches. 

-'  The  gland  was  named  after  the  shield-like  cartilage  of  the  larynx.  Since 
dvpeos  is  a  shield,  and  dvpos  a  door,  it  should  be  called  thyreoid.  The  name 
thyroid  given  by  British  anatomists  is  manifestly  erroneous. 


596 


VETERINARY   PHYSIOLOGY 


2.  Structure. — It  early  loses  its  connection  with  the  ali- 
mentary canal,  and  becomes  cut  up  by  fibrous  tissue  into  a 
number  of  small  more  or  less  rounded  cysts  or  follicles,  each 
lined  with  cubical  epithelium  and  filled  with  a  mucus-like 
colloid  substance,  with  a  marked  affinity  for  acid  stains 
(fig.  229).  It  is  enormously  vascular  and  has  a  rich  supply 
of  lymphatics. 

3,  Chemistry — The  colloid  substance  is  characterised  by 
containing-  iodine,  but  the  amount  varies  in  different  animals 
and  in   the  same  animal  according  to    the  mode   of  feeding. 


Fig.  229.— Section  through  Part  of  the  Thyreoid  (TA.)  and  a  Parathyreoid  (P.) 
of  a  Mammal. 


A  very  stable  organic  compound  of  iodine,  known  as  iodothyrin, 
may  be  prepared,  but  this  is  actually  combined  in  a  globulin- 
compound  which  is  the  active  constituent  of  the  organ,  and  is 
known  as  iodothyreoglobulin.  Kendall  has  described  a  crystal- 
line product  of  definite  composition  containing  the  indol  nucleus 
(p.  330)  with  iodine  attached  to  the  benzene  ring — a  thyreo- 
oxyindol. 

4.  Physiology. — In  1873  Gull  described  a  peculiar  disease 
chiefly  affecting  women  which  has  received  the  name  of  myx- 
cedema  (p.  597),  and  in  1877  Ord  was  able  to  show  that  it  is 
associated   with   atrophy   of  the   thyreoid.      Kocher   and   the 


I 


I 


REGULATORS  597 

Reverdins  in  1882  described  a  somewhat  similar  condition  after 
removal  of  the  thyreoids  in  operations  for  goitre.  In  the  last 
decade  of  the  nineteenth  century  the  experimental  investiga- 
tion of  the  effects  of  removal  was  taken  up. 

(1)  Removal. — A  diflSculty  is  experienced  in  studying  the 
effects  of  removal,  inasmuch  as  the  parathyreoids  lie  embedded 
in  the  thyreoid  of  most  animals,  and  in  close  juxta-position  to 
it  in  others,  and  care  is  required  to  leave  a  sufficient  amount  of 
these  to  carry  on  their  functions. 

When  proper  precautions  are  taken,  it  is  found  that  the 
effect  of  removal  of  the  thyreoid  in  young  animals  is  to  check 
the  growth,  and  especially  to  check  the  growth  of  cartilage 
in  developing  bone  (p.  46).  This  leads  to  marked  shortening 
of  the  long  bones,  causing  stunted  growth.  The  basis  cranii  is 
also  aflFected,  and,  since  the  intra-membranous  bones  continue 
to  grow,  the  frontal  bones  tend  to  arch  forward.  The  gonads 
do  not  develop,  but  remain  infantile.  The  animal  is  generally 
dull — lethargic. 

In  adults  the  changes  are  less  prominent.  Muscular  weak- 
ness and  apathy  are  marked.  The  hair  falls  out,  and  the  tem- 
perature is  low.  There  is  often  a  peculiar  swelling  of  the  skin, 
which  does  not  pit  on  pressure.  The  rate  of  metabolism  is 
markedly  decreased.  The  mobilisation  of  carbohydrates  is 
lowered  and  the  carbohydrate  tolerance  is  raised.  Removal  of 
the  thyreoid  decreases  the  glycosuria  produced  by  removal  of 
the  pancreas,  and  in  this  respect  the  influence  of  the  thyreoid 
co-operates  with  that  of  the  chromaffin  tissue  in  facilitating 
the  mobilisation  of  carbohydrates,  which  is  held  in  check 
by  the  pancreas.  The  functions  of  the  sexual  organs  are 
disturbed.  The  condition  is  sometimes  known  as  cachexia 
strumipriva. 

(2)  Hypothyreoidism  (Decreased  Functional  Activity). — (a) 
Iji  the  IToung. — The  thyreoid  may  be  congenitally  imperfectly 
developed  in  man  and  animals,  and  all  the  conditions  described 
under  the  eff"ects  of  removal  of  the  gland  are  in  a  very  marked 
degree,  (b)  In  the  adult  human  subject. — When  the  thyreoid 
atrophies,  the  disease  myxcedema  is  produced.  The  sufferer 
manifests  the  symptoms  described  above  as  cachexia  strumi- 
priva. 

(3)  Transplantation. — If,  when  the  symptoms  following  re- 


598  VETERINARY   PHYSIOLOGY 

moval  have  developed,  a  small  piece  of  the  tissue  of  the  thyreoid 
is  grafted  in  a  suitable  part  of  the  body,  blood-vessels  may  grow 
in,  the  tissue  may  survive  and  increase,  and  this  may  bring  about 
a  disappearance  of  symptoms.  On  removing  the  graft  the 
symptoms  recur. 

(4)  Extracts. — The  administration  of  extracts  of  the  thyreoid 
subcutaneously,  or  by  the  mouth  as  was  shown  by  Murray, 
frequently  leads  to  the  disappearance  of  the  symptoms  of 
removal  or  deficiency,  and  this  line  of  treatment  is  now  uni- 
versally used  in  cases  of  cretinism  and  myxoedema  in  man. 
lodothyreo-globulin  or  thyreo-oxyindol  appears  to  be  the  active 
principle. 

The  Avay  in  which  the  thyreoid  acts  upon  the  metabolism 
is  demonstrated  by  the  study  of  the  effects  of  its  administra- 
tion to  normal  animals.  When  given  in  large  doses  over 
loug  periods,  all  the  symptoms  of  hyperthyreoidism  may  be 
produced. 

Tadpoles  fed  on  thyreoid  tissue  undergo  a  very  rapid  develop- 
ment, and  Hoskins  and  Herring  have  shown  that  in  young  white 
rats  the  continued  administration  of  thyreoid  causes  an  extra- 
ordinary increase  in  the  suprarenals,  heart,  kidneys,  and  pancreas 
with  a  decrease  in  the  size  of  the  pituitary  in  the  female.  The 
main  effect  is  to  increase  metabolism. 

A  further  effect  is  to  activate  the  terminations  of  the 
true  sympathetics  and  of  the  para-sympathetics  of  all  the 
visceral  nerve  fibres.  Hence  both  the  augmentor  and  the 
inhibitory  terminations  in  the  heart  are  activated,  and  the 
heart  responds  more  readily  to  stimulation  of  the  vagus  or  of 
the  augmentor.  The  same  seems  to  be  the  case  with  the  nerve 
terminations  in  the  blood-vessels,  e.g.  the  reflex  response  to  the 
depressor  nerve  (p.  418).  Since  these  thyreoid  preparations 
act  upon  the  true  sympathetic  termination,  they  facilitate  the 
action  of  adrenalin.  Further,  the  suprarenals  are  supplied  by  the 
splanchnic  nerves,  and  apparently  the  active  principle  of  the 
thyreoid  facilitates  the  action  of  these,  and  so  increases  the 
output  of  adrenalin. 

(5)  Hyperthyreoidism. — {Increased  Functional  Activity). — 
This  condition  in  man  is  known  as  Graves  Disease  or  exoph- 
thalmic goitre.  It  is  characterised  by  a  condition  of  hyper- 
excitability   and   sleeplessness,   rapid    action   of   the   heart,   a 


I 

I 


KEGULATORS  599 

tendency  to  flushing  from  increased  vaso- motor  activity, 
sweating,  increased  secretion  of  urine,  often  prominence  of  the 
eyeballs  and  enlargement  of  the  thyreoid  forming  a  soft  goitre. 
The  rate  of  metabolism  is  markedly  accelerated,  proteins  are 
more  rapidly  broken  down,  and  carbohydrates  are  too  rapidly 
mobilised  and  hence  sugar  may  appear  in  the  urine.  The 
symptoms  may  all  be  explained  in  terms  of  the  action  of 
thyreoid  extracts.  The  prominence  of  the  eyeballs  in  man  is 
probably  due  to  stimulation  of  visceral  muscular  fibres  in  the 
eyelids  by  which  they  are  unduly  opened  and  the  bulging  of 
the  eyeball  allowed  to  take  place.  It  may  occur  in  lower 
animals. 

In  simple  goitre  the  thyreoid  tissue  undergoes  a  slow  hyper- 
trophy and  no  general  symptoms  are  manifest. 

(6)  Nervous  Control. — Taking  as  indices  (a)  the  sensitising 
action  of  the  internal  secretion  of  the  thyreoid  on  the  abdominal 
sympathetic  nerve  ;  and  (6)  the  decrease  in  the  amount  of  iodine 
in  a  lobe,  it  has  been  found  that  stimulation  of  the  nerves 
supplying  the  gland  leads  to  an  increased  output  of  the 
internal  secretion. 

The  thyreoid  thus  seems  to  produce  an  internal  secretion 
rich  in  iodine  which  exercises  a  stimulating  effect  upon  the 
metabolism. 

Whether  it  does  so  by  a  direct  action,  or  whether  through 
the  autonomic  nervous  system,  upon  which  it  undoubtedly 
acts,  cannot  at  present  be  decided. 


4.  Pituitary. 
The  true  pituitary  is  the  anterior  part  of  the  pituitary  of 
anatomists. 

1.  Development.— It  is  formed  by  a  hollow  outgrowth  from 
the  roof  of  the  buccal  cavity,  and  it  lies  in  front  of  and  embraces 
the  hypophysis. 

2.  Structure. — It  consists  of  (1)  an  anterior  j^art,  composed  of 
dense  columns  of  cells  of  two  kinds— (a)  the  chief  cells,  which 
are  large  and  do  not  stain  readily;  (b)  the  chromophil  cells 
which  contain  granules,  some  staining  with  acid,  some  with 
basic  stains.  It  is  very  vascular.  (2)  A  2^^'^^^  intermedia, 
separated  from  the  former  by  a  cleft  and  applied  closely  to 


600  VETERINARY  PHYSIOLOGY 

the  hypophysis,  and  consisting  of  cells  with  colloid  material 
between  them. 

3.  Physiology. — (a)  Removal  of  the  true  pituitary  is  generally 
rapidly  fatal;  muscular  tremors,  slow  pulse  and  respiration,  and 
a  fall  of  temperature  preceding  death.  Partial  removal  in 
young  animals  is  followed  by  decreased  growth,  persistence  of 
the  infantile  characters,  and  arrested  growth  of  the  gonads. 
Often  there  is  an  accumulation  of  fat.  The  thyreoid  is  generally 
hypertrophied. 

(h)  Acromegaly,  a  disease  in  man  characterised  by  greatly 
increased  growth  of  the  bones,  and  more  especially  of  the 
intra-membranous  bones,   and   with    increased   growth    of  the 


fars  inlermed    \^ 


i>»Of 


'mi 


Pars  tuU  .     ^•'Y')j 


Fig.  230. — Longitudinal  section  through  the  Hypophysis  and 
Pituitary.      (Edinger.) 

subcutaneous  fibrous  tissue,  has  been  associated  with  disease  of 
the  pituitary.  According  to  Gushing,  it  shows  two  phases : — 
First,  the  irritative  phase,  in  which  there  is  an  increased  growth 
of  bone  and  a  premature  development  of  the  testis,  and,  second, 
the  destructive  stage,  in  which  the  testes  atrophy  and  the  sexual 
functions  are  in  abeyance.  Very  probably  the  development  of 
giantism  is  associated  with  increased  activity  of  the  pituitary, 
for,  in  most  cases,  an  enlargement  of  the  sella  turcica  has  been 
described. 

The  fatal  effects  of  removal  may  apparently  be  delayed  for 
a  time  by  the  transplantation  of  a  part  of  the  gland,  but  the 
grafts  do  not  persist.  Administration  of  extracts  of  the 
pituitary  has  not  given  conclusive  results.  It  has '  been 
claimed  that  a  substance  of  definite  composition  which  has 
been  called  tethelin  may  be  prepared  from  it,  which  first 
decreases  then  increases  the  growth  of  young  mice  and  causes 
a  persistence  of  the  soft  coat  of  the  young  animal. 


REGULATORS  601 

A  physiological  hypertrophy  occurs  in  pregnancy,  during 
which  the  ovarian  functions  are  in  abeyance,  the  chief  cells 
being  increased.  In  the  male,  removal  of  the  testes  leads  to 
hypertrophy  of  the  pituitary. 

While  the  pituitary  thus  exercises  a  stimulating  action  on 
the  growth  of  the  connective  tissues  and  of  the  gonads,  the 
latter  appear  to  have  a  checking  action  on  the  pituitary. 
More  work  upon  this  is  required. 


III.  From  the  Intestine. 
0.  Pancreas. 

The  development  and  structure  of  the  pancreas  have  been 
described  (p.  300). 

The  effects  of  removal  in  producing  the  condition  of  diabetes 
have  been  considered  (p.  357),  and  it  has  been  shown  that  the 
organ  produces  an  internal  secretion  which  checks  the  mobilisa- 
tion of  sugar  in  the  liver,  and  possibly  facilitates  its  utilisation 
by  the  muscles.  The  transplantation  of  a  piece  of  pancreas 
prevents  the  onset  of  these  symptoms,  but  the  administration 
of  pancreas  or  of  extracts  of  the  pancreas  does  not  do  so. 

The  internal  secretion  of  the  pancreas  acts  in  the  opposite 
direction  to  that  of  the  chromaffin  tissue  and  the  thyreoid. 
The  fact  that,  after  removal  of  the  pancreas,  adrenalin  causes 
dilatation  of  the  pupil  seems  to  indicate  that  its  internal 
secretion  inhibits  the  action  of  the  termination  of  the  true 
sympathetic  in  the  iris,  and  therefore  probably  also  in  the 
liver. 

It  is  probably  the  islets  of  Langerhans  which  yield  the 
active  principle.  The  true  islets  are  developed  early  in  foetal 
life  from  the  epithelium  of  the  ducts.  A  case  has  been 
described  in  which  excision  of  a  piece  of  pancreas,  which  had 
been  left  after  partial  removal  of  the  organ,  led  to  glycosuria, 
and  in  which  the  fragment  was  found  to  have  degenerated  and 
to  be  composed  entirely  of  islet  tissue. 

6.  The  Mucous  Membrane  of  the  Small  Intestine. 
The  production  of  secretin  and  its  action  on  the  pancreas 
have  been  dealt  with  on  p.  321. 


■f 


602 


VETERINARY   PHYSIOLOGY 


IV.  From  the  Branchial  Arches. 

7.  Thymus. 

1.  Development. — The  thymus  is  formed  as  epithelial  out- 
growths from  the  branchial  arches — in  mammals  from  the 
ventral  side  chiefly  of  the  third  and,  to  a  lesser  extent,  from  the 
fourth  cleft. 

2.  Position. — In  man  and  in  most  mammals  it  lies  in  the 
thorax  just  in  front  of  the  heart.     Islets  of  thymus  tissue  are 


'  T< 


Fig.  231. 


-Section  of  the  Lobules  of  the  Thymus  to  show  the  Lobules, 
with  Hassall's  Corpuscles  in  the  Central  Part. 


frequently  found  in  and  around  the  thyreoid.     In  the  guinea- 
pig  it  is  entirely  in  the  neck. 

3.  Structure — It  is  composed  of  two  lobes,  each  made  up 
of  a  series  of  separate  lobules  surrounded  by  a  fibrous  capsule 
and  showing  a  denser  cortical  and  a  less  dense  medullary  part. 
It  consists  essentially  of  a  network  of  epithelial  cells,  which,  in 
the  medullary  part,  are  here  and  there  massed  together  to  form 
concentric  agglomerations  of  cells,  some  in  a  state  of  degenera- 
tion— the  Hassall's  Corpuscles.  In  the  meshes  of  the  network 
are  lymphocyte-like  cells  which  are  probably  derived  from 
outside  the  gland,  but  which,  according  to  some  observers,  are 
formed  from  the  epithelial  cells  (fig.  231). 


REGULATORS  603 

4.  Life  History. — The  thymus  reaches  its  greatest  size,  in 
relationship  to  the  weight  of  the  body,  about  the  time  of 
birth  ;  it  continues  to  grow  till  puberty,  when  it  begins  to 
atrophy,  being  replaced  by  fatty  tissue.  In  adult  life,  it  is 
reduced  to  a  mass  of  adipose  tissue  with  only  some  islands  of 
thymus  substance.  Conditions  of  malnutrition  lead  to  a  tem- 
porary atrophy  of  the  gland. 

5.  Physiology. — (1)  Removal. — In  young  guinea-pigs  this 
produces  no  marked  symptoms.  In  young  dogs  very  different 
results  have  been  recorded  by  different  investigators  ;  but  the 
most  recent  series  of  experiments  shows  no  observable  differ- 
ence between  normal  pups  and  those  deprived  of  their  thymus. 
Some  investigators  describe  a  peculiar  sluggish  condition  with 
manifestations  of  muscular  fatigue ;  others  state  that  a  condi- 
tion of  decreased  calcification  of  the  bones  and  the  peculiar 
enlargement  of  their  ends,  characteristic  of  rickets,  are  produced  ; 
but  rickets  develops  very  readily  in  puppies,  and  just  as  readily 
in  those  with,  as  in  those  without,  the  thymus.  A  thymusless 
pup  may  escape  rickets  while  other  members  of  the  litter  may 
develop  it. 

(2)  Feeding  tadpoles  with  thymus  leads  to  continued  growth 
and  absence  of  development,  while  feeding  with  thyreoid  leads 
to  more  rapid  development, 

(3)  After  castration  of  male  animals,  the  thymus  persists  in 
adult  life.  If  thymus  and  testes  are  both  removed,  the  growth 
of  the  animal  is  delayed.  It  would  thus  seem  as  if  the  thymus 
and  testes  co-operate  in  stimulating  growth,  and  that,  if  one  of 
these  structures  is  removed,  a  compensatory  hypertrophy  of  the 
other  occurs.  As  the  testes  increase  in  size,  the  thymus  begins 
to  atrophy  and  to  play  a  less  important  part.  Similar  relations 
Avith  the  ovaries  have  not  been  established. 

8.  Parathyreoids. 

1.  Development. — These  are  formed  as  epithelial  outgrowths 
from  the  dorsal  aspect  of  the  third  and  fourth  branchial  clefts 
on  each  side,  there  being  thus  two  on  each  side. 

2.  Position. — The  parathyreoids  formed  from  the  third 
clefts,  in  most  animals,  lie  close  to  the  thyreoid  lobes,  but 
outside  of  them.  Those  from  the  fourth  clefts  are  generally 
embedded   in   them.     In   man,    both    sets    lie    outside   of  the 


60-i  VETERINARY   PHYSIOLOGY 

thyreoid.  Supplementary  parathyreoids  are  frequent,  and  in 
some  animals,  e.g.  cats,  they  are  embedded  in  the  thymus.  It 
is  therefore  impossible  to  be  sure  that,  after  removing  the  usual 
four  parathyreoid  bodies,  a  considerable  amount  of  parathyreoid 
tissue  is  not  left.  This  explains  the  negative  results  got  by 
some  experimenters. 

3.  Structure. — Each  consists  of  columns  of  cells  (a)  the  chief 
cells,  large  and  not  staining  readily,  (6)  oxyphil  cells,  smaller 
than  the  last,  and  with  granules  staining  with  eosin.  Masses 
of  colloid  material  may  occur,  giving  the  structure  somewhat 
the  appearance  of  the  thyreoid  gland  (fig.  229). 

4.  Physiology. — (1)  Removal. — Since  1882  it  has  been  known 
that,  after  removal  of  the  thyreoid  gland  for  goitre  in  the  human 
subject,  a  peculiar  condition  of  spasm  of  the  muscles,  and  even 
of  convulsions  leading  to  death,  may  occur.  This  was  first 
ascribed  to  removal  of  the  thyreoid,  but  in  1896  Vassal e  and 
Generali  definitely  showed  by  experiments  upon  animals  that  it 
is  due  to  removal  of  the  parathyreoids,  and  that,  if  a  sufficient 
amount  of  their  tissue  is  left,  the  symptoms  do  not  develop. 

The  symptoms  are  (i)  depression  and  emaciation ;  (ii)  tonic 
contraction  of  various  muscles,  chiefly  the  extensors ;  (iii) 
tremors  and  jerkings  of  the  muscles,  which  may  go  on  to 
a  general  convulsion.  Sometimes  these  disturbances  of  the 
neuro-muscular  system  are  accompanied  by  disturbances  of 
balancing.  (iv)  A  peculiar  increase  in  the  excitability 
of  the  peripheral  motor  neurons,  so  that  if  a  motor 
nerve  is  compressed  or  tapped  a  violent  convulsive  movement 
of  the  muscles  supplied  by  it  may  be  produced,  while  the 
response  to  galvanic  stimulation  is  enormously  heightened, 
(v)  In  the  dog  increased  rate  of  the  heart  and  of  the  respirations. 
The  symptoms  vary  greatly  from  time  to  time,  and  may  remain 
latent  except  for  the  increased  excitability  of  the  nerves  which 
persists. 

The  spasticity  and  tremors  are  due  to  the  implication  of  the 
motor  neurons  of  the  spinal  cord  and  are  arrested  by  cutting 
the  nerve  to  the  muscles.  The  increase  in  the  response  to 
the  stimulation  of  peripheral  nerves  is  due  to  an  increased 
excitability  of  the  nerve  endings  in  the  muscles.  These  two 
conditions  are  not  necessarily  proportional  to  one  another. 

All   the  symptoms  are  due  to  a   poison  developed  in  the 


\ 


REGULATORS  605 

body  and  present  in  the  blood.  They  are  all  temporarily 
removed  by  bleeding  and  transfusing  with  a  0*9  per  cent. 
NaCl  solution. 

NH 
II 
The  evidence  points  to  guanidin,  NH^  —  C  —  NHo 

NH  CH3 

II       I 
or  to  methyl  guanidin,  NHo  —  C  —  N  —  H,  as  the  toxic  sub- 
stance. 

The  administration  of  the  salts  of  these  reproduces  all  the 
symptoms  following  removal  of  the  parathyreoids,  while  the 
amount  in  the  blood  and  in  the  urine  is  increased  after 
parathyreoidectomy. 

The  parathyreoids  thus  seem  to  regulate  the  guanidin 
metabolism  of  the  body  and  to  prevent  such  an  increase  as  will 
lead  to  symptoms.  There  is  evidence  that  guanidin  or 
methyl  guanidin  liberated  in  the  body  is  linked  to  acetic  acid 
to  form  the  non-toxic  creatin  (p.  209). 

In  rats  which  have  survived  removal  of  the  parathyreoids 
lying  beside  the  thyreoid,  but  which  presumably  have  some 
parathyreoid  tissue  left,  defective  calcification  of  the  teeth  has 
been  observed.  This  is  probably  associated  with  a  decrease 
in  the  calcium  of  the  blood.  A  decrease  in  the  growth  of  the 
bones  and  changes  resembling  those  of  rickets  have  also  been 
described. 

(2)  Transplantation  of  parathyreoid  tissue  has  been  found  to 
abolish  the  symptoms,  and  they  recur  when  the  graft  is  excised, 

(3)  Extracts. — Beebe  has  isolated  a  nucleo-protein  which, 
according  to  some  observers,  suppresses  or  mitigates  the 
symptoms  of  tetany. 


V.  From  the  Mesotheliuin  of  the  Genital  Ridges. 

9.  The  Gonads,  or  Sex  Glands. 
1.  Development. — These  are  formed  by  ingrowths  of  meso- 
thelial  cells  over  the  genital  ridge  of  the  embryo. 

A.    Testes. — In  the  male,  these  cells,  for   the    most    part 


606  VETERINARY  PHYSIOLOGY 

become  arranged  in  tubules,  forming  (a)  the  spermatogonia, 
from  which  the  spermatozoa  are  produced  (p.  620),  and  (6) 
certain  larger  cells,  the  supporting  cells  of  Sertoli,  (c)  Some 
of  the  mesothelial  cells  remain  outside  and  between  the  tubules, 
and  form  the  interstitial  cells  of  Ley  dig.  These  are  large  cells, 
and  they  contain  a  large  amount  of  lipoids.  Some  observers 
maintain  that  they  are  really  connective  tissue  cells. 

B.  Ovaries. — In  the  ovaries  the  ingrowing  mesothelial  cells 
form  separate  masses,  the  Graafian  follicles,  (a)  One  of  the 
cells  enlarges  and  becomes  the  ovum,  the  female  gamete  (p.  619) ; 
while  (6)  the  others  remain  smaller  and  form  the  cells  of  the 
zona  granulosa,  (c)  In  many  animals  between  the  Graafian 
follicles  a  considerable  number  of  the  mesothelial  cells  remain 
as  the  interstitial  cells  of  the  ovary. 

The  interstitial  cells  of  the  testis  and  ovary  I'esemble  one 
another  very  closely  and  are  both  very  similar  to  the  cells  of 
the  inter-renal  organ  which  forms  the  cortex  suprarenalis. 
They  are  very  rich  in  lipoids  and  especially  in  cholesterol 
compounds. 

2.  Physiology. — The  part  played  by  the  gonads  in  the  process 
of  reproduction  will  be  considered  later  (p.  618).  At  jjresent 
their  action  as  endocrinetes  has  to  be  dealt  with. 

A.   Testes. 

(1)  Removal- — Removal  of  the  testes  in  young  boys  and  in 
young  animals  leads  to  a  persistence  of  the  infantile  type  of  body, 
and  to  the  absence  of  development  of  the  secondary  sexual 
structures,  such  as  the  prostate  gland,  the  hair  of  the  body  and 
face  in  men,  the  horns  of  sheep  and  cattle,  and  the  antlers  of  deer. 

The  temperament  is  generally  phlegmatic,  and  hence  a 
gelded  horse  is  more  easily  managed.  Castrated  animals  fatten 
more  readily.  The  cartilaginous  growth  of  bone  tends  to  persist, 
and  hence  the  bones  tend  to  be  longer  and  more  slender  than 
in  the  entire  animal. 

(2)  Precocious  Development. — In  man  this  is  accompanied 
by  premature  development  of  the  secondary  sexual  orgaa.s,  by 
abnormal  growth  of  hair,  prematui'e  union  of  the  epiphyses  of 
the  long  bones,  and  increased  growth  of  the  bones  in  thickness. 
These  conditions  have  been  observed  in  connection  with  tumour 


REGULATORS  607 

growths  of  one   testis,  and  removal   of  the  tumour  has  been 
followed  by  their  disappearance. 

A  similar  condition  is  associated  with  hypertrophic  changes 
in  the  cortex  suprarenalis  (p.  610),  and  with  irritative  changes  in 
the  pituitary  (p.  600).  Whether  these  act  through  the  testes 
or  directly  is  not  known, 

(3)  Transplantation — The  first  demonstration  of  the  action 
the  endocrinetes  was  afforded  by  Berthold,  who  showed  that 
transplanting  the  testis  into  a  capon  leads  to  the  develop- 
ment of  the  typical  sexual  characters  of  the  cock.  This  has 
been  fully  confirmed  by  other  observers  in  different  species  of 
animals. 

(4)  Extracts. — There  is  some  evidence  that  in  frogs  the 
development  of  sexual  character  may  be  produced  by  the  adminis- 
tration of  testicular  substance,  but  it  is  not  quite  satisfactory. 

The  testis  thus  exercises  an  important  influence  on  the 
growth  and  development  of  the  animal,  and  it  does  this  by 
yielding  an  internal  secretion.  That  the  source  of  this  is  the 
interstitial  cells  is  shown  by  the  fact  that  ligature  of  the  vas 
deferens  causes  atrophy  of  the  spermatogonia,  and  also  in 
course  of  time,  of  the  cells  of  Sertoli,  leaving  only  the  inter- 
stitial cells,  and  yet  there  is  no  arrest  of  the  development 
of  the  sexual  characters.  In  the  mole,  these  interstitial  cells 
reach  their  greatest  development  before  the  beginning  of  the 
breeding  season. 

In  man  they  are  well  developed  at  birth,  but  disappear  in 
the  course  of  the  early  weeks  of  life  to  reappear  again  at 
puberty  and  to  persist  throughout  life. 

B.  Ovaries. 

(1)  Removal  of  the  ovaries  has  the  same  effect  on  the  female 
as  removal  of  the  testes  has  on  the  male.  The  development 
of  the  secondar}'^  sexual  organs,  the  uterus,  mammee,  etc.,  is 
arrested.  But,  after  removal  of  the  ovaries,  there  is  frequently 
a  tendency  to  develop  the  characters  of  the  male.  Thus,  hinds 
with  diseased  ovaries  may  develop  horns  and  hen  pheasants 
and  ducks  may  develop  male  plumage.  The  ovaries  thus  seem 
to  check  the  development  of  male  characters. 

(2)  Transplantation  acts  in  the  same  way  in  the  female  as  in 
the  male  in  preventing  the  effects  of  removal.     In  most  animals 


608  VETERINARY   PHYSIOLOGY 

it  is  the  interstitial  cells  which  are  the  active  part ;  for,  after 
complete  degeneration  of  all  the  cells  of  the  Graafian  follicles, 
the  graft  of  ovarian  tissue  still  produces  the  development  of  the 
sexual  characters. 

The  transplantation  of  ovaries  into  young  castrated  male  rats 
and  guinea-pigs  has  led  to  the  development  of  female  characters, 
such  as  excessive  growth  of  the  nipples,  to  the  secretion  of  milk, 
and  in  rats  to  characteristic  sexual  reflexes. 


The  evidence  is  thus  quite  clear  that  the  gonads  exercise 
a  direct  influence  upon  the  soma!  cells  through  an  internal 
secretion. 

But  certain  peculiar  modifications  in  the  development 
of  the  sexual  characters  have  been  recorded  which  appear 
inexplicable  on  any  theory  of  the  action  of  an  internal  secretion. 
Bond  has  described  a  pheasant  with  male  plumage  on 
one  side  and  female  plumage  on  the  other,  and  a  similar 
condition  has  been  recorded  in  a  bullfinch.  In  the  pheasant 
an  atrophic  ovo-testis  was  present.  It  is,  of  course,  inconceiv- 
able that  an  internal  secretion  could  have  had  this  bilateral 
action.  In  insects  there  is  also  evidence  that  the  sexual 
characters  develop  after  castration  of  the  caterpillar. 

In  the  feinale  the  ovaries  not  only  act  in  the  young  in 
determining  the  development  of  the  sexual  characters,  but  in 
adult  life  they  have  an  important  influence  on — 

(1)  The  Course  of  Pregnancy. — In  the  bitch  if  the  ovaries 
are  removed  early  in  pregnancy  the  ovum  does  not  become 
embedded  in  the  mucous  membrane  of  the  uterus.  The  cells  of 
the  corpus  luteum  exercise  an  influence  on  the  uterus  which 
brings  this  about.  This  structure  is  formed  when  the  Graafian 
follicle  ruptures  and  discharges  its  ovum.  It  is  produced  essen- 
tially by  an  enormously  increased  growth  of  the  cells  of  the 
zona  granulosa,  which  become  loaded  with  lipoids.  If  the  ovum 
is  fertilised  the  corpus  luteum  grows  to  a  large  size ;  if  the 
ovum  is  not  fertilised  it  grows  to  a  less  extent.  When  the 
corpus  luteum  is  formed,  any  irritation  of  the  uterine  mucosa 
may  cause  the  development  of  the  typical  tissue  for  the 
emiaedding  of  the  ovum. 


REGULATORS  609 

Removal    of  the    ovaries    late   in   pregnancy   produces   no 
disturbances. 

(2)  The  Mammary  Gland. — When  the  ovaries  are  removed  in 
early  life  the  mammary  gland  does  not  grow.  The  growth  of 
the  gland  in  pregnancy  seems  to  be  associated  with  the  develop- 
ment of  the  corpus  luteum.  It  has  been  found  that,  in  the 
rabbit,  the  injection  of  extracts  of  the  foetus  leads  to  a  rapid 
growth  and  to  a  functional  activity  of  the  gland,  and  it  has  been 
deduced  that  a  secretion  from  the  foetus  is  the  specific  stimulus 
to  the  development  of  the  gland  and  to  milk  formation.  But 
milk  secretion  occurs  after  the  expulsion  of  the  foetus,  and  it  is 
more  probable  that  the  retention  in  the  mother  of  the  material 
which  was  formerly  passed  to  the  foetus,  and  which  is  virtually 
foetal  material,  is  the  stimulus  to  milk  secretion,  rather  than 
that  this  is  caused  by  a  reabsorption  from  the  foetus.  That  an 
internal  secretion  from  the  foetus  does  not  play  an  essential  part 
is  shown  by  the  fact  (i.)  that  milk  secretion  may  occur  in 
the  young  of  both  sexes;  and  (ii.)  that  it  may  be  caused 
by  stimulation  through  the  nervous  system,  since  it  has  been 
induced,  even  in  the  virgin  animal,  by  continued  stimulation 
of  the  nipple  by  sucking;  (iii.)  that  milk  may  be  produced  in 
bitches  some  weeks  after  oestrus  without  impregnation.  In 
such  bitches  development  of  the  corpus  luteum  and  of  the 
uterus  and  mammary  glands  occurs  and  retrogressive  changes  do 
not  appear  till  after  thirty  days.  This  may  explain  the  secre- 
tion of  milk  which  in  these  cases  seems  to  follow  atrophy  of 
the  corpus  luteum,  while  increase  of  the  mammarv  gland  is 
associated  with  its  growth. 

10.  Inter-renal  Tissue. 
In  mammals,  this  is  chiefly  massed  as  the  cortex  supra- 
renalis,  but  separate  masses  occur  along  the  course  of  the  aorta, 
in  the  epididymis  testis  and  near  the  ovaries.  In  some  animals, 
e.g.  the  rat,  they  are  more  abundant  than  in  others.  In  fishes 
the  inter-renals  lie  quite  separately  from  the  chromaffin  tissue. 

(1)  Development. — The  tissue  is  developed  from  ingrowths 
of  the  mesothelium  of  the  genital  ridge. 

(2)  Structure. — The  cells  are  large,  and  the  protoplasm  con- 
tains an  abundance  of  lipoids,  with  a  high  proportion  of 
cholesterol.     They  closely  resemble  the  interstitial  cells  of  the 

39 


610  VETERINARY   PHYSIOLOGY 

testes  and  ovaries  and  the  cells  of  the  corpus  luteum.  Brown 
granules  are  also  often  present  in  them. 

The  cortex  suprarenalis  is  covered  by  a  fibrous  capsule 
which  sends  trabeculge  inwards.  Under  the  capsule,  the  cells 
are  arranged  in  somewhat  fan-like  groups  to  form  the  zona 
glomerulosa.  Deeper,  they  run  in  parallel  rows  at  right  angles 
to  the  surface,  and  constitute  the  zona  fasciculata.  In  the 
deepest  layer,  the  zona  reticularis,  their  arrangement  is  in 
looser  and  less  regular  columns. 

(3)  Physiology.  —  (1)  Removal.  —  Biedl  has  succeeded  in 
removing  the  inter-renals  in  selachian  fishes,  and  he  finds  that 
the  animals  die.  In  mammals  it  is  impossible  to  remove  this 
tissue  without  also  removing  the  chromaffin  tissue  of  the 
medulla  suprarenalis  (see  p.  590). 

(2)  Extracts  of  this  tissue  are  without  the  action  of  extracts 
of  the  medulla.  Some  experiments  on  continuous  feeding  in 
young  rats  indicate  that  the  growth  of  the  testis  may  be 
stimulated. 

(3)  Relation  to  Gonads.— The  physiological  significance  of 
the  inter-renals  is  obscure,  but  that  they  are  probably  of  the 
same  nature  as  the  interstitial  and  other  ancillary  cells  of  the 
gonads  is  indicated  by  (i.)  their  common  origin  with  the  cells 
of  the  gonads  ;  (ii.)  their  close  resemblance  to  the  interstitial 
cells ;  (iii.)  the  fact  that  pieces  of  inter-renal  tissue  are 
frequently  found  in  close  relationship  to  the  gonads  ;  (iv.)  the 
fact  that  abnormalities  of  this  tissue  are  frequently  counected 
with  abnormalities  of  the  gonads.  Hypertrophy  seems  to  be 
associated  with  premature  sexual  development  in  the  male  and 
with  the  assumption  of  male  characters  by  the  female,  such  as 
the  typical  growth  of  hair  and  muscular  development ;  and 
(v.)  the  observation  that  the  cells  of  the  cortex  suprarenalis 
of  the  guinea-pig  undergo  marked  changes  in  pregnancy. 

It  has  been  found  that  in  infants  born  with  the  brain  un- 
developed, the  cortex  suprarenalis  is  also  defective. 

Addison's  Disease. — Addison  described  in  man  a  condition  of 
great  muscular  weakness  and  emaciation  with  a  curious  bronzing 
of  the  skin  which  ends  fatally,  and  he  was  able  to  associate  this 
with  destructive  lesions  of  the  suprarenals.  The  evidence  at 
present  forthcoming  as  to  whether  this  condition  is  due  to  an 
implication  of  the  inter-renal  tissue  of  the  cortex  is  inferential 


REGULATORS  611 

and  is  based  upon  Biedl's  experiments  upon  fish  (p.  610),  and 
Vincent's  experiments  on  mammals  (p.  591). 


The  Interaction  of  the  Endocrinetes. 

The  endocrinetes,  with  their  internal  secretions,  exercise  a 
balanced  injiuence  on  the  metabolism. 

This  is  very  clearly  shown  by  the  reciprocal  action  on  the 
mobilisation  of  sugar  of  the  chromaffin  tissue  and  thyreoid  on 
the  one  hand,  and  of  the  pancreas  on  the  other,  the  two  former 
stimulating  it,  the  latter  checking  it. 

There  is  also  evidence  that  they  act  upon  one  another. 
Thus,  while  thyreoidectomy  has  not  been  shown  to  have  any 
influence  upon  the  adrenalin  content  of  the  chromaffin  tissue, 
feeding  with  thyreoid  does  increase  it.  The  true  pituitary  and 
the  thyreoid  exercise  an  influence  on  the  growth  and  on  the 
functional  activity  of  the  gonads,  and  they  in  turn  are  acted 
upon  by  the  gonads. 

In  some  cases  these  structures  seem  to  supplement  one 
another.  The  thymus,  testis,  and  anterior  lobe  of  the  pituitary 
all  seem  to  combine  in  maintaining  growth  in  the  young. 


Mode  of  Action  of  Internal  Secretions. 

The  study  of  the  action  of  these  internal  secretions  involves 
the  question  of  how  far  they  act  independently  of  or  through 
the  nervous  system.  The  evidence  that  adrenalin  acts  through 
the  nerve  structures  appears  to  be  conclusive,  and  in  all  proba- 
bility the  products  of  the  hypophysis,  the  thyreoid,  and  the 
pancreas  act  in  the  same  way.  On  the  other  hand,  in  domin- 
ating the  growth  and  development  of  the  body  in  early  life,  it  is 
very  possible  that  the  thymus  and  gonads,  possibly  the  thyreoid 
and  pituitary,  act  directly  upon  the  tissues.  If  this  be  so,  the 
internal  secretions  may  act  (1)  as  primary  chemical  regulators 
and  (2)  as  neiiro-cheinical  regulators. 

The  influence  of  the  various  factors  dominating  metabolism, 
growth,  and  development,  might  be  represented  in  fig.  2.32, 
where  the  influence  of  hereditary  inertia  is  shown  as  dominant 
in  embryonic  life ;  where  the  influence  of  the  nervous  system 


612  VETERINARY  PHYSIOLOGY 

is  shown  as  gradually  increasing  in  importance;  where  the 
primary  chemical  regulators  are  indicated  as  playing  an  im- 
portant part  towards  the  end  of  foetal  and  the  beginning  of 
extra-uterine  life;  and  where  the  part  played   by   the  neuro- 


EmbryonLc  Life  .  Post-embryonic  Life 


d 


Fig.  232. — To  show  the  Relative  Parts  played  during  Embryonic  and  Post- 
embryonic  Life  in  the  Regulation  of  Metabolism  by  (a)  hereditary 
inertia,  (h)  the  primary  chemical  regulators,  (c)  the  nervous  system, 
and  id)  the  neuro-ohemical  regulators. 


chemical  regulators  is  shown  as  advancing  with  the  increased 
importance  of  the  nervous  system. 

Modifications  of  Metabolism  for  Protection 
against  Toxic  Agents. 

The  study  of  the  production  and  modes  of  action  of  these 
internal  secretions  leads  to  the  consideration  of  the  protection 
of  the  organism  against  the  action  of  various  poisons  of  animal 
or  bacterial  origin. 

This  will  be  dealt  with  very  briefly,  since  it  viust  be 
studied  fully  in  connection  luith  Pathology. 

The  question  may  be  most  simply  approached  by  consider- 
ing first  the  probable  mode  of  action  of  the  toxin  or  poison  of 
snake  venom  and  of  that  produced  by  the  diphtheria  bacillus, 
and  the  way  in  which  protection  against  these  is  established 
by  the  development  oi  antitoxins. 

1.  Snake  Venom  and  Diphtheria  Toxin. — By  injecting,  under 
the  skin  of  the  horse,  increasing  doses  of  such  toxins  the  animal 
is  made  quite  resistant  to  the  poison.  A  certain  quantity  of 
its  serum  can  then  neutralise  a  definite  quantity  of  the  toxin, 
so  that,  when  the  mixture  of  serum  and  toxin  is  injected  into 
another  animal,  the  latter  is  uninjured.     Something  has  been 


REGULATORS  613 

formed  in  the  horse  which  seizes  on  the  molecules  of  the  toxin 
and  makes  them  harmless,  just  as  when  soda  is  added  to  sul- 
phuric acid  a  neutral  salt  is  formed. 

The  two  molecules  have  a  definite  chemical  affinity  for  one 
another,  so  that  the  toxin  or  antigen  is  no  longer  free  to  seize 
upon  the  protoplasm  of  the  animal's  body.  To  explaiu  this, 
Ehrlich  has  suggested  that  the  protoplasm  molecule  (fig.  233) 
is  made  up  of  a  central  core  with  a  number  of  side-chains  or 
rece'ptors,  which  play  an  important  part  in  taking  up  nourish- 
ment of  different  kinds,  special  receptors  being  developed  for 
each  kind  of  material.  He  supposes  that  some  of  these  side- 
chains  fit  the  toxin  molecule,  and  are  thus  capable  of  anchoring 
it  to  the  cell  and  allowing  it  to  exercise  its  toxic  action.  The 
production  of  antitoxin  he  explains  by  supposing  that,  as  these 
side-chains  get  linked  to  the  toxin  and  are  thus,  as  it  were, 
thrown  out  ot  action,  others  are  produced  to  take  their  place, 
since  they  are  necessary  for  the  nourishment  of  the  protoplasm. 
If  the  toxin  is  continually  administered  in  small  doses  this 
production  of  side-chains  may  be  so  increased  that  they  get 
thrown  off  into  the  blood,  and  in  it  they  are  capable  of  linking 
to  the  toxin  and  so  preventing  it  from  fixing  itself  to  the  cells. 
If,  therefore,  some  of  the  blood  be  injected  into  an  animal 
which  afterwards  receives  a  dose  of  the  toxin,  that  toxin  will 
not  act,  and  the  animal  will  be  immune. 

2,  Enteric  Toxin. — But  immunity  may  also  be  established 
not  merely  against  toxins  separate  from  organisms,  but  against 
organisms  which  hold  their  toxin,  as  in  the  case  of  the  bacillus 
of  enteric  fever.  Here,  repeated  injections  of  increasing  doses  of 
the  dead  bacilli  cause  the  production  of  a  serum  which  has  the 
power  of  destroying  the  organism  when  added  to  it  even  outside 
the  body.  This  is  not  a  simple  combination;  because,  if  the  serum 
be  heated  to  55°  C,  it  loses  its  power,  but,  if  a  few  drops  of  the 
fresh  serum  even  of  an  unimraunised  animal  be  added,  the  power 
is  restored.  Obviously  the  anti-body  which  destroys  the  organ- 
ism^ — the  bactericidal  or  bacteriolytic  body, oiten  called  the  ambo- 
ceptor— requires  the  co-operation  of  another  body  to  enable  it 
to  act,  and  this  body  has  been  called  the  complement  or  alexine. 
This  is  readily  destroyed  at  a  comparatively  low  temperature. 
Ehrlich  supposes  that  the  immune  body  does  link  to  the  proto- 
plasm of  the  organism,  but  that  it  must,  in  its  turn,  be  linked 


614 


VETERINAEY   PHYSIOLOGY 


to  the  complement  before  it  can  act.  The  figure  may  help  to 
explain  this  (fig.  234). 

3.  Cytotoxins. — Similar  anti-bodies,  acting  upon  the  cells 
of  the  animal  body,  may  be  produced  by  injecting  the  particular 
kind  of  cell  into  an  animal  of  another  species.  Thus,  if  human 
blood  be  repeatedly  injected  into  a  rabbit,  the  serum  of  the 
rabbit's  blood  becomes  hcemolytie — i.e.  acquires  the  power  of 
dissolving  the  erythrocytes  in  human  blood.  In  this  case  too, 
the  immune  body  requires  the  presence  of  a  complement, 
readily  destroyed  at  a  comparatively  low  temperature,  to  enable 
it  to  act  (see  p.  486). 

If  such  hgemolytic  serum  be  injected  into  another  animal 
an   anti-hceniolysin   may   be  developed  —  a  body  which  will 


sc 

Fig.  233.— To  illustrate  the  For- 
mation of  Side-chains  or  Recep- 
tors, sc,  bywhich  the  toxin  mole- 
cules, T,  are  either  anchored 
to  the  cell  or  neutralised. 
When  the  side-chains  are  set 
free  an  anti-toxin  is  formed. 


Fig  234.— To  illustrate  the 
Anchoring  of  the  Anti- 
body or  Amboceptor,  a, 
to  the  cell  by  a  side-chain 
or  receptor,  sc,  and  the 
action  upon  it  of  comple- 
ment, com. 


antagonise  the  action  of  the  hsemolysin.  Possibly  this  is  a 
body  which  links  with  the  amboceptor  to  prevent  its  linking 
to  the  complement. 

4.  Precipitins. — By  the  injection  of  the  proteins  of  the  blood 
of  any  particular  animal  into  an  animal  of  another  species,  a 
serum  is  developed  which,  in  association  with  the  proteins  of 
the  blood  of  the  first  species,  forms  a  precipitate.  This  action  is 
specific,  unless  in  the  case  of  closely  allied  species  when  it  may 
occur  to  a  slight  extent. 

5.  Agglutinins. — These  constitute  another  group  of  anti- 
substances.     They  may  be  developed  towards  bacteria,  erythro- 


REGULATORS  615 

cytes,  etc.,  by  repeated  injections  of  these  substances  into  an 
aaimal.  Their  action, is  shown  by  their  causing  clumping  or 
agglutination  when  added  to  a  suspension  of  the  corresponding 
antigen.  Agglutinins  may  appear  in  the  blood  during  an 
attack  of  certain  diseases,  e.g.  enteric  fever,  and  the  reaction 
may  be  used  for  diagnosis. 

In  these  various  cases,  the  active  body  is  produced  by  the 
throwing  off  of  side-chains  from  the  protoplasm,  and,  since  these 
products  are  carried  away  in  the  blood,  the  process  is  analogous 
to  the  formation  of  internal  secretions. 

Opsonins. — Many  bacteria,  after  treatment  with  the  serum 
of  the  animal,  are  taken  up  by  the  leucocytes,  but  if  not  treated 
with  serum  are  not  taken  up.  Apparently  the  serum  contains 
something,  which  has  been  called  an  oi)sonin,  which  prepares 
the  bacteria  to  be  devoured.  The  action  of  opsonins  is  destroyed 
by  temperatures  between  55°  and  65°  C.  The  opsonic  power 
of  the  serum  for  a  particular  bacterium  is  often  increased  by 
the  injection  of  small  quantities  of  that  organism  in  the  dead 
condition  (Vaccines). 


PART   IV. 
THE   ANIMAL  AS   PART   OF   THE   SPECIES. 

A.  Repeoduction. 

So  far  the  animal  has  been  studied  simply  as  an  individual. 
But  it  has  also  to  be  regarded  as  part  of  a  species,  as  an  entity 
which  has  not  only  to  lead  its  own  life,  but  to  transmit  that 
life  to  offspring  from  generation  to  generation  and  thus  to 
secure  the  survival  of  the  species. 

When  a  unicellular  organism  reaches  such  a  size  that  the 
proportion  of  mass  to  surface  begins  to  interfere  with  nutrition, 
the  metabolism  undergoes  alterations  which  lead  to  the  division 
of  the  cell  (p.  28). 

In  certain  cases  conjugation  between  two  cells  may  occur 
and  this  appears  to  precipitate  the  process  of  division. 

In  multicellular  organisms  the  gametic  cell  or  cells  early 
throw  off  the  somal  cell  or  cells  from  which  all  the  tissues  of 
the  body  are  developed.  In  Ascaris  this  occurs  at  the  very 
first  division. 

The  purpose  of  somal  development,  of  the  development  of 
the  body  as  a  whole  in  all  its  marvellous  complexity,  is  simply  for 
the  nutrition  and  protection  of  the  gametic  cells.  These  latter 
are  eternal,  going  on  from  one  generation  to  another  and  in 
each  building  up  a  body.  The  body  is  mortal,  perishing  with 
the  death  of  the  individual. 

In  all  higher  forms  of  animals  conjugation  of  two  gametic 
cells  precedes  division  and  development.  In  many  inverte- 
brates this  is  not  always  necessary  and  reproduction  without 
conjugation — i^arthenogenesis — may  occur. 

In  vertebrates  and  in  a  large  number  of  invertebrates  two 
sets  of  gametes  are  formed — one  the  ovum  which  is  generally 


EEPRODUCTION  617 

large  and  which  manifestly  undergoes  division,  multiplication, 
and  development  to  form  the  new  individual,  and  one  the  sperm 
or  spermatozoon,  which  is  generally  smaller  and  which  con- 
jugates with  the  former. 

In  vertebrates  these  are  produced  in  separate  individuals  of 
the  species  which  have  the  distinctive  anatomical  and  physio- 
logical characters  of  the  female  and  male. 

The  dependence  upon  the  gonads  of  the  development  of 
these  distinctive  characters  has  been  considered  on  p.  606  et  seq. 


I.  Determination  of  Sex. 

The  problem  of  what  determines  in  any  ovum  its  develop- 
ment either  into  ova  associated  with  the  female  type  of  body  or 
into  spermatozoa  associated  with  the  male  type  is  a  most  difficult 
one.  A  consideration  of  importance  is  that,  under  normal  con- 
ditions of  sexual  reproduction,  the  number  of  males  and  of 
females  produced  is  approximately  equal. 

(1)  In  some  animals  the  sex  is  determined  before  the  ovum  is 
impregnated.  Among  vertebrates  this  seems  to  be  the  case  in 
birds.  A  hen  with  barred  pattern  of  feathers  transmits  this 
character  to  male  chicks  only,  unless  the  father  is  also  barred. 
Hence  it  seems  clear  that  there  must  be  two  kinds  of  ova — one 
to  produce  males  and  one  to  produce  females. 

(2)  In  other  animals,  and  almost  certainly  in  mammals,  the 
sex  seems  probably  to  be  determined  by  there  being  two  types 
of  spermatozoa.  It  is  probable  that  certain  special  chromosomes, 
often  called  the  X  chromosomes,  are  the  determining  factors. 
When  two  of  these  are  present,  a  female  develops,  when  one  is 
present,  a  male  ;  and  the  gametic  cells  developed  in  each  sex  will 
be  characterised  by  the  possession  of  these  numbers.  When  these 
cells  undergo  their  reduction  division  (pp.  619  and  620),  before 
forming  ova  and  spermatozoa,  each  female  cell  is  left 
with  one  X  chromosome,  while  half  the  male  cells  contain 
an  X  chromosome  and  half  do  not  contain  one.  When  one  of 
the  former  impregnates  an  ovum,  that  ovum  has  two  X  chromo- 
somes and  develops  into  a  female;  when  one  of  the  latter  per- 
forms the  impregnation,  the  ovum  has  only  one  X  chromosome 
and  develops  into  a  male. 

The  existence  of  these  two  kinds  of  spermatozoa  and  the 


618  VETERINARY   PHYSIOLOGY 

fact  that  only  those  ova  which  have  the  paternal  X  chromosome 
will  develop  into  females  explains  the  transmission  of  certain 
paternal  characters  through  the  daughters.  In  the  human  sub- 
ject this  is  seen  in  colour  blindness  and  in  haemophilia,  a  disease 
characterised  by  excessive  bleeding  from  slight  wounds.  These 
conditions  are  common  in  the  male  but  very  rare  in  the  female. 
But  since  the  female  gamete  alone  contains  the  paternal  chromo- 
some, the  abnormal  conditions  can  pass  only  through  the  females 
to  become  manifest  in  the  male  progeny. 

Such  observations  suggest  that  the  determination  of  sex 
may  be  co-related  with  the  Mendelian  hypothesis. 

(3)  In  some  animals,  e.g.  the  frog,  the  nutritional  condition  of 
the  ovum  may  determine  its  sex.  By  varying  the  condition  of 
the  eggs  before  fertilisation  the  proportion  of  males  to  females 
may  be  modified. 

It  would  appear  that  in  some  animals  the  course 
of  development  of  the  gamete  is  firmly  established  from 
the  first,  that  in  others  it  may  be  determined  by  the  X 
chromosome  in  impregnation,  while  in  a  third  group  it  may  be 
modified  by  the  nutrition  of  the  cell  either  before  impregnation 
or  at  the  time  of  impregnation.  How  far  this  last  factor  acts 
in  the  case  of  mammals  we  do  not  at  present  know. 

There  is  some  evidence  in  invertebrates  that  the  number  of 
X  chromosomes  in  a  cell  may  be  reduced  under  certain  con- 
ditions, and  thus  a  potential  female  cell  changed  into  a 
male  cell. 

The  problems  of  Heredity  are  fully  dealt  with  in  all  text- 
books of  Biology,  and  therefore  they  need  not  be  considered 
here. 

II.  The  Gonads. 

{The  structure  of  the  organs  ofreproductionmust  he  studied 
practically.) 

The  development  of  the  true  gametic  cells  of  the  gonads 
and  of  their  ancillary  cells  has  been  already  considered  on 
p.  605. 

While  the  individual  is  actively  growing,  the  reproductive 
organs  are  quiescent ;  but,  when  puberty  is  reached,  they  begin 
to  perform  their  functions — the  testes  to  produce  spermatozoa. 


REPRODUCTION  619 

the  ovaries  to  produce  ova,  and,  as  they  become  functionally 
active,  the  secondary  sexual  characters  develop  (p.  606). 

A.  Ovary. — The  ovaries  are  oval  structures  lying  in  a  fold  of 
peritoneum — the  broad  ligament.  The  cells  covering  the  genital 
ridge  grow  downwards  as  the  oogonia.  These  become  disposed 
(1)  as  a  covering  layer  of  columnar  cells  ;  (2)  as  interstitial  cells 
(p.  608) ;  (3)  as  clumps  of  cells  forming  the  Graafian  follicles. 
The  central  cell  of  each  of  these  undergoes  further  growth, 
becomes  larger  than  the  surrounding  cells  and  forms  the 
primary  oocyte.  This  cell  throws  out  the  first  polar  body 
and   becomes   a   secondary  oocyte.     A    second   polar   body   is 


Proliferation 


Fig.  235.— Scheme  of  Spermatogenesis  and  Oogenesis. 

then  thrown  out,  and  half  the  chromosomes  are  thus  elimi- 
nated and  the  mature  ovum  is  formed  (fig.  235).  This  becomes 
surrounded  by  a  capsule,  the  zona  j^ellucida.  The  surround- 
ing cells  forming  the  zona  granulosa  multiply,  and  a  fluid, 
the  liquor  folliculi,  appears  among  them,  dividing  them 
into  a  set  attached  to  the  capsule  of  the  follicle  and  a  set 
surrounding  the  ovum.  When  the  follicle  is  ripe,  it  projects  on 
the  surface  of  the  ovary,  and  finally  bursts,  setting  free  the 
ovum,  which  escapes  into  the  peritoneal  cavity  and  passes  into 
the  trumpet-shaped  fimbriated  upper  end  of  the  Fallojnan  tube 
through  which  it  reaches  the  uterus.  The  ruptured  Graafian 
follicle  generally  becomes  filled  with  blood,  and  later  with  the 


620 


VETERINARY   PHYSIOLOGY 


proliferated  cells  of  the  zona  granulosa  and  forms  the  corpus 
luteum.  If  the  ovum  is  fertilised  and  pregnancy  occurs  the 
corpus  luteum  goes  on  growing,  attains  a  considerable  size, 
and  plays  an  important  part  in  the  course  of  gestation 
(p.  608). 

B.  Testis. — The  testis  is  enclosed  in  a  dense  fibrous  capsule 
— the  tunica  albuginea.  Posteriorly  this  is  thickened,  and 
forms   the    corpus    Highmori.      Processes    extend    from    this 


Fig.  236. — I,  To  show  this  development  of  spermatozoa  from 
spermatogonia  ;  II  and  III,  heads  of  spermatozoa  dipping 
into  cells  of  Sertoli ;  IV,  mature  spermatozoa.  Two  inter- 
stitial cells  shown  below.     (Mott.) 

and  form  a  supporting  framework.  In  the  spaces  are  situated 
the  seminiferous  tubules,  which  open  into  irregular  spaces  in 
the  corpus  Highmori — the  7'ete  estis.  From  these  the  efferent 
ducts,  vasa  eferentia,  pass  away  and  join  together  to  form  the 
vas  deferens,  which,  after  receiving  the  duct  from  a  diverticulum 
of  the  seminal  vesicle,  opens  into  the  urethra. 

In  the  seminiferous  tubules,  the  spermatozoa  are  produced. 
Some  of  the  lining  cells  divide  into  two,  forming  a  supporting 
cell  next  the  membrane  and  a  spermatogonium.  The  latter 
divides  and  subdivides  till  a  group  of  cells,  the  primary 
spermatocytes,  are  formed.  Each  of  these  undergoes  a  division, 
in  which  the  number  of  chromosomes  in  each  cell  formed  is 


REPRODUCTION  621 

reduced  to  one  half,  and  thus  secondary  sjjermatocytes  are  pro- 
duced. These  again  divide  to  form  the  spermatids  (fig.  235).  In 
each  spermatid  the  nucleus  elongates  and  passes  to  the  attached 
extremity,  the  protoplasm  decreases  in  amount,  and  a  long 
cilium  develops  from  the  free  end,  and  the  spermatozoon  is 
thus  pro'lnced.  As  they  develop  they  seem  to  bore  into  the 
cells  ol  Sertoli  (fig.  236),  the  lipoids  of  which  probably  act  as 
nutr'ont  material. 

The  interstitial  cells  of  Leydig  have  been  already  described 
(p.  606).     They  are  very  rich  in  phospholipins. 

III.  The  Secondary  Sexual  Organs. 
A.  In  the  Male. 

1.  The  Prostate  consists  of  a  dense  framework  of  fibrous 
tissue  with  visceral  muscular  fibres  enclosing  dense  branching 
glands  which  secrete  a  fluid  which  undergoes  coagulation. 
In  some  animals,  e.g.  the  guinea-pig,  it  is  ejaculated  after 
the  rest  of  the  seminal  emission  and  forms  a  plug  in  the 
vagina. 

Semen. — When  the  testes  have  become  active,  the  glands 
of  the  prostate  increase  and  produce  a  fluid  which,  with  the 
spermatozoa,  forms  the  semen. 

2.  The  Penis. — Thisconsistsof  erectile  tissue — a  dense  fibrous 
tissue  filled  with  irregular  blood  spaces,  into  which  arteries  open. 
The  penis  of  the  bull  is  pointed,  while  that  of  the  ram  terminates 
in  a  filiform  process.  In  these  animals  the  semen  is  passed 
into  the  mouth  of  the  uterus. 

B.  In  the  Female. 

1.  The  Fallopian  Tubes. — A  tube,  with  a  trumpet-like 
fimbriated  upper  end,  lying  close  to  the  ovary  on  each 
side,  leads  to  the  uterus.  It  is  lined  by  a  mucous  mem- 
brane raised  into  complex  ridges  and  covered  by  a  ciliated 
epithelium. 

2.  The  Uterus  is  a  hollow  organ  with  a  wall  composed  of 
visceral  muscle  fibres.  In  the  sheep  and  cow  and  bitch  it  con- 
sists of  two  distinct  cornua — in  the  mare  of  a  central  part.  It 
is  lined  by  a  mucous  membrane,  which  is  covered  by  a  ciliated 
epithelium,  and  in  which  are  tubular  glands  that  extend  down 


622  VETERINARY   PHYSIOLOGY 

to  the  muscular  coat.     The  nature  of  their  secretion  requires 
investigation.     The  nerve  supply  is  considered  on  p.  633. 


IV.  The  (Estrous  Cycle. 

The  female  of  all  species  of  mammals,  and  the  male  of  some, 
pass  through  a  cycle  of  sexual  activity  and  sexual  rest.  This 
has  been  called  the  osstrous  cycle. 

In  the  anoestriim  the  ovaries,  uterus,  and  other  sexual  organs 
are  quiescent,  and  their  blood-vessels  more  or  less  contracted. 
In  the  procestrum  one  or  more  ova  ripen,  the  blood-vessels 
dilate,  haemorrhages  occur  into  the  mucous  membrane  of  the 
uterus,  and  blood  may  escape  by  the  vagina.  In  the  (esiriim 
these  conditions  reach  their  maximum,  and  in  the  lower  animals 
coitus  is  allowed. 

In  the  mare,  cow,  pig,  and  sheep  the  cestrum  may  recur  at 
short  intervals  at  some  special  season  of  the  year,  while  in 
carnivora,  e.g.  the  dog,  it  occurs  only  once  at  a  given  sexual 
season. 

In  the  mare  the  usual  sexual  season  is  early  summer.  Each 
cestrum  may  last  4  or  5  days. 

In  the  cow  the  season  varies,  and  in  it  cestruon  may  recur  at 
intervals  of  about  20  days. 

In  the  sheep  the  season  is  in  autumn,  and  oestrum  may 
recur  repeatedly  at  intervals  of  about  12  days,  lasting  each  time 
for  only  about  a  day  or  less. 

In  the  sow  oestrum  may  recur  at  intervals  of  about  20 
days,  and  the  whole  cycle  is  generally  repeated  twice  a 
year. 

In  the  bitch  an  oestrum  cycle  recurs  generally  twice  a  year. 
CEstrum  usually  lasts  for  about  a  week.  Even  if  not  impreg- 
nated, the  ovary,  uterus,  and  mammary  gland  show  the  same 
chano-es  as  occur  in  pregnancy  and  which  may  be  secreted. 

In  the  postc£strum  the  organs  return  to  their  normal  condi- 
tion, and  the  corpus  luteum  or  corpora  lutea  grow  in  the 
ovaries. 

In  some  animals,  e.g.  the  rabbit,  rupture  of  the  follicle  occurs 
only  if  copulation  takes  place.  If  this  does  not  occur  the  follicle 
atrophies. 


REPRODUCTION 


623 


V.  Impregnation. 

Impregnation  is  effected  by  the  transmission  of  spermatozoa 
into  the  genital  tract  of"  the  female.  For  this  purpose  erection 
of  the  penis  is  brought  about  reflexly  through  a  centre  in  the 


Fig.  237. — Ovum,  after  Segmentation,  showing  the  Formation  of  the  Ectoderm 
(.4.)  and  Endoderm  (B.).  From  the  cells  of  the  latter  the  Blastoderm 
is  formed.     (Ellenberger.) 

lumbar  enlargement  of  the  cord,  the  outgoing  nerves  being  the 
nervi  erigentes,  or  pelvic  nerves  which  dilate  the  arterioles,  and 
the  internal  pudics  supplying  the  transversus  perinei  and  bulbo- 
cavernous muscles  by  which  the  veins  of  the  penis  are  constricted. 
The  semen  is  ejected  by  a  rhythmic  contraction  of  the  bulbo- 


A.  B.  C. 

Fig.  238. — To  show  A.,  the  Spreading  out  of  the  Endoderm  Cells  to  Form  the 
Blastoderm  ;  B.,  the  Formation  of  Epiblast  and  Hypoblast ;  and  C,  of 
Mesoblast.     In  B.  and  G.  the  ectoderm  is  not  shown.     (Ellexberger.) 

cavernous  and  other  perineal  muscles,  an  action  which  is  also 
presided  over  by  a  centre  in  the  lumbar  region  of  the  cord 
(p.  89). 

The  spermatozoon  meets  the  ovum  in  the  Fallopian  tube  or 
upper  part  of  the  uterus. 


624 


VETERINARY    PHYSIOLOGY 


B.   Development. 
I.   Early  Stages. 

As  the  ovum  passes  down  the  Fallopian  tube  it  is  surrounded 
by  cells  of  the  zona  granulosa,  and  these  probably  serve  as  a 
source  of  nourishment  and  may  prevent  the  ovum  from  becom- 
ing attached  till  it  reaches  the  uterus  and  absorption  of  the 
cells  is  completed.  Sometimes  implantation  occurs  in  the  tube 
and  a  tubal  pregnancy  may  ensue. 

It  is  unnecessary  here  to  describe  the  changes  in  the  ovum 
before  or  immediately  after  its  conjugation  with  the  spermato- 


FiG.  239. — Transverse  Section  of 
more  advanced  Blastoderm,  to 
show  Epiblast,  Mesoblast,  and 
Hypoblast,  formation  of  Neural 
Groove  and  splitting  of  the 
Mesoblast. 


Fig.  240.  —  Longitudinal  Section 
through  Embryo  to  show  it  Sinking 
Down  into  Ovum  and  the  Formation 
of  the  Amnion,  am.  In  the  Meso- 
blast round,  all.,  the  allantois,  the 
blood-vessels  grow  out  to  form  the 
placenta. 


zoon,  since  they  are  so  fully  dealt  with  in  all  works  on  Biology 
(p.  29). 

The  mammalian  ovum  is  holoblastic,  that  is,  undergoes  com- 
plete segmentation,  and  forms  a  mulberry-like  mass  of  cells  (fig. 
237,  A.).  The  cells  then  get  disposed  in  two  sets,  a  layer  of 
small  surrounding  cells  and  a  set  of  large  central  cells  (fig.  237, 
jB.).  The  former  constitute  the  Ectoderm  and  take  part  in 
forming  the  processes  or  'primitive  villi  by  which  the  ovum 
becomes  attached  to  the  maternal  mucous  membrane.  The 
latter  spread  out  at  one  pole  to  form  the  blastoderm  (fig.  238,  A.) 
and  dispose  themselves  in  three  layers — the  epiblast,  meso- 
blast, and  hypoblast  (fig.  238,  B.  and  C).  From  these  layers  the 
various  parts  of  the  body  are  derived  as  follows : — 

I.  Epiblast. — Nervous  system ;  epidermis  and  appendages  ; 
epithelium  of  the  mouth,  nose,  naso-pharynx,  and  all  cavities 
and  glands  opening  into  them,  and  the  enamel  of  teeth. 


DEVELOPMENT 


625 


II.  Hypoblast. — Epithelia  of  (a)  the  alimentary  canal  from 
the  back  of  the  mouth  to  the  anus  and  of  all  its  glands  ;  (h)  of 
the  Eustachian  tube  and  tympanum  ;  (c)  of  the  trachea  and 
lungs ;  (d)  of  the  thyreoid  and  thymus  ;  and  (e)  of  the  urinary 
bladder  and  urethra. 

III.  Mesoblast. — All  other  structures. 

By  the  formation  of  a  vertical  groove  down  the  back  of  the 
blastoderm,  a  tube  of  epiblast  cells  (the  neural  canal)  is  enclosed, 
from  which  the  nervous  system  develops  by  the  conversion  of 


Fig.  241. — Longitudinal  Section  through  the  Human  Uterus  and  Ovum  at  the 
Fifth  Week  of  Pregnancy.  D.S.,  deoidua  serotina,  which  will  become 
the  placenta  ;  D.R.,  decidua  reflexa  ;  D.  V.,  the  uterine  mucous  mem- 
brane called  the  decidua  vera. 

some  of  the  cells  into  neurons,  and  others  into  neuroglia  cells 
(fig.  -239). 

The  mesoblast  on  each  side  of  this  splits,  and  the  outer  part, 
with  the  epiblast,  goes  to  form  the  body-wall  (Somatopleur),  while 
the  inner  part  with  the  hypoblast  gets  tucked  in  to  produce 
the  alimentary  canal  (Splanclinopleur)  (fig.  239). 

The  developing  embryo  sinks  into  the  blastocyst,  and,  as  a 
result  of  this,  the  somatopleur  folds  over  it  and,  uniting  above,  en- 
closes it  in  a  sac — the  amniotic  sac  (fig.  240,  am.),  which  becomes 
distended  with  fluid — the  amniotic  fluid,  in  which  the  embryo 
40 


626  VETERINARY   PHYSIOLOGY 

floats  during  the  latter  stages  of  its  development,  and  which  acts 
as  a  most  efficient  protection  against  external  violence.  The 
source  of  this  fluid  has  been  much  debated.  In  birds  it  is 
certainly  of  foetal  origin.  In  mammals  it  has  been  contended 
that  it  is  derived  from  the  maternal  circulation.  But,  since  in 
herbivora  it  resembles  urine  more  than  a  blood  transudate  and 
since  the  urethra  of  the  foetus  opens  into  the  amniotic  sac,  it  is 
probably  chiefly  derived  from  the  foetal  kidneys.  In  rabbits, 
when  the  foetus  is  killed  in  utero,  no  fluid  is  formed  in  the  sac, 
although  the  maternal  part  of  the  placenta  persists. 

A  very  significant  fact  is  that  in  herivora  it  contains  a  sugar, 
Isevulose,  which  is  present  in  the  fcetal  blood  but  not  in  the 
maternal  blood. 


11.  Attachment  to  the  Mother. 

(1)  By  the  action  of  its  ectoderm  cells  the  ovum  burrows 
its  way  into  the  mucous  membrane  of  the  uterus  which 
is  hypertrophied  and  very  vascular.  These  ectoderm  cells 
grow  outwards  as  syncytial  masses  of  protoplasm  forming  the 
trophoblast  layer.  The  burrowing  action  may  be  due  to  the 
development  of  some  powerful  proteolytic  enzyme,  although 
definite  proof  of  its  existence  is  not  forthcoming. 

Certainly,  in  some  way  the  maternal  tissues  are  killed  and 
digested.  When  a  maternal  blood-vessel  is  opened  into,  the 
blood  is  hsemolysed,  thus  probably  rendering  the  iron  of  the 
heemoglobin  available  for  absorption  by  the  embryo. 

At  this  stage  of  development  the  embryo  is  a  parasite  upon 
the  mother  living  upon  her  substance. 

The  important  part  played  by  the  corpus  luteum  in  de- 
termining the  implantation  of  the  ovum  has  been  discussed  on 
p.  608. 

(2)  Later,  the  mesoblast  of  the  embryo  extends  out  in  a 
number  of  finger-like  processes  into  the  trophoblast  layer,  and 
soon  afterwards  blood  -  vessels  shoot  into  these,  and  the 
chorionic  villi  are  formed.  These  are  at  first  covered  by  a 
definite  layer  of  cubical  cells,  the  layer  of  Langhans  with 
outside  it  a  syncytial  trophoblast  layer  of  protoplasm. 
Later  the  layer  of  Langhans  disappears  and  the  syncytium 
becomes  extremely  thin  (fig.  242). 


DEVELOPMENT 


627 


The  origin  of  the  first  blood-vessels  in  the  villi  is  not 
known,  but  ultimately  they  are  derived  from  the  allantoic 
arteries  which  pass  out  from  near  the  posterior  end  of  the  hind  gut. 

As  the  villi  grow,  the  blood-vessels  of  the  maternal  mucosa 
in  the  decidua  serotina  (fig.  241,  B.S.)  dilate,  and  the  capillaries 
form  large  sinuses  or  blood  spaces.  Into  these  the  chorionic 
villi  pass,  and  thus  the  loops  of  foetal  vessels  hang  free  in  the 
maternal  blood,  and  an  exchange  of  material  is  possible  between 
the  mother  and  foetus. 

The  distribution  of  these  villi  is  different  in  different 
animals.  In  the  horse  they  are  diffusely  scattered  ;  in  the  cow 
and  sheep  they  are  arranged  in  circular  patches,  the  cotyledons ; 
in  carnivora  they  are  generally  in  a  zone. 


Fig.    242. — Longitudinal    Section    through    the    Tip  of    a    Villus   of   the 

Placenta,  covered  by  its  trophoblast  layer,  and  containing  a  loop  of 

blood-vessels,  and  projecting  into  a  large  blood  sinus,  J.  V.S.,  in  the 
maternal  mucosa. 


In  the  pig,  horse,  and  in  ruminants,  the  connection  of  the 
foetal  blood-vessels  with  the  maternal  structures  is  not  very 
intimate,  and  when  the  young  are  born  the  foetal  part  of  the 
placenta  separates  from  the  maternal  part,  which  is  thus  not 
shed.     Hence  such  animals  are  called  non-deciduata. 

In  rodents,  insectivora,  apes,  man,  and  carnivora,  the  associa- 
tion is  so  intimate  that  at  birth  the  maternal  part  of  the 
placenta  is  shed  along  with  the  foetal.  Hence  these  are  called 
deciduata. 

In  the  mesoblast,  through  which  the  allantoic  arteries  pass 


628 


VETERINARY   PHYSIOLOGY 


out,  a  vesicle,  filled  with  fluid,  and  at  first  communicating  with 
the  posterior  gut,  is  developed  (fig.  240,  all.).  This  is  the 
allantois.     In  man  it  never  attains  any  size,  but  in  most  of  the 


Fig.  243. — Schematic  section  through  the  pregnant  uterus  of  the  Mare  to 
show  the  large  allantoic  sac,  All.,  filled  with  fluid  surrounding  the 
amniotic  sac  ;  Am.,  the  fluid  in  which  the  fa?tus  floats. 

lower  animals  it  spreads  all  around  and  encloses  the  amnion, 
and  is  distended  with  a  large  quantity  of  fluid.  This  fluid  has 
all  the  characters  of  urine,  and  when  fluorescin  is  injected  into 


c   A 
Fig.  244. — Schematic  section  of  one  cornu  of  the  uterus  of  a  ruminant  at  an 
early  stage  of  gestation  to  show  the  elongated  umbilical  vesicle.  A, 
and  allantois,  B,  and  the  embryo  in  the  amniotic  sac,  G. 

the  maternal  circulation,  it  appears  in  the  foetus  before  it  shows 
in  the  allantoic  fluid.  It  is  almost  certainly  produced  from  the 
first  by  the  foetal  kidney. 


III.  The  Nourishment  of  the  Foetus. 
The  Placenta  is  formed  on  the  foetal  side  by  these  processes; 
on  the  maternal  side  by  the  increased    growth  and  increased 
vascularity  of  the  maternal  mucosa. 


DEVELOPMENT  629 

In  the  deep  layer  of  the  mucosa  under  the  placenta  the 
connective  tissue  cells  enlarge  and  form  a  thick  mass  of 
decidual  cells.  Possibly  these  are  protective,  preventing  the 
enzymes  or  other  products  of  foetal  metabolism  from  invading 
the  mother, 

A  new  stage  in  the  physiological  relationship  of  the  foetus 
to  the  mother  is  now  established.  It  is  no  longer  a  parasite 
but  rather  a  guest  which  shares  with  the  mother  the  supply  of 
nourishment  in  the  maternal  food.  The  placenta  becomes  (1) 
the  foetal  lung,  giving  the  embryo  the  necessary  oxygen  and 
getting  rid  of  the  waste  carbon  dioxide,  (2)  The  foetal 
alimentary  canal  supplying  the  necessary  material  for  growth 
and  development ;  and  (3)  the  foetal  kidney  through  which  the 
waste  nitrogenous  constituents  are  thrown  off. 

When  this  stage  of  gestation  is  reached,  it  is  generally  found 
that  the  maternal  body  shows  the  same  increased  power  of 
storing  the  constituents  of  the  food  as  is  seen  during  the  period  of 
growth.  The  pregnant  animal  stores  material  like  the  growing 
young,  generally  taking  just  what  is  required  to  satisfy  the 
normal  growth  of  the  foetus,  but  frequently  retaining  more  and 
storing  it  in  its  body.  Hence  the  practice  of  having  cows 
rendered  pregnant  in  feeding  them  for  market. 

Carbohydrates  are  early  stored  as  glycogen  in  the  cells  of  the 
maternal  placenta,  and,  since  no  glycogen  is  found  in  the  foetus 
till  much  later,  it  must  be  taken  up  by  the  chorionic  villi  as 
sugar.  Whether  the  diastase  for  this  conversion  is  formed  by 
the  mother  or  by  the  foetus  is  not  known. 

Fats  appear  earlier  in  the  chorionic  villi  than  in  the 
maternal  placenta,  and  there  is  no  evidence  that  fats  stained 
with  Sudan  III.  are  passed  to  the  foetus.  Probably  the  foetal  fats 
are  formed  from  carbohydrates. 

Iron  also  appears  earlier  in  the  foetal  j)lacenta  than  in  the 
maternal,  and  it  is  probably  taken  up  from  the  hasmolysed 
maternal  blood  (p.  626). 

As  regards  the  passage  of  proteins  nothing  is  known  with 
any  certainty.  It  may  be  that  the  proteins  of  the  maternal 
blood  are  passed  to  the  fostus  unchanged.  Since  amino-acids 
are  found  in  the  fcetal  blood  it  has  been  argued  that  the  pro- 
teins may  be  digested  to  this  condition  before  passing  to 
the  foetus.     But  it   must   be   recognised   that   possibly   these 


630  VETERINARY  PHYSIOLOGY 

amino-acids  are  the  results  of  the  protein  metabolism  of  the 
fcBtus. 

Chemical  examination  of  the  allantoicfluid  of  ungulates,which 
is  foetal  urine  (p.  626),  shows,  in  the  early  stage  of  development,  a 
high  proportion  of  nitrogen  in  amino-acids,peptides  and  allantoin. 
This  seems  to  indicate  a  less  complete  catabolism  of  protein 
and  a  more  active  nuclear  metabolism  than  in  extra-uterine  life. 

The  placenta  manifests  little  power  of  regulating  the 
materials  which  are  passed  to  the  foetus,  and  most  drugs  and 
toxic  substances  reach  it.  Even  the  micro-organisms  which 
are  the  cause  of  various  diseases  may  pass  through  the 
placenta.  In  this  respect  it  seems  to  differ  from  the  cells  of 
the  choroid  plexus,  which  manifest  a  selective  action  on  the 
materials  which  it  passes  to  the  cerebro-spinal  fluid  (p.  511). 

IV.  Growth  of  the  Foetus. 

The  growth  of  the  foetus  is  steady  and  slow,  and  the  daily 
demands  on  the  mother  are  comparatively  small.  At  birth  the 
human  foetus  is  less  than  8  per  cent,  of  the  weight  of  the  mother, 
while  in  some  animals,  e.g.  the  dog,  the  litter  may  weigh  20  or 
25  per  cent,  of  the  maternal  weight. 

Considering  the  fact  that  the  power  of  fixing  material  in  the 
body  is  increased  during  pregnancy,  the  amount  of  the  food 
consumed  which  has  to  be  transmitted  to  the  foetus  is  com- 
paratively trivial. 

Only  in  the  later  period  of  intra-uterine  life  is  the 
demand  for  proteins,  and,  more  especially,  for  fats  in  any 
way  considerable. 

It  may  thus  be  said  that  under  all  conditions  of  normal 
nutrition  it  is  the  surplus  of  nourishment  which  is  passed  from 
the  mother  to  the  foetus,  and,  if  in  the  later  months  of 
pregnancy  her  nourishment  is  limited,  the  size  of  the  young  may 
be  reduced. 

The  maternal  tissues  part  more  readily  with  some  sub- 
stances than  with  others.  Thus  the  demands  of  the  foetus  for 
calcium,  when  the  supply  to  the  mother  is  inadequate,  may  be 
met  by  removal  of  calcium  from  her  bones,  which  may  thus  be 
softened.  On  the  other  hand,  the  maternal  tissues  do  not 
become  depleted  of  iron  in  the  same  way  to  meet  the  require- 
•ments  of  the  young. 


I 


DEVELOPMENT  631 

V.  Metabolism  in  Pregnancy. 

During  pregnancy  the  increase  in  the  metabolism  is  pro- 
portionate to  the  increase  in  the  weight  of  the  mother.  This 
increase  is  due  not  only  to  the  growth  of  the  foetus,  but  also  to  the 
growth  of  the  uterus  and  mammary  glands  and  to  the  formation 
of  the  amniotic  and  allantoic  fluids  which  are  inert.  Hence 
probably  the  metabolism  of  the  foetal  tissues  is  more  active 
than  that  of  the  maternal.  Experiments  upon  guinea-pigs 
support  this  conclusion. 

It  has  been  found  by  means  ot  the  respiratory  calorimeter 
(p.  259)  that  the  total  metabolism  of  the  mother  just  before 
delivery  is  practically  the  same  as  that  of  the  mother  and 
young  after  delivery. 

VI.  The  Young  Animal  at  Birth. 

At  birth  the  young  animal  is  suddenly  precipitated  from  its 
prolonged  bath  in  the  warm  amniotic  fluid  Avhere  its  temperature 
has  been  maintained,  and  where  it  has  received  a  steady  supply 
of  oxygen  and  of  food  without  any  exertion  on  its  part  into  the 
chill  air  of  the  outer  world  where  it  has  to  secure  its  oxygen 
by  the  efforts  of  breathing,  to  get  its  food  by  sucking  and 
digesting  and  to  maintain  its  temperature  by  its  own  meta- 
bolism. No  wonder  that  this  sudden  change  proves  too  much 
for  the  less  robust,  and  that  the  mortality  during  the  first  week 
of  life  is  high.  The  power  of  heat  regulation  is  not  at  once 
developed,  and  the  young  animal  at  first  tends  to  react  to 
the  temperature  of  its  surroundings  in  the  manner  of  a 
cold-blooded  animal.  In  a  day  or  two  its  power  of 
adaptation  improves  and  the  rate  of  its  chemical  changes 
increases  so  that  from  this  time  onwards  throughout  the  period 
of  active  growth  they  are  in  excess  of  those  of  the  adult  (p.  266). 

VII.  Fcetal  Circulation. 

The  performance  of  its  functions  by  the  placenta  is  associated 
with  a  course  of  circulation  of  the  blood  somewhat  different  to 
that  in  the  post-natal  state  (fig.  245). 

The  blood  coming  from  the  placenta  to  the  foetus  is  collected 
into  a  single  umbilical  vein,  u.v.,  which  passes  to  the  liver,  I. 
This   divides   into   the    ductus  venosus,   d.v.,   passing   straight 


632 


VETERINARY  PHYSIOLOGY 


through  the  organ,  and  into  a  series  of  capillaries  among  the 
cells.  From  these  the  blood  flows  away  in  the  hepatic  vein  to 
the  inferior  vena  cava,  'p.v.c,  and  mixed  blood  passes  to  the 
right  auricle.     In  this  it  is  directed  by  a  fold  of  endocardium, 


A.y. 

Fig.  245. — Scheme  of  Circulation  in  the  Foetus,  u.v.,  umbilical  vein  ;  d.v., 
ductus  venosus  ;  p.v.c,  inferior  vena  cava  pouring  blood  through  the 
right  auricle  and  through  the  foramen  ovale,  f.o.,  into  the  left  heart ; 
a.v.c,  superior  vena  cava  bringing  blood  from  the  head  to  pass  through 
the  right  side  of  the  heart,  and  through  the  ductus  arteriosus,  d.a.  ; 
Jit. v.,  portal  vein.  The  degree  of  impurity  of  the  blood  is  indicated  by 
the  depth  of  shading. 

through  the  foramen  ovale,  f.o.,  a  hole  in  the  septum  between 
the  auricles,  and  it  thus  passes  to  the  left  auricle,  and  thence  to 
the  left  ventricle,  l.v.,  which  drives  it  into  the  aorta,  a.a.,  and 
chiefly  up  to  the  head,  ant.a.  From  the  head  the  blood  returns 
to  the  superior  vena  cava,  a.v.c,  and,  passing  through  the  right 


DEVELOPMENT  683 

auricle,  enters  the  right  ventricle,  r.v.,  which  drives  it  into  the 
pulmonary  artery,  p.a.  Before  birth  this  artery  opens  into  the 
aorta  by  the  ductus  arteriosus,  d.a.,  while  the  branches  to  the 
lungs  are  still  very  small  and  unexpanded.  In  the  aorta,  this 
impure  blood  from  the  head  mixes  with  the  purer  blood  from 
the  left  ventricle,  and  the  mixture  is  sent  to  the  lower  part  of 
the  body  through  the  descending  aorta,  yo.a.  From  each  iliac 
artery,  i.a.,  an  umbilical  artery,  u.a.,  passes  off,  and  these  two 
vessels  carry  the  blood  in  the  umbilical  cord,  u.c,  to  the 
placenta. 

When  the  animal  is  born,  the  flow  of  blood  between  it  and 
the  mother  is  arrested.  As  a  result  of  this,  the  respiratory 
centre  is  no  longer  supplied  with  pure  blood,  and  is  stimulated 
to  action.  The  lungs  are  thus  expanded  and  the  blood 
flows  through  them.  In  the  ductus  venosus  a  clot  forms 
and  the  vessel  becomes  obliterated.  The  ductus  arteriosus  also 
closes  up,  and  the  foramen  ovale  is  occluded.  The  circulation 
now  takes  the  normal  course  in  post-natal  life. 

VIII.  Gestation  and  Delivery. 

The  length  of  gestation  is  different  in  different  animals — 


Mare  . 
Cow  . 

.     11  months  (330  to  340  days) 
.       9        „        (270  to  290     „   ) 

Sheep 
Sow    . 

.       5       „        (145  to  155     „   ) 
.       4       „        (115  to  120     „   ) 

Dog    . 

Cat     . 

.     62  days 
.     63     „ 

At  the  end  of  this  period  labour  occurs,  and  the  foetus  and 
its  membranes  are  expelled. 

The  mechanics  of  labour  must  be  studied  with  obstetrics. 

Nervous  Control. — The  uterus  is  supplied  by  fibres  leaving 
the  spinal  cord  by  true  sympathetic  fibres  from  the  splanchnics. 
These  fibres  pass  to  the  inferior  mesenteric  ganglion  and  on  in 
the  hypogastric  nerves  to  end  in  two  large  plexuses  or  ganglia, 
containing  numerous  nerve  cells,  one  on  each  side  of  the  cervix. 
From  these,  fibres  pass  to  the  uterus  and  to  the  vagina. 

There  is  evidence  that,  in  lower  animals  at  least,  the 
contents  of  the  uterus  may  be  expelled  after  complete  separa- 


634  VETERINARY  PHYSIOLOGY 

tion  of  the  organ  from  the  central  nervous  system.  The  peri- 
pheral mechanism,  like  that  of  the  intestine  and  bladder,  is 
capable  of  independent  action.  But  normally  a  centre  in  the 
lumbar  enlargement  of  the  spinal  cord  appears  to  be  excited 
reflexly.  This  centre  is  further  acted  upon  by  the  brain,  and 
various  disturbances,  accompanied  by  emotional  changes,  may, 
for  a  time,  arrest  uterine  contraction. 

IX.  Lactation. 

1.  The  Mammary  Gland. — The  mammary  glands  consist 
essentially  of  a  collection  of  specially  developed  sebaceous 
glands,  the  function  of  which  has  been  modified  to  yield  a 
nutritive  secretion  for  the  young.  In  some  of  the  lower 
mammals  (Monotremes)  the  secretion  is  provided  by  the 
enlarged  skin  glands  which  open  direct  on  to  the  surface  of 
the  abdominal  wall,  and  the  young  are  nourished  by  licking 
the  wall  where  aggregations  of  these  occur.  In  the  higher 
mammals  the  glands  are  more  highly  developed  and  more 
definitely  collected  into  groups  forming  the  mammar}^  glands, 
with  sinuses  to  act  as  reservoirs  for  the  secreted  fluid,  and  a 
teat  for  convenience  of  the  young  in  sucking. 

The  number  of  glands  present  is  roughly  in  proportion 
to  the  usual  number  of  young  born  at  a  time.  The  mare, 
the  sheep,  and  the  goat  have  two  glands.  The  pig  has  ten 
to  fourteen,  the  dog  eight  to  twelve.  The  udder  of  the  cow 
is  usually  spoken  of  as  having  four  "quarters."  A  fibrous 
septum  in  the  median  line  divides  the  udder  into  two 
halves.  There  is  no  dividing  line  between  the  two  quarters 
of  the  same  side,  though  the  sinuses  of  the  same  side  do  not 
communicate. 

2.  Physiology — (1)  Development.  —  Rudimentary  glands 
are  present  in  both  male  and  female  animals.  In  the  male, 
normally  they  remain  undeveloped.  In  the  female,  as 
sexual  maturity  is  reached,  the  gland  increases  in  size,  the 
increase  consisting  chiefly  of  fibrous  tissue  with  a  large 
amount  of  fat.  In  pregnancy,  proliferation  of  the  glandular 
tissue  occurs.  The  tubules,  which  were  solid  blocks  of  cells, 
grow  outward  and  alveoli  develop.  As  the  cells  of  the 
tubules  and  the  alveoli  divide,  some  remain  attached  to  the 


DEVELOPMENT  635 

basement  membrane,  and  some  come  to  lie  in  the  lumen  of 
the  tubules  and  the  cavities  of  the  alveoli.  These  latter 
undergo  fatty  degeneration  and  are  shed  with  the  first 
milk — colostrum.  It  is  the  cells  left  on  the  basement  of 
the  membrane  of  the  alveoli  that  elaborate  the  constituents 
of  the  milk. 

(2)  Regulation  of  Activity — ((()  Chemical  Stimulation. — This 
subject  is  dealt  with  on  page  609.  (6)  Nerve  Stimulation. — 
The  extent  to  which  the  secretion  of  milk  is  influenced  by 
the  nervous  system  has  not  been  determined  with  certainty. 
After  secretion  of  all  the  nerves  passing  to  the  gland,  if  the 
animal  be  lactating,  secretion  continues,  though  the  amount 
may  be  diminished  ;  if  the  animal  be  pregnant,  glandular 
development  proceeds,  and  at  parturition  normal  secretion 
of  milk  occurs.  On  the  other  hand,  pain  or  excitement 
reduces  the  quantity  of  milk.  Whatever  influence  the 
nervous  system  does  exert  is  probably  produced  through 
vasomotor  nerves  and  intrinsic  nerves  of  the  gland.  On  the 
whole,  it  seems  certain  that  control  through  the  nerves  is 
subsidiary  to  chemical  control  through  the  blood. 

The  flow  of  the  milk  can  be  influenced  by  the  central 
nervous  system.  The  walls  of  the  ducts  contain  muscle 
fibres  which  can  act  by  constricting  the  lumen  and  stopping 
the  flow.  In  this  way  the  cow  is  able  to  "hold  up"  its 
milk,  as  often  occurs  when  milking  is  attempted  by  a  person 
to  whom  the  animal  is  unaccustomed,  or  when  a  cow  that 
has  been  sucked  by  its  calf  is  milked  by  hand.  (c) 
Mechanical  Stimulation. — The  distension  of  the  ducts  with 
milk  inhibits  further  secretion.  The  periodical  emptying  of 
the  udder,  therefore,  by  sucking  or  milking  is  necessary  to 
maintain  functional  activity.  The  influence  of  the  sucking 
is  not  entirely  due  to  the  relief  of  the  distension.  The 
mechanical  stimulation  probably  induces  secretion  through  a 
nervous  reflex,  as  by  this  means  a  flow  of  milk  may  be 
produced  in  a  virgin  animal  (p.  609). 

3.  Composition  of  Milk — (1)  Adaptation  for  Needs  of  Young 
Animal. — Milk  is  produced  to  supply  material  and  energy 
to  a  rapidly  growing  animal.  The  materials  present,  there- 
fore, are  in  proportion  to   the  requirements  for   growth.      It 


636  VETERINARY  PHYSIOLOGY 

has  already  been  shown  (p.  372)  that  the  percentage  of 
proteins  present  varies  with  the  rate  of  formation  of  new  tissue. 
The  relationship  between  the  salts  of  the  milk  and  those  con- 
tained in  the  tissues  of  the  growing  animal  is  shown  by  the 
following  table  given  by  Bunge.  The  percentage  composi- 
tion of  the  chief  inorganic  constituents  of  the  tissues  of  a 
young  rabbit,  of  the  milk  it  was  receiving,  and  of  the  serum 
of  the  mother's  blood  are  compared  :— 


Potash    . 

Kabbit 
14  days  old. 

10-8 

Kabbit's 
Milk. 

10-1 

Mother's 
Blood  Serum 

3-2 

Soda 

6-0 

7-9 

54-7 

Lime 
Magnesia 

35-0 
2-2 

35-7 

2-2 

1-4 
0-6 

Iron  Oxide 

0-23 

0-08 

0- 

Phosphoric  Acid 
Chlorine 

41-9 
4-9 

39-9 
5-4 

3-0 

47-8 

This  close  adaptation  of  the  composition  of  the  milk  to 
the  needs  of  growth  affords  an  explanation  for  the  difference 
in  percentage  composition  of  the  milk  of  different  species. 
It  also  accounts  for  the  well-recognised  fact  that,  after  wean- 
ing, especially  if  this  takes  place  too  early,  the  rate  of  gain 
of  weight  per  day  suffers  a  marked  decrease,  since,  in 
practice,  no  combination  of  food-stuffs  can  yield  the  perfect 
proportion  of  the  necessary  materials  which  is  present  in 
milk, 

(2)  Constituents — (i.)  Protein. — The  chief  protein  of  milk 
is  casein,  a  phospho-protein.  It  exists  in  milk  in  the  form 
of  a  calcium  salt.  Casein  contains  all  the  amino-acids, 
except  one,  necessary  for  building  up  the  various  proteins  of 
the  young  animal.  The  one  absent — glycine — can  be  easily 
formed  in  the  body  from  other  amino-acids.  The  other 
proteins  present  are  an  albumin  and  a  globulin,  which  closely 
resemble  those  found  in  the  blood. 

(ii.)  Fat. — Milk  fat  consists  of  olein,  palmitin,  and  stearin 
to  the  extent  of  nearly  90  per  cent.  The  remaining  10 
per  cent,  consists  of  fats  of  lower  molecular  weight.  It  is 
the  latter  that  gives  the  characteristic  flavour  to  butter. 
Lecithin  and   cholesterin   are    also   present.      The   phospho- 


DEVELOPMENT  G37 

lipins  of  the  milk  are  much  more  abundant  than  those  in 
the  blood  serum. 

The  fat  occurs  in  the  form  of  minute  globules.  Coal- 
escence is  prevented  by  the  surface  tension  of  the  spheres, 
and  by  the  adsorption  of  protein  on  the  surface.  It  cannot 
be  separated  from  milk  by  ether  till  the  globules  are  broken 
down  by  an  acid  or  an  alkali. 

The  specific  gravity  of  the  fat  is  "93,  while  that  of  the 
milk  free  from  fat  is  about  I'OSo.  The  globules  of  fat, 
therefore,  tend  to  rise  to  the  surface  to  form  cream.  The 
mechanical  agitation  in  churning  causes  the  globules  to 
coalesce  with  the  formation  of  butter. 

(iii.)  Carbohydrates. — The  carbohydrate  of  the  milk  is 
lactose  (p.  286).      It  only  occurs  in  milk. 

(4)  Ash. — The  ash  of  milk  contains  the  same  inorganic 
substances  as  are  found  in  the  tissues  (p.  636).  The 
substance  present  in  greatest  amounts  are  calcium  and 
phosphorus.      Iron  is  present  only  in  traces. 

3.  The  proportion  in  which  constituents  are  present  in 
the  milk  of  some  species  is  shown  in  the  following  table  : — 

(8)  Percentage  Composition. 


Cow. 

Goat. 

Mare. 

Sow. 

Bitch. 

Water  . 

86-3 

85-7 

91-6 

84-0 

75 

Protein 

3-0 

4-3 

10 

7-3 

10 

Fats       . 

3"5 

4-8 

1-3 

4-5 

11 

Carbohydrates 

45 

4-4 

5-7 

3-1 

3 

Ash       . 

0-7 

0-8 

0-4 

1-1 

1 

The  cells  of  the  mammary  gland  transfer  to  the  milk 
many  substances  administered  Avith  the  food,  hence  the  taste 
of  the  milk  may  be  altered  and  certain  drugs  given  by  mouth 
may  appear  in  the  milk. 

(4)  Colostrum. — The  fluid  drawn  from  the  gland  for  the 
first  two  or  three  days  after  parturition  difl^ers  from  true 
milk.      It  is  characterised  by^ 

(1)  A  large  proportion  of  solids — 25  to  30  per  cent. 

(2)  A  high  percentage  of  albumin  and  globulin. 

(3)  The  presence  of  multinuclear  bodies,  probably  leuco- 
cytes and  desquamated  glandular  cells. 


638  VETERINARY   PHYSIOLOGY 

The  percentage  of  fat  and  carbohydrates  are  not  markedly 
different  from  those  of  milk. 

Colostrum  is  valuable  to  the  newly  born  animal.  Its 
food  value  is  high,  and  it  stimulates  evacuation  of  the  bowels. 

(5)  Factors  affecting  Milk  Production  in  Cows — (1)  Breed- — 
The  amount  and  composition  of  the  milk  produced  is  partly 
dependent  upon  the  breed  of  cows.  Thus  the  milk  of  the 
Jersey  is  especially  rich  in  fat,  frequently  containing  over 
5  per  cent.,  while  that  of  the  Friesian  Holstein  and  the 
Holderness  has  a  comparatively  low  fat  content.  In  general, 
the  breeds  noted  for  a  large  yield  give  milk  with  a  low 
percentage  of  fat.  The  differences  due  to  breeds  are,  how- 
ever, comparatively  small — less  than  what  is  often  found 
between  individuals  of  the  same  breed. 

(2)  Milking. — It  has  been  definitely  proved  that  in  a 
cow  with  a  large  yield  the  cavities  of  the  udder  have  not 
the  capacity  to  contain  all  the  milk  that  can  be  obtained  at 
a  milking.  There  must  be  rapid  secretion  during  the  time 
of  milking.  In  the  interval  between  the  milkings  secretion 
is  most  active  when  the  udder  is  empty.  As  the  cavities 
get  filled  the  distension  appears  to  inhibit  secretion.  The 
more  frequent  the  emptying  of  the  udder  therefore  up  to  the 
point  where  the  prejudicial  influence  of  over-action  of  the 
gland  cells  appears,  the  greater  the  quantity  of  milk  obtained. 
It  is  estimated  that  three  milkings  per  day  may  yield  nearly 
5  to  20  per  cent,  more  milk  than  two,  and  two  may  yield 
nearly  50  per  cent,  more  than  one. 

When  the  intervals  between  the  milkings  are  of  an 
unequal  length,  milk  with  a  higher  fat  content  is  got  after 
the  shorter  period.  It  has  been  suggested  that  this  is  due 
to  the  distension  of  the  ducts,  which  occurs  in  the  longer 
interval,  preventing  the  extrusion  of  the  fat  globules  from 
the  secreting  cells  and  consequently  slowing  the  synthesis  of  far. 

Marked  differences  occur  in  the  fat  content  of  the  milk 
drawn  at  different  stages  of  milking.  The  first  portion  may 
contain  less  than  1  per  cent.,  the  last  portion  drawn,  "  the 
strippings,"  may  contain  over  10  per  cent.  The  fat  only  is 
involved,  the  other  constituents  remain  uniform  throughout 
the  period  of  milking. 


DEVELOPMENT  639 

The  complete  emptying  of  the  udder  at  milking  is 
necessary,  not  only  to  obtain  the  valuable  fat  in  the  last 
portion,  but  to  maintain  the  full  activity  of  the  gland.  In 
incomplete  milking  the  amount  secreted  diminishes. 

(3)  Food. — (1)  The  percentage  composition  of  tlie  milk  of 
any  individual  animal  is  within  wide  limits  independent  of 
the  relative  proportions  of  the  constituents  of  the  food.  Only 
small  deviations  from  the  normal  standard  can  be  obtained 
by  feeding  excessive  amounts  of  one  constituent.  So  great 
is  the  tendency  to  secrete  milk  of  normal  composition  that 
when  one  of  the  constituents  is  deficient  in  the  food,  the  animal 
draws  upon  its  own  body  for  material,  salts  being  supplied  by 
the  skeleton  and  protein  and  fat  by  the  tissues.  The  amount 
secreted  is  however  diminished.  This  prevents  undue 
depletion  of  the  body.  The  body  possesses  only  a  small 
reserve  store  of  carbohydrate  (p.  353).  In  an  experiment  in 
■which  the  carbohydrate  of  the  food  was  reduced,  and  the 
reserve  store  depleted  by  drawing  off  sugar  through  the 
kidneys  by  means  of  phloridzin,  it  was  found  that  instead  of 
milk  deficient  in  sugar  being  secreted  the  quantity  decreased. 


ilk  CO. 

Lactose  per  cent. 

230 

3-9 

238 

4-0 

238 

3-9  )     Sugar  drawn  off 
3-8  j      bv  phloridzin. 

218 

170 

3-8 

(2)  Nature  of  Fat. — The  composition  of  fat  can  be 
modified  by  the  food.  Abnormal  fats  fed  may  appear  in  the 
milk.  Feeding  stuffs  rich  in  oils,  such  as  linseed  cake, 
produce  butter  which  is  deficient  in  the  higher  fatty  acids 
and  consequently  soft  at  a  low  temperature. 

(3)  Yield. — So  long  as  food  is  given  to  supply  (1)  the 
maintenance  requirement  of  the  animal  and  (2)  sufficient 
material  for  milk  formation,  the  yield  depends  upon  the 
capacity  of  the  animal  as  a  milk  producer  much  more  than 
on  the  food.  When,  however,  cows  are  put  to  graze  on 
young  pasture-grass  increased  flow  almost  invariably  occurs, 
and   in   certain    experiments,    increasing    the    proportion    of 


640  VETERINARY  PHYSIOLOGY 

protein  in  the  diet  increased  the  yield.  Excess  of  carbo- 
hydrate or  fat  tends  to  fat  formation  and  deposit  in  the 
animal's  body. 

The  food  requirements  for  milk  cows  are  given  on 
page  375. 

(4)  Housing. — It  was  formerly  a  custom  to  restrict 
ventilation  for  the  purpose  of  maintaining  heat  in  byres, 
the  idea  being  that  in  warm  byres  the  milk  flow  Avas 
increased.  Spier  and  Hendrick  have  shown  that  cool  byres 
and  free  ventilation  do  not  reduce  milk  production.  The 
following  are  results  obtained  at  different  temperatures  : — 


Aver.  temp. 

Milk  lbs. 

Fat 

of  byre. 

per  cow. 

per  cent. 

Cool  byres,  free  ventilation     41-2°  F. 

29-0 

3-51 

Warm  byres,  restricted 

ventilation     .         .         .     61  "T"  F. 

28-9 

3-48 

m 


The  animals  in  the  cool  byres  had  better  coats  and  were 
better  condition  at  the  end  of  the  winter. 

It  has  been  found  however  that  a  marked  decrease  in  the 
temperature  reduces  milk  secretion,  and  that  animals  exposed 
in  winter  eat  more  food  than  those  comfortably  housed. 

The  temperature  below  which  more  food  is  required,  or 
at  which  the  milk  secretion  begins  to  diminish,  depends  upon 
the  critical  temperature  (p.  2  7 1 )  of  the  animal.  Unfortunately 
comparatively  little  work  has  been  done  to  determine  the 
critical  temperature  of  dairy  cows.  Owing  to  the  stimulus 
to  metabolism  caused  by  the  large  amount  of  food  eaten  it  is 
probably  comparatively  low.  From  the  evidence  available  it 
would  appear  to  be  not  higher  than  7°  to  S''  C.  To  have 
animals  housed  in  an  atmosphere  above  the  critical 
temperature  should  lead  neither  to  a  saving  in  food  nor  to 
an  increased  production  of  milk. 


APPENDICES 


I.  SOME   ELEMENTARY   FACTS   OF   ORGANIC  CHEMISTRY 

The  following  elementary  facts  may  help  the  student  who  has  neglected 
the  study  of  the  outlines  of  Organic  Chemistry  in  understanding  the 
chemical  jJi'oblems  of  physiology. 

Organic  compounds  are  built  round  the  four-handed  carbon  atom 

! 
— c— 

I 

When  each  hand  links  to  the  one-handed  hydrogen  atom, 

Methane—  H 

i 
H— C— H         is  formed. 

I 
H 

By  taking  away  a  hydrogen  atom  from  two  Methane  molecules  and 
linking  the  two  molecules  together 

Ethane—  H   H 

I       ! 
H — C— C— H         is  produced. 

I       I 
H    H 

By  further  linking  more  and  more  of  those  molecules  together,  similar 
molecules  containing  three,  four,  five  or  more  carbon  atoms  are  jsroduced. 

When  the  carbons  are  arranged  in  a  straight  line  the  normal  series  is 

produced — 

I       I       I       I 
— C— C-C-C— 

I    I    I    I 

Where  the  line  is  branched  the  series  is  known  as  iso — 

I 
'C— 


I       I 


When   each   carbon   has  its  due  proportion  of  hydrogen  atoms   it   is 
41  6U 


642  APPENDICES 

saturated,  but  if  two  hydrogen  atoms  are  let  go,  the  unocsupied  hands  of 
the  carbon  may  join  and  form  an  unsaturated  molecule,  thus  : — 

Ethane  becomes  Ethylene         H   H 

!     I 
H— C  =  C— H 


When  one  hydrogen  atom  is  taken  away,  and  the  molecule  has  a  hand 
ready  to  link  with  some  other  substance,  a  radicle  is  constituted,  and  these 
are  known  as  Methyl,  Ethyl,  etc. 

Alcohols- — If  one  of  the  hydrogen  atoms  of  Methane  is  oxidised  to 
hydroxyl  (  -OH),  an  alcohol  is  formed — 

The  hydroxyl  group  ( — OH)  is  characteristic  of  alcohols. 

H 

H— 0— OH         Methvl  Alcohol. 
I 
H 


Similarly  Ethane  gives 


H   H 

i      I 
.C-C— OH        Ethvl  Alcohol, 

I      I 
H   H 


and  so  on  for  compounds  having  a  longer  carbon  chain. 

If  only  one  — H  is  oxidised  to  — OH,  the  alcohol  formed  is  termed 
Monohydric. 

If  tAvo  — H  atoms  are  oxidised,  the  alcohol  is  Dihydric. 

If  more  than  two  — OH  groups  are  present,  the  alcohol  is  Polyhydric,  e.g. 

H    H    H 

I       I       I 
HO— C— C— C— OH         Glycerol  (Glycerine). 

1       I       I 
H  OH  H 

Propane  C^H^,  and  compounds  having  more  than  three  carbon  atoms, 
may  form  more  than  one  monohydric  alcohol,  according  to  the  C  atom  on 
which  the  — OH  is  placed. 

Thus,  there  are  two  propyl,  four  butyl,  and  eight  amyl  alcohols. 

Primary  Alcohols  are  those  in  which  a  terminal  carbon  is  oxidised. 

Secondary  Alcohols  have  one  or  more  of  the  middle  carbons  oxidised. 

Polyhydric  alcohols  may  contain  primary  or  secondary  groups  or 
both. 

Aldehydes — When,  from  a    Primary    Alcohol,   two    hydrogens    are 


APPENDICES  6VS 

removed,  the  vacant  hand  of  the  oxygen  links  to  the  vacant  hand  of  the 
terminal  carbon  to  form  an  Aldehyde — 
H 
I 
H— C— C  =  0         Ethyl  Aldehyde. 

I       I 
H    H 

Ketones- — These  are  formed  in  the  same  way  from  the  Secondary 
Alcohols,  a  carbon  atom,  which  is  not  the  terminal  one,  being  involved, 
thus  :— 

H    O    H 

TT     J,      '1      I      rr         Acetone,  the  Ketone  of  Secondary 
I  I  Propyl  Alcohol. 

H  H 

Acids. — If  the  hydrogen  of  the  terminal  carbon  atom  of  the  Aldehyde 
is  replaced  by  hydroxyl  —OH  an  acid  is  produced — 
H  :  0 
I    i   II 
H— C--C— 0— H        Acetic  Acid. 
I 
H 

The  carhoxyl  group  (to  the  right  of  the  dotted  line)  is  characteristic  of  the 
acids. 

The  oxidation  may  be  carried  on  at  each  end  of  the  line  ;  divalent  acids 
being  thus  produced — 

0     0 

H— 0— C— C— 0— H        Oxalic  Acid. 

If,  in  the  radicle  of  one  of  these  acids,  a  hydrogen  is  replaced  by  hydroxyl 

— OH,  an  oxy-acid  is  formed,  thus  : — 

H    H    0 

I       I      II 
H— C— C— C— 0— H         Propionic  Acid. 

I       I 
H    H 

Tins  may  be  converted  to  the  two  Lactic  acids  called  respectively  a  and 

3  hydroxy-propionic  acid,  according  to  the  carbon  whif^li  is  oxidised. 

H  OH  0 

I      I      II 
a         H-C-C— C— 0-H 


and 


I       i 
H    H 


OH  H    O 

!     I     II 

H-C— C— C-O-H 


H    H 

Similarly  oxy-acids  are  formed  from  the  divalent  acids. 


644 


APPENDICES 


Ethers. — These  are  formed  by  the  union  of  two  alcohol  molecules, 
with  dehydration. 


CH.  .  CHo 
CH.,  .  CH. 


OH 


CHo  .  CH.3 


CH.,  .  CH. 


Esters. — These  are  formed  by  linking  an  alcohol  and  an  acid  mole- 
cule with  dehydration. 

CH3 . CHo    OH 


CH.,  .CO      OH 


CH.J  .  CH. 
CH.,  .  CO 


^0 


Anhydrides.— These  are  formed  by  the  union  of  two  acid  molecules 
with  dehydration. 


CH3 . CO .  ;  OH 
CH. .  CO .  oIh 


CH, . CO 


CH.,  .  CO 


Cyclic  Compounds. 
(1)  An  important  series  of  carbon  compounds  contains  a  ring  of  six 
carbons,  each  with  an  unsatisfied  affinity,  thus : — 

1 
C 


c 

I 

When  each  hand  holds  a  hydrogen,  Benzene  is  formed. 

These  hydrogens  may  be  replaced  by  yarious  molecules  giving  rise  to 
a  large  series  of  different  compounds. 

(2)  If  a  ring  contains  atoms  of  other  elements  besides  carbon,  it  is  called 
heterocyclic.     One  of  the  most  important  of  these  is  Pyrrol — 
H— C— C— H 

H-C    C— H 


N 

which  occurs,  linked  to  a  benzene  ring,  in  certain  important  constituents 
of  the  protein  molecule. 


APPENDICES  645 


Nitrogen-containing  Compounds. 

Ammonia. — The  three-handed  Nitrogen  by  linking  with  three  hydro- 
gens forms  Ammonia, 

H 

i 

H— N— H 

If  one  of  these  hydrogens  is  removed,  Amidogen,  — NH„,  which  can  link 
with  other  molecules,  is  produced. 

Amino  Acids- — If  one  of  the  hydrogen  atoms  directly  joined  to 
carbon  in  the  radicle  of  an  acid  is  replaced  by  amidogen,  a  mon-amino 
acid  is  formed,  thus  : — 

H     O 

/N — C — C — 0 — H         Amino  acetic  acid. 
H  i 

H 

When  two  hydrogen  atoms  are  thus  replaced,  a  di^amino  acid  is  produced — 
NH„NH„   0 

r  r  ii 

H — C  —  C  —  C — 0 — H  or,  «,  3^  di-amino  propionic  acid. 

i        I 
H     H 

Amines. — In  these,  NHa  takes  the  place  of  OH  of  an  alcohol ;  or,  looked 
at  in  another  way,  they  are  formed  from  NH3,  by  replacement  of  hydrogen 
atoms  by  alkyl  groups. 

H  H  H    H 

I 


1      I 
H— C— |0-H     H 


-N        =H— C— N         Methvl-amine. 

I  !      I 


H  H  H    H 

Methyl  alcohol.  Ammonia. 

Di-  and  tri-amines  may  be  formed  thus — 

CH3 
CH3  \^^ 

]]i:r=NH     Di-methvl  amine.  CH3  J>N     Tri-methvl  amine. 

CH3  '  ^^^^ 

CH3 

1  The  carbon  atoms  are  named  a,  ,3,  and  7  from  their  position  in  relation- 
ship to  the  carboxyl  group. 


646 


APPENDICES 


Amides.— If  the  amidogen  molecule  takes  the  place  of  the  hydroxyl : 
the  carboxyl  of  an  acid,  an  amide  results,  thus  : — 

HO  H 


H-iJ_ 

1 

0_H— H 

1 

H 

H 

Acetic  acid. 

H    0    H 

1      11      1 
H— C-C-N 

1 

1 
H          H 

Acetamide. 

Compare  'Methylanmie,  CHgNH.,  with  Acetamide,  CH3 — CO — NH.,. 
From  the  divalent  carbonic  acid — 


H-0— C-O-H 

0 

H\         ii              H 

>N-U-Nv 
H/                       -H 

the  important  substance  Urea  or  Carbonic  Diamide. 
Urea  molecules  may  link  together — 
(a)  By  dropping  NH3  when  Biuret  is  produced- 

0     H     0 

Hx  II       I       II         /H 

\N— C— N— C— N( 


H' 


^H 


(6)  By  holding  on  to  an  intermediate  radicle  of  an  acid,  e.g.  an  unsatu- 
rated three  carbon  acid.  These  are  Diureides,  or  Purins,  of  which  the  most 
important  is  Uric  Acid — 

0 


H— N- 

I 

o=c 

I 

H— N- 


-N— H 
^C=0 

-N--H 


APPENDICES  647 


II.    CLASSIFICATION    OF   THE   PROTEINS. 

I.  Native  Proteins. 

A.  Uncombined. 

1.  Poor  in  Di-amino  Acidi. 

(i.)  Albumins.— Coagulated  on  heating;  soluble  in  water ; 

not  precipitated  by  half  saturation  with  (NHJ.^SO^. 

(ii.)  Globulins.— Coagulated    on   heating;    insoluble    in 

water ;     precipitated     by    half    saturation     with 

(NHJ.SO,. 

2.  Rich  in  Di-amino  Acids. 

(i.)  Protamines  from  the  heads  of  spermatozoa, 
(ii.)  Histones  from  blood  corpuscles,  e.g.  globin  of  haemo- 

L'lobin. 


B.  Combined. 

1.  Phospho-proteins.— Yield     phosphoric    acid    on    decom- 

position, but  not  purin  bases,  e.g.  vitellin  of  yolk  of  egg 
and  carcinogen  of  milk. 

2.  Nucleo-proteins.— Compounds  of  protein  with  nucleic  acid. 

The  nucleo-proteins  with  the  largest  amount  of  nucleic 
acid  are  called  nucleins.  Nucleic  acid  may  be  broken 
down  into  phosphoric  acid,  a  carbohydrate,  purin  bases 
and  pyrimidin  bases. 

3.  Gluco-proteins,  e.g.  mucin  (p.  35). 

4.  Cbromo-proteins,  e.g.  haemoglobin  (p.  486). 

II.  Modified  Proteins.    The  Sclero-proteins. 

1.  Collagen  with  its  hydrate  gelatin  (p.  39 j. 

2.  Elastin  (p.  39). 

3.  Keratin  fp.  32). 


III.  Products  of  Digestion  of  the  Proteins. 

1.  Proteoses  (p.  16  et  seq.). 

2.  Peptones  (p.  16  et  seq.). 

3.  Polypeptides  (p.  19). 


648  APPENDICES 


III.  SOME  ELEMENTARY  FACTS  OF   PHYSICS. 
Diffusion. 

The  molecules  of  a  gas  are  in  continual  movement,  and  if  two 
different  gases  be  brought  into  contact  the  molecules  of  the  two  gases 
freely  intermingle  till  a  homogeneous  mixture  results.  The  molecules  of  a 
liquid  are  also  in  continual  motion,  the  only  difference  from  the  gaseous 
state  being  that,  owing  to  the  greater  concentration  of  the  molecules,  the 
paths  are  more  restricted.  When  two  miscible  fluids,  e.g.  alcohol  and 
water,  are  brought  into  contact,  intermingling  of  molecules  occurs  as  in 
the  case  of  the  gases  with  a  resulting  homogeneous  mixture.  This 
phenomenon  whereby  two  gases  or  two  miscible  fluids  in  contact  become 
uniformly  mixed  is  known  as  diffusion.  It  may  occur  through  a 
membrane  permeable  to  the  molecules. 

In  solids,  molecules  are  also  in  motion,  but  the  paths  are  so  restricted 
and  the  mutual  attraction  of  the  molecules  so  great  owing  to  their 
proximity  to  each  other,  that  difi'usion  between  solids  in  contact  is 
extremely  difficult,  and  only  proceeds  at  a  very  slow  rate.  That  it  can 
occur,  however,  has  been  shown  by  the  fact  that  if  gold  and  lead  be  kept 
in  contact  for  several  years  some  gold  is  found  to  be  deposited  on  the  lead. 

Kinetic  Energy  of  Molecules. 

As  molecules  have  a  definite  mass,  their  movement  endows  them  with 
kinetic  energy  which  is  in  proportion  to  their  velocity.  When  the 
temperature  of  a  substance  is  raised  the  velocity  of  the  molecules  is 
increased,  and  as  a  result  their  kinetic  energy  is  increased.  Consequently 
when  a  gas  is  heated  the  molecules  impinge  on  the  containing  vessel  with 
gi  eater  force,  tending  to  distend  it.  In  other  words,  the  pressure  of  the 
gas  is  increased. 

When  liquids  are  heated  the  kinetic  energy  of  the  molecules  is 
increased  so  that  they  may  break  loose  from  the  surface  of  the  fluid  and" 
become  free,  forming  gas,  as  occurs  when  water  reaches  100°  C.  at  ordinary 
atmospheric  pressure.  In  the  same  way,  under  the  influence  of  heat,  the 
molecules  of  a  solid  may  increase  their  movement  and  the  solid  become  a 
liquid.  Conversely,  reducing  the  temperature  decreases  the  kinetic 
energy  and  the  velocity  of  the  molecules,  and  consequently  a  gas  may 
become  a  liquid,  and,  on  still  further  cooling,  a  solid. 

The  heat  of  a  substance  is  thus  identical  with  its  molecular  energy. 
As  heat  is  added  the  velocity  and  consequently  the  energy  of  the  mole- 
cules are  increased  ;  as  heat  is  withdrawn  the  velocity  is  decreased. 

Solutions. 

If  a  piece  of  solid  cane  sugar  be  put  in  water  some  of  the  molecules  of 
the  sugar  break  loose  from  the  surface  of  the  solid  and  move  among  the 


APPENDICES  649 

molecules  of  the  water.  As  in  the  case  of  two  miscible  fluids  in  contact  a 
uniform  mixture  is  produced.  The  sugar  is  said  to  go  into  solution  in  the 
water.  The  sugar  is  termed  the  solute  and  the  water  the  solvent.  The 
solvent  is  usually  a  licjuid.  The  solute  may  be  a  solid,  e.g.  sugar  in  water, 
a  liquid,  e.g.  alcohol  in  water,  or  a  gas,  e.g.  CO,  in  water. 

In  their  erratic  flight  some  of  the  molecules  of  the  sugar  in  solution 
impinge  upon  the  solid  sugar.  When  the  free  molecules  of  the  solute  have 
reached  such  a  degree  of  concentration  in  the  solvent  that  as  many 
molecules  are  striking  the  solid  as  are  leaving  it,  no  higher  concentration  is 
possible,  and  the  result  is  termed  a  saturated  solution.  If  the  solution 
be  heated  the  kinetic  energy  of  the  molecules  of  the  sugar  both  in 
solution  and  in  the  solid  mass  is  increased  and  more  break  free  from  the 
solid,  and  consequently  the  degree  of  concentration  is  increased.  This 
explains  why,  under  the  influence  of  heat,  a  substance  dissolves  more 
rapidly  and  a  higher  degree  of  concentration  is  obtained. 

According  to  the  foregoing  description  a  solution  can  be  regarded  as  a 
homogeneous  mixture  of  two  substances,  the  molecules  of  which  are  in  free 
movement  throughout  the  whole  of  the  mixture. 

Colloids  (see  p.  12). 

The  essential  character  of  the  colloidal  state  consists,  in  the  existence 
together  in  a  physical  combination,  of  two  substances,  one  of  which  is  in 
the  form  of  ultra-microscopic  particles  dispersed  in  the  other.  The 
dispersed  particles  are  separated  from  each  other  by  the  containing  sub- 
stance which  forms  a  continuous  film  or  medium  surrounding  the  particles. 
Each  particle  has  thus  a  surface  of  contact  with  the  substance  forming  the 
continuous  medium.  At  that  surface  the  dispersed  particle  is  internal 
and  the  continuous  substance  external.  In  this  colloid  state,  therefore, 
matter  is  in  two  forms  or  "phases" — (1)  dispersed  or  internal,  and  (2) 
continuoxis  or  external. 

When  the  two  substances  are  immiscible  fluids  the  colloid  complex 
forms  an  emulsion.  In  protoplasm,  however,  the  condition  is  not  simply 
a  dispersion  of  one  fluid  in  another  immiscible  fluid,  since  in  it  matter  may 
exist  in  all  degrees  between  the  liquid  and  the  solid  condition.  Protoplasm 
is  termed  an  Emulsoid. 

The  main  points  of  difference  between  a  solution  and  a  colloid  may 
be  noted  : — 

Solution.  Colloid. 

Molecules  small.  Molecules  very   large    or    molecules 

aggregated  into  particles. 

Free    movement    of    molecules   of  Dispersed  particles  separated  by  con- 

solute    which    possess     kinetic  tinuous     phase.       Little     or     no 

energy.  movement,   therefore   little   or  no 

kinetic  energy  of  particles. 

No  surface  phenomena.  Large  size  of   molecules    introduces 

phenomena  of  surface  tension  and 
adsoi-ption  between  the  dispersed 
and  continuous  phases. 


650  APPENDICES 

While  tliere  is  sucli  a  marked  difference  between  a  true  solution  and 
the  colloid  state,  there  is  no  clear  line  of  demarcation  between  the  two 
conditions.  As  the  size  of  the  molecules  or  agj]>regates  of  molecules  of  the 
dispersed  phase  of  a  colloid  becomes  smaller,  the  colloid  comes  to  take  on 
the  character  of  a  solution,  so  that  the  particles  or  molecules  of  the  colloid 
show  movement,  and  therefore  kinetic  energy. 

Substances  that  go  into  a  true  solution  and  pass  through  an  animal 
membrane  {see  Osmosis)  have  been  called  crystalloids  to  distinguish  them 
from  colloids.  There  is,  however,  no  fixed  dividing  line  between 
crystalloids  and  colloids.  Some  colloids,  e.g.  htemoglobin,  can  be 
obtained  in  the  crystalloid  form. 

Ions. 

When  salts  are  dissolved  in  water  they  become  ''dissociated."  Thus 
NaCl  in  a  dilute  solution  becomes  Na,  carrying  a  positive  charge  of 
electricity  usually  indicated  by  the  sign  +  or  by  a  dot,  e.g.  Na+  or  Na-, 
and  CI  carrying  a  negative  charge  usually  indicated  by  a  dash,  e.g.  Cl~  or 
Cr.  Na  does  not  exist  as  an  atom  of  sodium  nor  CI  as  an  atom  of  chlorine. 
Each  is  combined  with  a  definite  quantity  of  electricity.  A  monovalent 
atom  or  group  of  atoms  carries  one  unit  charge,  a  divalent  two  and  so  on. 
Thus,  if  a  phosjihate  be  dissociated,  the  PO4  part,  being  trivalent,  carries 
three  unit  charges  and  is  written  PO4'".  In  an  ammonium  salt  the  NH^  is 
monovalent.  It  carries  one  unit  charge  and  is  written  NH4-.  The  atom 
or  group  of  atoms  thus  dissociated  and  carrying  a  quantity  of  electricity  is 
termed  an  Ion.  It  is  the  presence  of  ions  that  enables  a  salt  solute  to 
conduct  electricity. 

Substances  that  become  dissociated  into  ions  when  in  solution  are 
called  Electrolytes,  to  distinguish  them  from  substances  such  as  sugar 
that  are  not  dissociated  when  dissolved.  Nearly  all  salts,  acids,  and  bases 
are  electrolytes. 

The  Reaction  of  Watery  Fluids. 

The  Hydrogen  Ion  Concentration. 

Water  between  22'  and  23°  C.  is,  to  a  slight  extent,  ionised.  That  is, 
some  of  the  water  molecules  are  dissociated  into  hydrogen  and  hydroxyl 
ions.  According  to  the  law  of  mass  action,  the  ratio  of  the  product  ot  the 
ions  to  the  undissociated  water  is  a  constant.     This  may  be  written — 

IHWOHi^   ,,,„,^,,^, 
This  constant  has  been  found  experimentally  for  the  temperature  of  23°  C. 

to    1)6  — 


100,000,000,000,0(10 


or  10" 


APPENDICES 


651 


It  has  also  been  found  that  in  pure  water  at  this  temperature  there  is 
an  equal  number  of  H  an.i  OH  ions,  i.e.  [H]  =  [OH].  Therefore  the 
concentration  of  hydrogen  ions 


or  Ch  =  J^10~'^*  =  10~'^ 
and  Coh  =  Ch  =10~^ 

For  short,  the  H  ion   concentration  is   often  denoted  by  the  logarithmic 
exponent,  i.e. 

Ch  of  10-'=  pH  of  7. 

It  has  been  found  that  all  substances  which  make  a  solution  acid  do 
so  by  mcreasiiig  the  number  of  H  ions,  e.g. 


HCi  \ 
H,oJ 

n.o     j 


H+ 

+  ci- 

H+ 

+  OH 

H  + 

+       HCO; 

H- 

\-   OH 

NaH.PO^  ^ 
HoO  J 


H+        +  Na+  +  HP04 
H+    1    +  0H~ 


and  as  [H]  x  [OHj  is  a  constant,  [OH]  -.uust  be  correspondingly  decreased. 

Similarly,  alkalies  and  alkaline  salts  cause  an  increase  in  the  number  of 
free  OH  and  a  decrease  in  the  free  H  ions. 

Any  increase  in  the  concentration  of  the  H  ions  will  be  denoted  by  a 
decrease  in  the  denominator  of  the  fraction  e.xpressing  concentration. 

At  the  neutral  point 

Therefore,  if  the  pH  is  denoted  by  a  figure  less  than  7  the  solution  is 
acid,  if  by  a  figure  greater  than  7  the  solution  is  alkaline. 

A  value  lying  between  two  whole  numbers  may  be  written  in  either  of 
two  ways.     For  example,  water  just  slightly  alkaline  may  have  a  concen- 

0'5 

tration  of  hydrogen  ions  of  1  in  20  million  =  ^^  ^^^  q^^-     This  may  be 

written  as  Ch  =  0-5x10-7  or  the  fraction  may  all  be  put  as  a  power  of 
10  =  10-7-3  or  pH  =  7-3. 


652  APPENDICES 

To  convert  one  system  of  notation  to  the  other  is  a  simple  matter  of 
logarithms. 

For  example,  to  convert  pH  7-6  to  other  notation— 

pH  7-6  =-10-7-6=  10-7x10-0-6 

(the  antilogarithm  of  -0-6  is  0-25)  .  •,  =0-25  x  10-7. 
Conversely,  Ch  5x  10-6  =  log  5  +  log  10-6. 

=  0-6990 +  (-6-0000). 

=      5-3, 
cr   5  X  10-6  =  10-699  X  10-6  =  10-5-3  =  pH  5-3. 

(The  student  will  remember  that  in  dealing  with  logs,  multiplication  is 
done  by  addition  division  by  subtraction,  and  squaring  by  multiplication 
by  2,  etc.) 

Se.mipkrmeable  Membrane. 

A  membrane  is  simply  a  thin  film  of  substance.  It  may  be  composed 
of  almost  any  material,  and  therefore  may  have  all  degrees  of  permeability. 
When  a  membrane  allows  water  to  pass  through,  but  prevents  the  passage 
of  a  substance  in  solution  in  the  water,  it  is  said  to  be  semipermeable  to 
the  solution.  Thus  water  passes  through  a  membrane  of  copper-ferro- 
cyanide,  but  cane  sugar  in  solution  is  kept  back,  Copper-ferrocyanide  is 
therefore  said  to  be  semipermeable  to  a  solution  of  cane  sugar  in  water. 

Osmosis  and  Dialysis, 

The  passage  of  water  through  a  semipermeable  membrane  is  termed 
Osmosis.  The  passage  of  substance  in  solution  through  such  a  membrane 
is  termed  Dialysis. 

Osmotic  Pressure. 

If  a  solution  of  a  substance  in  water  be  placed  in  a  bag  which  is  semi- 
permeable to  the  solution,  and  the  bag  with  its  contents  be  immersed  in 
pure  water,  the  molecules  of  water  will  have  free  passage  through  the  walls 
of  the  bag,  but  the  molecules  of  the  solute  will  be  held  back.  No  pressure 
will  be  exercised  by  the  water,  since  it  has  free  passage.  The  molecules 
of  the  solute,  however,  will  impinge  on  the  inside  surface  of  the  bag  with 
a  certain  amount  of  force,  which  is  not  balanced  by  molecules  of  solute 
impinging  on  the  outside.  There  will  thus  be  a  force  exerted  tending  to 
distend  the  bag.    This  force  is  called  Osmotic  Pressure, 

It  has  been  found  that  the  osmotic  pressure  of  a  solution  is  the  same 
as  the  pressure  exerted  by  a  gas  having  the  same  number  of  molecules  per 
unit  of  volume  as  the  solute.  The  solute  thus  behaves  as  if  it  were  a  gas 
and  the  solvent  absent.  As  in  the  case  of  a  gas,  the  pressure  is  proportional 
to  the  temperature  and  to  the  concentration.     This  is  easily  understood, 


APPENDICES  653 

since  the  greater  the  concentration  the  more  molecules  hit  the  containing 
membrane,  and  the  higher  the  temperature  the  greater  the  velocity,  and 
consequently  the  greater  the  force  -with  wliich  they  impinge  on  the 
membrane. 

The  osmotic  pressure  may  be  very  high.  A  10  per  cent,  solution  of 
cane  sugar  has  an  osmotic  pressure  of  over  ten  atmospheres. 

If  a  solution  be  separated  by  a  semipermeable  membrane  from  pure 
water  or  a  less  concentrated  solution,  water  passes  from  the  pure  water  to  the 
solution  or  from  the  dilute  to  the  more  concentrated  solution.  Osmosis,  there- 
fore, always  occurs  towards  the  more  concentrated  solution.  On  the  other 
hand,  if  the  molecules  of  the  solute  are  free  to  pass  through  the  membrane 
the  passage  is  from  the  concentrated  to  the  dilute  solution,  the  tendency 
being  to  establish  a  uniform  mixture  (see  Diffusion).  Dialysis,  therefore, 
always  occurs  towards  the  more  dilute  solution. 

Surface  Tension. 

In  the  body  of  a  fluid  the  molecules  are  equally  attracted  on  all  sides 
by  other  molecules,  and  the  resultant  is  zero.  At  the  surface,  however, 
the  attraction  is  one-sided — towards  the  liquid.  The  molecules  are  there- 
fore subjected  to  unbalanced  forces,  and  the  surface  is  consequently  in  a  state 
of  tension.     It  is  this  tension  that  makes  a  drop  spherical. 

The  same  state  of  tension  exists  at  the  interface  of  two  Huids.  If  the 
fluids  are  miscible  a  solution  occurs  and  there  is  no  interface.  If,  however, 
the  fluids  are  immiscible  an  interface  exists  (see  Colloids)  and  surface 
tension  is  present.  In  a  colloidal  state  formed  by  two  immiscible  fluids, 
there  is  an  enormous  extent  of  surface  (p.  13).  Surface  tension  plays  an 
extremely  important  jjart  in  many  physiological  processes. 

The  surface  tension  at  the  interface  between  immiscible  fluids  or 
between  a  solid  and  a  fluid  is  lowered  by  the  presence  of  substances 
in  solution. 

Adsorption. 

According  to  the  second  law  of  thermo-dynamic,  any  process  that 
diminishes  free  energy  always  tends  to  take  place.  Surface  tension  is 
diminished  by  substances  in  solution.  These,  therefore,  tend  to  concentrate 
at  the  interfaces  where  the  tension  exists.  This  depositing  of  a  solute  at 
a  surface  of  contact  is  termed  Adsorption.  In  the  colloidal  state  where 
there  is  a  great  extent  of  .surface,  adsorption  is  of  great  physiological 
importance.  It  facilitates  chemical  reaction  between  the  substance 
composing  the  dispersed  phase  and  those  in  solution  in  the  continuous 
phase. 

Thermopile. 

In  a  circuit  composed  of  two  difterent  metals,  if  one  of  the  junctions  of 
the  metals  be  at  a  different  temperature  from  the  other  junction,  a  current 


654 


APPENDICES 


of  electricity  is  produced  in  the  .circuit.  The 
strength  of  the  current  is  in  proportion  to  the 
difference  of  temperature  of  the  junctions,  and  it 
can  be  measured  by  means  of  a  galvanometer.  By 
increasing  the  number  of  junctions  the  strength  of 
the  current  produced  by  changes  of  temperature 
is  proportionally  increased. 

In  the  figure,  if  the  junctions  B  B  be  warmer 
tlian  A  A  A,  a  current  flows  in  the  direction  in- 
dicated and  the  needle  is  deflected,  the  degree  of 
deflection  being  in  proportion  to  the  strength  of  the 
current. 

The  thermopile  thus  converts  a  heat  change 
which  is  difficult  to  measure  directly  by  a  thermo- 
meter, into  an  electrical  change  which  can  be 
detected  and  measured  with  great  accuracy  by 
means  of  a  sensitive  galvanometer. 

When  the  strength  of  the  current  is  known, 
the  amount  of  heat  can  be  calculated,  since  for 
each  thermopile  there  is  a  direct  relationship 
between  the  degree  of  diff"erence  of  the  tempera- 
ture uf  the  junctions  and  strength  of  the  resulting  current. 


Fio.  o-ie. 

Diagram  of  Thermopile. 
^■  =  Iron. 
[131=  German  Silver. 
A,  B  =  Junctions. 
G  =  Galvanometer. 
N  =  Needleof  G. 


INDEX 


Abdomen,  nerves  of,  198 
Abdominal  breathing,  520 

muscles  of  respiration,  520 
Abducens  nerve,  201 
Aberration,  spherical,  147 

chromatic,  157 
Abomasum,  294 
Absorption,  channels  of,  348 

from  the  stomach,  318 

mode  of,  348 

of  carbohydrates,  347  et  seq. 

of  fats,    347 

of  food,  347 

of  proteins,  347 
Accessory  factors,  281 
Accommodation,  far  point,  147 

near  point,  146 

positive,  146 

range  of,  149 
Acetone,  358 
Achroridextriii,  303 
Acid  sulphur  in  urine,  563 
Acidosis,  481 
Acids,  divalent  (see  Appendix  I.) 

organic  (see  Appendix) 
Acinus  of  gland,  34 
Acromet,'aly,  600 
Action  of  limbs  of  horse,  237 
Addison's  disease,  610 
Adenin,  556 
Adipose  tissue,  42,  351 
Adrenalin,  591 

as  vaso-constrictor,  591 

action  of,  591 
Adsorption,  14  (see  Appendix  III.) 
^rotonometer,  541 
After-discharge  in  reflex  arcs,  84 
After-image,  negative,  154 

positive,  154 
Agglutinins,  614 

Air  breathed,  influence  of  respira- 
tion on,  537 

complemental,  523 

interchange  by  diffusion,  523 

passages,  structure  of,  514 


Air,  residual,  523 

reserve,  523 

respired,  amount  of,  522 

tidal,  522 

vesicles  in  the  lungs,  514 
Alanin,  17 
Albumin  (see  Apjjendix  II.) 

„        nucleo  (see  Appendix  II.) 
Alcapton,  555 

Alcohols,  chemistry  of  (see  Appen- 
dix I.) 
Aldehydes  (see  Appendix  I.) 
Aldoses  (see  Appendix  I.) 
Alexine,  613 
Alimentary  canal,  291  et  seq. 

bacterial  acticm  in,  329 

nerve  supply  of,  300 

structure,  291 
Alkaline  reserve,  481 
Alkalosis,  531 
Allantoic  arteries,  627 
Allantoin,  561 
Allantois,  628 
Amble,  241 
Amboceptor.  613 
Amides,  17  (see  Appendix  I.) 
Amines,  330  (see  Appendix  1.) 
Amino-acetic  acid  (see  Appendix  I.) 
Amitotic  division,  30 
Ammonio-magnesium   phospha  te, 

559 
Ammonium  carbonate,  554 
Ammonium  salts  in  urine,  559 
Amniotic  fluid,  625 

sac,  625 
Amoeboid  movement,  24,  484 
Amount  of  respired  air,  522 
Amylolytic  period  of  gastric  diges- 
tion, 311 
Anabolism,  9 
Anacrotic  pulse,  442 
Anal  canal,  296 
Analysis  of  alveolar  air,  540 
Anelectrotonus,  63 
Anhydrides  (see  Appendix  I.) 


656 


INDEX 


Animal  products  as  food,  290 
Antipepsiii,  313 
Antiperistalsis,  335,  345 
Antithi'Oinbin,  477 
Antitoxin,  612 
Aortic  area,  407 

valve,  392 
ApncBa,  531 
Apocodeine  and  suprarenal  extract, 

592 
Aqueous  humour,  140 
Arc,  cerebellar,  95 

cerebral,  95 

spinal,  82 
Areolar  tissue,  40 
Arginin,  17,  555 
Armsby's  standard,  378 
Arterial  pressure,  449 

pulse,  cause  of,  435 
Arteries,  431 

factors  controlling  pressure,  435 

pressure  in,  434 
Arterioles,  nervous  mechanism,  454 

normal  state  of,  453 
Artificial  respiration,  549 
Arytenoid  cartilages,  550 
Ash  of  food,  288 
Asparagine,  284 
Asphyxia,  548 
Astigmatism,  151 

Atmospheric    pressure   on    respira- 
tion, 543 
Auditory  centre,  173 
Auerbach's  plexus,  30 1 
Augmentor  nerve  of  heart,  421 
Auricle,  386 
Auricular  flutter,  429 
Auriculo-ventricular  rings,  385 

nodes,  385 

valves,  390 
Availability  of  food-stuffs,  363 
Axon,  56 

reflex,  93 

Bacterial  action  in  the  alimentarv 

canal,  329 
Balance  sheet  experiments,  369 
Barotaxis,  25 
Basal  ganglion,  189 

metabolism,  264 
Bases  of  the  urine,  565 
Basilar  membrane,  170 
Basophil  leucocytes,  484 
Bell's  paralysis,  201 
Benzene  compounds  (see  Appendix) 
Benzoates,  562 


Bi-amide  of  carbonic  acid  {see  Urea) 
Bidder's  ganglion,  389 
Bile,  323  et  seq. 

flow  of,  326 

flow,  influence  of  nerves  on,  326 

functions  of,  327 

mode  of  secretion,  327 

pigments,  324 

salts  of,  323 
Bilirubin,  324.  491 
Biliverdin,  324 
Binocular  vision,  158 
Birth,  young  at,  631 
Biuret  (see  Appendix  I.) 
Bladder,  reflex,  93 

urinary,  581 
Blastoderni,  624 
Blind  spot,  151 
Blood,  474  et  seq. 

amount  of,  501 

carbon  dioxide,  495 

cells,  483 

changes  in  respiration,  538 

clotting,  475 

constituents,  fate  of,  503 
source  of,  498 

corpuscles,  483 

deftbrinated,  476 

distribution,  503 

erythrocytes,  485 

flow  of,  464 

velocity  of,  465 

flow  through  heart,  404,  411 

flow  through  different  organs, 
473 

gases  of,  492 

general  characters,  474 

influence  of  respiration  on,  538 

inorganic  constituents,  480 

laked,  474 

leucocytes,  483 

mean  pressure,  447 

methods  of  recording,  447 

opacity  of,  474 

plasma,  479 

platelets,  484 

pressure,  432 

diastolic       (see      Diastolic 

pressure) 
general  distribution,  432 
rhythmic  variations,  434 

serum,  479 

sodium  bicarbonate,  481 

specific  gravity,  474 
Bodv,  connections  with  brain  stem, 
112 


INDEX 


657 


Body  receptors,  97 
Bone,  45  et  seq. 

chemistry  of,  49 

development,  45  et  seq. 

diaphysif?  of,  49 

epiphysis  of,  49 

Haversian  system  of,  48 

marrow,  500 

metabolism  of,  50 

ossification,  centre  of,  45 

osteoblasts,  46 

osteoclasts,  46 
Bowman's  capsule,  571 
Bradycardia,  429 
Brain  (see  Cerebrum,  etc.) 
Breath  sounds,  524 
Breathing  shallow,  530 
Bronchial  muscle,  515 

sound,  524 
Bronchioles,  515 
Brunner's  glands,  295 
Bundle  of  His,  385 

Cachexia  strumipriva,  597 
Ctecum,  296,  344 
Caffeine,  578 
Calcium  in  clotting,  477 

in  food,  289 
Calories,  definition  of,  248 
Calorimeter,  256 

bomb,  256 

respiratory,  259 
Calorimetry,  direct,  259 

indirect,  260 
Canaliculi,  46 
Canals,  semi-circular,  117 
Cancellous  tissue,  46 
Cane  sugar,  286 
Canter,  242 
Capillaries,  431 

pulse  in,  444 

pressure  in,  460 
Carbohydrates,  20,  285 

absorption  of,  347,  350 

determination  of   amount  oxi- 
dised, 258 

in  foods,  288 

storage  of,  352 

tolerance,  355 
Carbon  dioxide  in  blood,  481,  495 

haemoglobin,  490 

in  the  urine,  566 

passage  from  tissues  to  blood, 
546 
Carboxyhsemoglobin,  489 
Cardiac  contraction,  nature  of,  427 
42 


Cardiac  nerves,  function  of,  418 

propagation  of,  wave,  427 

starting  mechanism,  424 

cycle,  393,  401 

impulse,  398 

muscle,  205 

branches  of  the  vagus,  418 
Cardiac  rhythm,  control,  424 
Cardiogram,  398 
Cardiograph,  398 

Cardio-pneumatic  movements,  536 
Cartilage,  43 

elastic  fibro-,  45 

hyaline,  43 

white  fibro-,  45 
Cartilages  of  the  larnyx,  550 
Casein,  636 

Castration,   effect   of,    on    thymus, 
603 

effect  of,  on  body,  606 
Catalytic  action,  8 
Catelectrotonus,  63 
Cathode,  63 
Cells,  23  et  seq. 

activity  of,  24 

chalice,  34 

nucleus,  26 

oxyntic,  310 

peptic,  311 

reproduction  of,  28 
Cellulose  in  food,  287 
Centre,  auditory,  173 

for  dilator  pupillas,  148 

for  smell,  136 

for  sphincter  pupillse,  148 

oculo-motor,  161 

of  ossification,  45 

respiratory,  526 

taste,  132 ' 

thermal  sense,  116 

touch,  116 

vaso-constrictor,  457 

vaso-dilator,  460 

visual,  164 
Centrosome,  23 
Cerebellar  arc,  95 

synapses,  117 
Cerebellum,  structure,  125 

celk  of  Purkinje,  125 

connections  of,  126 

functions  of,  127 

removal,  128 

stimulation,  129 
Cerebral  action,  time  of,  186 
Cerebral  arc,  95 

cortex  {see  Cortex  cerebri) 


658 


INDEX 


Cerebral  arc,  discharging  side,  189 

mechanism,  fatigue  of,  186 

time  of  action,  186 
Cerebrosides,  58 
Cerebro-spinal  fluid,  511 
Cerebrum  development,  181 

discharging  mechanism  of,  189 

evolution,  181 

fatigue,  186 

localisation  of  functions,  1 76 

removal  of,  127 

storing  mechanism,  185 
Chalice  cells,  34 
Check  ligaments,  237 
Chemical  regulation  of  growth  and 

function,  588  et  seq. 
Cheniiotaxis,  25 
Cheyne-Stokes  respiration,  531 
Chlorine-containing  bodies  in  urine, 

565 
Cholalic  acid,  323 
Cholesterol,  19,  58,  325 
Cholin,  20 
Chondro-mucoid,  44 
Chondroitin,  44 
Chondroitin-sulphuric  acid,  44 
Chordae  tendinese,  388 
Chorda  tympani  nerve,  304 
Chorionic  villi,  626 
Choroid,  139 
Choroid  plexus,  511 
Chromaffin  tissue,  590 
Chromatic  aberration,  157 
Chromatin,  27 

Chromo-proteins  {see  Appendix  II.) 
Chromosomes,  617 
Chyle,  508 
Chyme,  319 
Cilia,  37 
Ciliary  muscle,  f39,  147 

processes,  139 
Ciliated  epithelium,  37 
Circulation,  tlie,  382  et  seq. 

factors,  extra-cardiac,  in,  470 

fcetal,  631 

"  head  down  and  up  position," 
471 

in  bone-marrow,  470 

in  cranium,  468 

influence  on  respiration,  536 

in  heart  wall,  469 

in  kidney,  f)72 

in  lungs,  468 

in  spleen,  469 

in  vessels,  431 

schema  of,  383 


Circulation,  time  taken  by,  472 
Classification     of     food-sturt's     for 

herbivora,  290 
Coagulation  of  the  blood,  475 
Cochlea,  170 

connections   with   central  ner- 
vous Hvstem,  172 
Coefficient  of  oxidation,  254 
Cold  spots,  104 
Collagen,  39 

Collecting  tubules  of  kidney,  572 
Colloids,  12 
Colon,  296 
Colostrum,  637 
Colour-blindness,  157 
Columnse  carnese,  387 
Columnar  epithelium,  33 
Common  path,  86 
Complemental  air,  523 

pleura,'  518 
Concentrates,  290 
"  Conditioned  '■'  reflexes,  131 
Conducting  paths  {see  Spinal  cord, 

etc.) 
Conduction  of  nerve  impulse,  69 
Cones  of  I'etina  {see  Retina) 
Connective  tissues,  37 
Consciousness,  185 

relations  to  cerebral  action,  185 
Contraction     of     muscle,     changes 

during,  219 
Convoluted  tubules  of  the  kidney, 

572 
Cooking,  effects  of,  on  food,  366 
Co-operative  antagonism  of  groups 

of  muscles,  231 
Coprosterol,  332 
Cord,  spinal  {see  Spinal  cord) 
Cornea,  139 

Corona  radiata  {see  Cerebrum) 
Coronary  arteries,  406 
Corpus  Arantii  {see  Valves  of  heart) 

Highmori,  620 

luteum,  608 

striatum,  189 

trapezoideum,  172 
Corpuscles,  blood,  483  et  seq. 

of  Hassall,  602 

tactile,  102 
Cortex  cerebri,  synapses,  115 

discharging  area,  189 

receiving  areas  (how  localised), 
176 

fibres,  194 

functional  development,  182 

storing  and  associating  part,  185 


INDEX 


659 


Cortex  cerebri,  structural  develop- 
ment, 181 
Corti,  organ  of,  171 
Coughing,  522 
Cranial  nerves,  200 

nuclei  of,  200 
Creatin,  209,  560 
Creatinin,  560 
Cricoid  cartilage,  550 
Critical  temperature,  271 
Crossed  pyramidal  tract,  194 
Crude  fat,  285 

fibre,  288 
Crystalline  lens,  141 
Crystalloids,  20 
Curare  experiment,  217 
Current  of  action,  213 

injury,  212 
Cutaneous  nerves,  influence  on  re- 
spiration, 534 

receptors,  99 
Cystin,  32,  564 
Cytase,  339 
Cytotoxins,  614 

Dairy     cows,    food     requirement, 

375 
Death,  10 

of  muscle,  214 
Decerebration  rigidity,  113 
Decidua  reflexa,  625 
Deftecation,  335 
Degeneration,  Nissl's,  77 
Deiters'  nucleus,  126 
Delivery,  633 
Dendrites,  53,  56 

axon  of,  56 
Dentate  nucleus  of  the  cerebellum, 

125 
)r  nerve  {see  Cardiac  branches 

of  vagus) 
Deutero-proteose,  312  {see  Appendix 

II.) 
Development,  624 
Dextrose,  286 

Diabetes  {see  Glycosuria),  357 
Diacetic  acid,  358 
Di-amino  acids,  17 
Diapedesis,  468 
Diaphragm,  517 
Diaphysis,  49 
Diastase,  320 
Diastole  of  heart,  394 
Diastolic  pressure,  449 
Dicrotic  notch,  441 
wave,  440 


Diets,  influence  of  various,  on  gastric 

secretion,  313 
Diffusion  of  gases  in  lungs,  523 
Digestibility  of  food,  364 

apparent,  364 
Digestion,  291  et  seq. 

experiments,  364 

fate  of  the  secretions  of,  331 

gastric,  308 

in  carnivora,  302 

in  herbivora,  336 

in  horse,  341 

in  mouth,  302 

in  ruminants,  337 

intestinal,  319 

of  stomach  wall,  313 
Digital  torus,  233 
Dilator,  pupillje,  140 
Dioptric  mechanism,  144 

imperfections,  149 
Diphtheria  bacillus,  612 

toxin,  612 
Diploe  of  bone  {see  Bone) 
Diplopia,  162 
Disaccharids,  286 
Disc,  optic,  141 
Dissociation     of     oxy-hiiemoglobin, 

494 
Diureides  {see  Appendix  I.) 
Divalent  acids  {see  Appendix  I.) 
Dobie's  line,  204 
Douglas  bag,  261 
Dropsy,  462 
Ductus  arteriosus,  633 

venosus,  631 
Dynamometer,  225 

Ear,  anatomy  of,  167 

annular  ligament,  168 

cochlea,  connections  with  central 
nervous  system,  173 

external,  167 

internal,  170 

middle,  167 

osseous  labyrinth,  170 

ossicles  of,  167 
Eck's  fistula,  360 
Effectors  (see  Muscle),  202 
Efferent  nerves,  195 
Elastic-fibro  cartilage,  45 
Elastin,  39 

Electrical  changes  in  muscle,  212 
Electricity      induced,     action     on 

muscle,  219 
Electrocardiogram,  428 
Electrotonus,  63 


660 


INDEX 


Eleventh  nerve  (see  Spinal  accessory) 
Embryo,  attachment  to  mother,  626 
Emmetropic  eye,  146 
Empedocles,  1 

Emulsion,  13  (Appendix  III.) 
Endocardium,  390 
Endocrinetes,  588  et  seq. 

classification,  589 

interaction,  611 
Endogenous  purins,  561 
Endolymph  (see  Ear) 
Endothelium,  40 
Energy  requirements,  264  et  seq. 

factors  modifying,  266 
Energy  value  of  carbohydrates,  257 

determination  of,  356 

fats,  257 

proteins,  257 
Enteric  fever,  toxin  of,  613 
Entero-hEsemal  circulation,  331 
Entero-hepatic  circulation,  331 
Enterokinase,  321 
Enzymes,  7 
Eosinophils,  483 
Epiblast,  624 

parts  developed  from,  624 
Epiglottis,  307,  550 
Epiphysis,  49 
Epithelium,  ciliated,  37 

columnar,  33 

excreting,  36 

glandular,  34 

mucin-secreting,  34 

secreting,  34 

simple  squamous,  32 

stratified  squamous,  32 

transitional,  33 

zymin-secreting,  36 
Equilibrium  of  body,  130 
Erection,  623 
Erepsin,  328 
Erythroblasts,  501 
Erythrocytes,  500,  505 
Erythro-dextrin,  303 
Esters  {see  Appendix  I.) 
Ethane  (see  Appendix  I.) 
Ethereal  sulphates  in  urine,  563 
Ether  extract,  285 
Ethers  (see  Appendix  I.) 
Ethylene  (see  Appendix  I.) 
Euglobulin  (see  Blood  serum),  480 
Eustachian  tube,  167,  169 
Excito-motor  nerves,  74 

reflex  nerves,  74 

secretory  nerves.  74 
Excreting  epithelium,  36 


Excretion,  by  the  lungs  (see  Respira- 
tion) 

by  the  skin,  583 

by  the  intestine,  363 
Exogenous  purins,  561 
Exophthalmic  goitre,  598 
Expiration,  521' 
Expiration,  forced,  521 
Expired  air,  537 
Extensor  thrust,  crossed,  85 
Exteroceptive  receptors,  96 
Eye,  139  et  seq. 

anatomy  of,  139 

astigmatic,  151 

blind  spot,  151 

connection   with  central  nerv- 
ous system,  162 

dark  adapted,  153 

emmetropic,  146 

fluids  of,  140,  143 

hyaloid  membrane  of,  140 

hypermetropic,  149 

myopic,  150 

physiology,  143 

presbyopic,  149 
Eyeballs,  glance  movements  of,  160 

movements  of,  159 

muscles  of,  159 

nervous  mechanism  of,  161 

Facial  nerve,  201 
Faeces,  361 

composition  of,  363 
Fainting,  472 
Fallopian  tube,  621 
Faraday-Tyndall  phenomenon,  13 
Faradic  stimulation,  67 
Far  point  of  accommodation,  147 
Fascia,  40 

dentata,  135 
Fasting,  metabolism  during,  272 

rate  of  waste  during,  273 
Fat  cells,  41 

in  foods,  285 

storage  of  surplus,  351 

soluble  A,  281 
Fat-splitting    enzyme    of    pancreas 

(see  Lipase) 
Fate  of  food  absorbed,  350 
Fatigue  of  cerebral  mechanism,  186 

nerve,  69 

of  muscle,  225 
Fats,  19,  41,  285 

absorption  of,  347,  350 

determination  of  amount  oxi- 
dised, 258 


INDEX 


661 


Fats,  energy  value  of,  257 

regulation  of  supply,  358 
Fattening,  373 
Fatty  acids,  29 

lower,  359 

tissue,  351 
Feeding,  standards,  375 
Fenestra  ovalis,  167 

rotunda,  167 
Fermentation  in  rumen,  340 

losses,  365 
Fetlock  joint,  237 
Fibres,  elastic,  39 

nerve,  56 

non-medullated  nerve,  56 

splanchnic,  54 

somatic,  54 

white,  39 
Fibrillar  twitching,  220 
Fibrillation  of  heart,  430 
Fibrin,  476 
Fibrinogen,  477,  480 
Fibroblasts,  38 
Fibro-cartilage,  elastic,  45 

white,  45 
Fibrous  tissue,  38 
Field  of  vision,  152,  158 
Fifth  cranial  nerve,  201 
Filiform  papillae  {see  Tongue) 
Flocculus  of  cerebellum,  125 
Flow  of  blood,  464 

to  heart,  404 
Fluids,  swallowing  of,  308 
Fodder,  green,  290 

dry,  290 
Fcetal  circulation,  631 
Fcetus,  growth  of,  630 

nourishment  of,  628 
Food,  283  et  seq. 

absorption  of,  347 

absorbed,  fate  of,  350 

availability  of,  363 

effect  of,  on  metabolism,  272 

not  yielding  energy,  288 

requirements,  368  et  seq. 

methods    of    determining 

369 
for  fattening,  373 
for  growth,  371 
for  maintenance,  371 
for  milk  production,  374 
for  meat  production,  374 
for  work,  375 

residues,  363 

storage  of  surplus,  351 

yielding  energy,  283 


Food    stuffs,    classification    of,    for 

herbivora,  290 
Foot,  horse's,  233  ft  seq. 
Foramen  ovale,  632 

occlusion  of,  633 
Formamide  {see  Apj^endix  I.) 
Formic  acid  {see  Appendix  I.) 
Fourth  cranial  nerve,  201 
Fovea  centralis  {see  Retina) 
Frauenhofer's  lines,  487 
Frog  of  horse's  foot,  233 
Frog's  heart,  393 

Galactose,  286 

Galactosides,  58 

Galen,  1 

Gall-bladder,  323 

Gallop,  241 

Galvanic  current,  stimulation  by,  62 

Galvanometer,  212 
string,  214 

Galvanotaxis,  26 

Ganglia,  nicotine  on,  198 
terminal,  55 

Gas  pump,  496 

Gases  of  the  blood,  492 

Gastero-colic  reflex,  335 

Gastric  digestion,  amylolvtic  period, 
311 
proteolytic  period,  311 

Gastric  glands,  295 

Gastric  juice,  310 

action  on  gelatin,  312 
action  on  proteins,  311 
antiseptic  action,  313 
influence  of  diet  on,  313 
nervous  mechanism  of,  314 
source  of  constituents,  310 

Gastric  movement,  nervous  mechan- 
ism of,  317 

Gelatin,  39 

action  of  trypsin  on,  320 
value  as  a  food,  276 

Gels,  13 

Gemmules,  56 

Generative  organs  {see  Gonads) 

Gennari,  layer  of,  164 

Gestation,  633 

Giant  cells,  501 

Giantism,  600 

Gills  of  aquatic  animals,  513 

Glance  movements  of  eyeballs.  160 

Glands,  34 

Glaucoma,  143 

Gliadins,  284 

Globin,  490 


662 


INDEX 


Globulin  (see  Appendix  II.) 

Glomerulus  of  kidney,  572 

Glossopharyngeal  nerve,  201,  305 

Glucosamine,  35 

Glucose,  288 

Glutamine,  281 

Glutelins,  284 

Glycaemia,  355 

Glycerol,  41  (Appendix  I.) 

Glycero-phosphates  in  urine,  565 

Glycin,  16 

Glycocholate  of  soda,  323 

Glycocholic  acid,  323 

Glycogen,  287 

Glyocogenic   function  of  the  liver, 

354 
Glyco-proteins  (see  Appendix  II.) 
Glycosuria,  355 

alimentary,  355 

experimental,  356,  357 
Glycuronic  acid,  44 
Glycyl-glycin,  19 
Gmelin's  test,  324 
Goitre,  exophthalmic,  598 
Goitre,  simple,  599 
Golgi,  organs  of,  105 
Gonads,  605 
Graafian  follicle,  619 
Granules,  in  cells,  23 

Nissl's,  86 
Grass,  290 
Graves'  disease,  598 
Gravity,  influence  on  capillary  pres- 

"'sure,  463 
Grey  matter  (see  Cerebrum,  etc.) 
Grooming,  585 
Growth,  essentials  for,  6 

rations  for,  371 

regulation  of,  586 
Guanidin,  209,  555 

methyl  guanidin,  605 

HyEMATIN,  490 
Hsematoidin,  491 
Hivmatoporphyrin,  491,  566 
Hsemautograph,  44(t 
Hfemin,  491 
Hasmocytometer,  485 
Haemoglobin,  486 

decomposition  of,  589 

derivatives,  489 
Htemoglobinometer,  Haldane's,  488 
Hteinolymph  glands,  500 
Hemolysin,  486 
Hceniolysis,  21,  486 

organs  connected  with,  504 


Hemosiderin,  504 

Hair,  583 

Haller,  2 

Harvey,  2 

Hassall,  corpuscles  of  (see  Thymus) 

Haversian      canals,     spaces,     etc., 

48 
Hay,  290 
"  Head   down    position,''   effect   on 

blood  pressure,  471 
Hearing,  166 

Heart  action,   influence    on    blood 
pressure,  451 

attachments      and       relations, 
393 

auriculo-ventricular  node,  385 

band  of  His,  385 

block,  429 

changes  in  shape,  397 
in  i^osition,  399 
in  pressure,  399 

chordae  tendineje,  388 

columnaj  carnese,  387 

compensation,  417 

connection  with  central  nerv- 
ous system,  418 

endocardium,  390 

failure,  462 

fibrillation,  430 

fibrous  rings  of,  385 

hypertrophj',  417 

influence  of  nerves  upon,  418 

intra-cardiac  nervous  mechan- 
ism, 388 

intra-cardiac  pressure,  399 

muscle,  205 

nerves  of,  388,  418 

neurons,  388 

impillary  muscle,  387 

pericardium,  389 

physiology  of.  393  et  seq. 

pulmonic,  383 

relations  of,  393 

sino-auricular  node,  385 

sounds  of,  407 

structure,  384 

sympathetic  fibres  to,  421 

tone,  428 

valves  (see  Valves) 

work  of,  410 
Heat  elimination,  267,  271 

elimination    from     respiratory 
passages,  268 

elimination  in  urine  and  faeces, 
268 

production,  268 


INDEX 


Heat  production  in  glands,  268 
in  muscle,  268 
regulation  of,  269 
chemical,  269 
physical,  269 
units  (see  Calories) 
Hemispheres,    cereljral    (see    Cere- 
brum) 
Henle's  loop,  572 

sheath,  57 
Hepatic  vein,  299 
Herbivora,  digestion  in,  336 
Hereditary  inertia,  586 
Herpes  zoster,  91 
Hexoses,  28 
Hiccough,  522 
High  tension  pulse,  443 
Highmori,  corpus  (see  Corpus  High- 

mori) 
Hippocampus,  135 
Hippocrates,  1 
Hippuric  acid,  562 
Hirudin,  action  on  blood,  478 
His,  bundle  of  (see  Bundle  of  His) 
Histamine,  451 
Histidin,  17,  555 
Histones  (see  Appendix  II.) 
Holoblastic  segmentation,  624 
Homogentisic  acid,  555 
Hoof,  horse's,  234  et  seq. 
Hormones,  588 
Horse,  action  of  limbs,  237  et  seq. 

amble,  241 

caecum,  297,  344 

canter,  242 

capacity  for  work,  252 

colon,  298,  344 

diet,  maintenance,  371 
for  work,  375 

digestion,  341 

efficiency  as  machine,  252 

foot,  237 

gallop,  241 

grooming,  585 

jump,  242 

legs,  conformation  of,  235 

lying,  239 

mastication,  293 

overwork,  253 

rising,  239 

standing,  237 

stomach,  295 

sweat,  585 

sweating,  267 

teeth,  292 

trot,  240 


Horse,  vomiting,  346 

walk,  239 

watering,  367 

work,  375 
Hot  spots,  104 
Humour,  aqueous,  140 

vitreous,  140 
Hunger,  99 

contraction,  309 
Hyaline  cartilage,  43 
Hyaloid  membrane,  140 
Hydrocarbons,      unsaturated      (-see 
Appendix  I.) 

saturated  (see  Appendix  I.) 
Hydrogen    ion    concentration    (see 

Appendix  III.) 
Hydroquinon,  555 
Hydrosols,  15 

Hydroxyl    molecule     (-^ee     Appen- 
dix III.) 
Hypermetropia,  149 
Hyperpncea,  528,  531 
Hyperthyreoidism,  598 
Hypnosis,  188 
Hypnotic  drugs,  187 
Hypoblast,  625 

parts  developed  from,  625 
Hypoglossal  nerve,  200 
Hypophysin,  595 
Hypophysis  cerebri,  594 
Hypothyreoidism,  597 
Hypoxanthin,  556 

Ileo-c^cal  valve,  296,  334 

Impregnation,  623 

Impulse,  nature  of  nervous,  74 

Incompetence  (cardiac),  410 

Incus,  167 

Indican,  563 

Indol,  329 

Indoxy],  563 

Induced  electricity,  stimulation,  62 

Inferior  oblique  muscle,  159 

Infundibula,  pulmonary,  514 

Infundibular  passages,  514 

Inhibitory  action  of  vagus,  420 

Inorganic  salts  in  food,  288 

Inosite,  209 

Insalivation,  302 

Inspiration,  516 
forced,  520 

Interchange  of  air  in  lungs,  537 

Intercostal  muscles   in   respiration, 
519 
external  intercostals,  519 
internal  intercostals,  521 


664 


INDEX 


Intermittent  muscular  exercise,  470 
Internal  respiration,  547 

secretions,  mode  of  action,  611 
Interoceptive  receptors,  96 
Interrenal  tissue,  609 
Intestinal  wall,  enzymes  of,  328 

mechanism  of  secretion,  328 
Intestine,  295 

absorption  from,  348 

capacity,  301 

digestion  in,  319  ef  seq. 

large,  296 

length,  301 

movements  of,  332 

secretion,  328 
Intracardiac  pressure,  399 
Inverted  image  rectified,  166 
lodothyreoglobulin,  596 
lodothvrin,  596 
Iris,  140 

Islets  of  Langerhans,  300,  601 
Isometric  contraction,  225 
Isotonic  contraction,  225 

Jacksonian  epilepsy,  191 
Jacobson's  nerve,  305 

Karyoplasm,  27 
Karyosomes,  27 
Kellner's  standards,  376 
Keratin,  32 

Ketones  (see  Appendix  I.) 
Kidney,  571  et  seq. 

influence    of    nervous    system, 
580 

nerves  of,  572 

regulation  of  ions,  556 

structure,  571 
Kilogram-metre,  242 
Kinsesthetic  sense  {see  Muscle-joint 

sense) 
Knee-jerks,  89 

Labyrinth,  osseous,  117,  170 

connections  with  central  nerv- 
ous system,  119 
physiology,  121 

Labyrintho-cerebellar     mechanism, 
117 
physiology,  121 

Lactase,  328 

Lactation.  634 

Lactose,  286 

Lacuna,  46 

Lsevulose,  286 

Lamellae  (see  Bone) 


Laminitis,  253 

Langerhans,  islets  of,  300,  601 

Lanolins,  585 

Large  intestine,  296,  334 

Laryngoscope,  552 

Larynx,  cartilages  of,  550 

centre,  552 

ligaments  of,  550 

mucous  membrane  of,  551 

muscles  of,  551 

nervous  mechanism  of,  552 

position  in  deglutition,  307 

structure  of,  550 
Latent  period  of  muscle,  216 

time  of  reflex  action,  83 
Lateral  fillet,  172 
Lecithin,  19 

Leg,  horse's  conformation  of,  235 
Lens,  capsule,  141 

crystalline,  141 

physiological,  145 
Lenticular  nucleus  (see  Cerebrum) 
Leucin,  17 
Leucoblasts,  500 
Leucocytes,  483 

fate  of,  503 

phagocytic  action,  484 

source  of,  499 
Leucocytosis  digestion,  348 
Leuwenhoek,  2 
Levers,  231 
Leydig's  cells,  606 
Lieberkiihn's  follicles,  295 
Life,  5 

Ligaments,  40 
Linin  network,  27 
Lipase,  321,  328 
Lipoids,  19 
Liquor  folliculi,  619 

sanguinis,  474 
Live  weight  test,  369 
Liver,  299 

circulation  in,  353 

development  of,  353 

formation  of  urea  in,  359 

glycogenic  function,  353 

regulator  of  sujjply  to  muscles, 
353 

relation  to  fats,  358 

relation  to  general  metabolism, 
353  et  seq. 

relation  to  proteins,  359 

storage  of  carbohydrates,  354 
Living  matter,  essentials  of,  5 
Lobus  pyriformis  (see  Smell) 
Loudness  of  sounds,  553 


INDEX 


665 


Lungs,  514 

circulation  in,  468 

interchange    between    air    and 
blood  in,  537 

physiology  of,  515  et  seq. 
Lymph,  507 

formation  of,  508 

in  disease,  508 
Lymphatic  glands,  499 
Lymphatics,  pressure  in,  464 
Lymphocytes,  483 
Lysin,  17,  278 

Macula  lutea  {see  Retina) 

Maize  as  food,  276 

Malleus,  167 

Malpighi,  2 

Malpighian  bodies  of  kidney,  573 

corpuscles  of  spleen,  504 
Maltose,  286 
Mammary  glands,  609 
Manometer,  papers,  400 
Manurial  values,  380 
Marchi's  method  of  nerve-staining, 

76 
Marey's  muscle  forceps,  225 
Mariotte's  experiment,  151 
Mastication,  302,  337 
in  horse,  293,  342 
in  ruminant,  337 
Maternal   attachment  of  ovum  {see 

Placenta) 
Meat  production,  374 
Meconium,  363 

Medulla  of  kidney  {see  Kidney) 
Medullary  sheath  {see  Nerve) 
Meeh's  formula,  265 
Megaloblasts,  500 
Meissner's  plexus  {see  Intestine) 
Melanin,  43 
Melanoidins,  43 
Membrana  nictitans,  140 
reticularis,  172 
tectoiia,  172 
tympani,  167,  169 
Membranous  labyrinth,  118,  171 
Memory,  180 
Mesoblast,  625 

parts  developed  from,  625 
Metabolism,  basal,  264 
during  fasting,  272 
effect  of  food  on,  272 
factors  modifying,  266 
general,  264 

method  of  investigating,  257 
of  fats  and  carbohydrates,  257 


Metabolism  of  protoplasm,  9 
protein,  257 
regulation  of,  586 
semi-starvation  in,  274 
Metaphosphoric  acid,  28 
Meth^emoglobin,  489 
Methane  (see  Appendix  L) 
Methyl-guanidin,  209,  605 
Micro-organisms  in  rumen,  339 

in  intestine,  329 
Micturition,  581 
Milk,  635 

composition  of,  637 
enzyme  causing  curdling,  310 
fats,  636 
proteins  of,  636 
production,  374 
secretion  of,  635 
sugar  of,  637 
Mitotic  division,  29 
Mitral  area,  408 

valve,  390 
Molecular     layers    of     retina     (see 

Retina) 
Mon-amino  acids  {see  Appendix  I.) 
Monaster  stage  of  cell  division,  30 
Monocular  vision,  144 
Monosaccharids,  286 
Motor  areas,  191 

nerves  {see  Nerves) 
Mountain  sickness,  543 
Mouth,  291 
Mucin,  35 

secreting  epithelium,  34 
Mucoid  tissue,  37 
Mucus,  35 
Midler,  2 
Murmurs,  cardiac,  409 

simulation  of,  by  breath  sounds, 
537 
Muscarin,  20 
Muscle,  202  et  seq. 

absolute  force  of,  224 
action,  mode  of,  231 
carbohydrates  of,  208 
cardiac,  205 
changes  in  shape,  219 
chemical  changes  in,  254 
chemistry  of,  207 
ciliary,  147 
colour  of,  210 
contraction,  222 
contracture,  226 
corpuscles,  204 
course  of  contraction,  222 
death  of,  214 


666 


INDEX 


Muscle,  deaeneration  of,  66 

development,  202 

direct  stimulation,  217 

Dobie's  line,  204 

efficiency,  248,  250 

elasticity,  210 

electrical  changes  in,  212 

electrical  condition  of,  212 

electrotonus,  63 

extent  of  contraction,  223 

factors   modifying  contraction, 
225 

fatigue,  226 

force  of  contraction,  223 

forceps,  Marey's,  225 

galvanotonus,  62 

heart,  205 

heat  production  in,  212,  246 

in  action,  215 

isometric  tracings,  225 

isotonic  tracings,  225 

latent  period  of  contraction,  223 

lever  arrangement  of,  231 

metabolism.  254 

mode  of  action,  231 

optimum  load,  244 

oxidation,  254 

material  oxidised,  255 
mode  of,  256 

paradoxical  contraction,  72 

physical  characters  of,  210 

physiology  of,  210 

pofar  exciWion  of,  62,  67,  219 

proteins  of,  207 

red,  205 

sarcolemma  of,  204 

sarcous  substance,  204 

skeletal,  217 

spindles,  105 

stapedius,  167,  169 

stimulation  bv  galvanic  current, 
219 

stimulation    by  induced    elec- 
tricity, 219  " 

stimulation,  direct,  217 

stimulation,  methods  of,  218 

stimulation  temperature,  219 

storage  of  food  in,  352 

structure  of,  202 

successive  stimuli,  228 

tension    developed,    224,    242, 
248 

tensor  tympani,  167,  169 

tetanus  of,  229,  230 

tonus  of,  211 

visceral,  203,  215 


Muscle,  voluntary  contraction,  230 

white,  205 

work,  242 

work  measurement,  245 

work    production,    relationship 
to  heat  production,  246,  248 
Muscles,  and  joint  sense,  105 

co-operative  antagonism  of,  231 

of  eyeball,  159 

of  larynx,  551 
Muscular  sense,  105 

work,  effect  on  metabolism,  266 

work,  effect  on  respiratory  in- 
terchange, 258  et  seq. 
Myelocytes,  484,  501 
Myenteric  plexus,  94 
Myocardium,  384 
Myoglobulin,  208 
Myohsematin,  210 
Myoids,  202 
Myopia,  150 
Myosin,  207 
Myosinogen,  207 
Myostromin,  208 
Myxcedema,  596,  597 

Native  proteins  (see  Appendix  II.) 
Near  point  of  accommodation,  1 46 
Necrobiosis,  10,  214 
Neopallium,  181 
Nephridium,  570 
Nerve,  58 

automatic  action  of,  75 

cells,  75 

centres  {see  Centres) 

changes   in   disease  and  injury 
in,  66 

chemistry  of,  57 

conduction,  76 

corpuscles,  56 

degeneration  of,  67 

development,  53 

electrical  changes  in,  60 

excitability,  variations  in.  68 

exposed,  stimulation  of,  62 

fibres,  56 

fibres,  medullary  sheaths  of,  56 

fibres,  non-medullated,  56 

grey,  56 

impulse,  nature  of,  74 

stimulous,  relationshijj  of,  68 

structure  of,  55 

under  skin,  stimulation  of,  64 
Nerves,  afferent,  74 

anterior  roots,  107 

auditory,  172 


INDEX 


667 


Nerves,  augmentor,  73 
cranial,  200 
degeneration  of,  76 
etterent,  73,  195 
excite- motor,  74 
excito-reflex,  74 
excito-secretory,  74 
excito-vaso  dilator,  74 
factors    modifying    conduction 

in,  71 
ingoing,  106 
inhibitory,  73 
mixed,  74 
motor,  73 

nature  of  impulse  in,  74 
optic,  162 
posterior  root,  107 
rate  of  conduction  in,  70 
regeneration  of,  76 
secretory,  73 
sensory,  74 
somatic,  54 
splanchnic,  54,  196 
thoracico-abdominal,  197 
vagus,  200 
vaso-constrictor,  456 
vaso-dilator,  458 
visceral,  195 
Nervous   mechanism,  intra-cardiac, 
388 
extra-cardiac,  418 
gastric  movement,  318 
regulation  of  metabolism,  587 
system,  53 

development  of,  53 
structure  of,  55 
Neural  arcs,  78  et  seq. 
cerebellar,  81 
cerebral,  80 
general  action  of,  80 
spinal,  78 
Neurilemma,  56 
Neuroblast,  53 
Neuro-keratin,  57 
Neuromyal  junction,  205 
Neurons"^  of  heart,  388 
Neurons,  53 
axon  of,  56 
cell  of,  55 

cell,  Nissl  granules  in,  55 
classification  of,  73 
conduction  in,  69 
degeneration  of,  77 
dendrites  of,  56 
excitation  of,  60 
function  of  cell,  76 


Neurons,  gemmules  of,  56 
in  series,  77 

manifestations  of  activity,  59 
means  of  stimulating,  61 
nature  of  impulse,  74 
neuro-myal  junction,  205 
nutrition  of,  76 
physiology,  58 
regeneration  of,  77 
stimulation  of,  61  et  seq. 
structure,  55 
synapsis,  54 
Neutral  sulphur  in  urine,  564 
Nicotine,  effect  of  painting  ganglia, 

198 
Ninth  nerve,  201 
Nissl's  degeneration,  77 

granules,  55 
Nitric  oxide,  489 
Nitrogen  in  the  urine,  559 
free  extract,  288 
non-protein,  284 
Noci-ceptive  receptors  {see  Pain) 
Nocuous  stimuli,  100 
Nodes  of  Eanvier,  57 
Non-medullated  nerve  fibres,  56 
Non-polarisable  electrodes  (see  Elec- 
trodes) 
Normoblasts,  501 
Nuclei  of  the  cranial  nerves,  200  _ 
Nucleic  acid  {see  Appendix  I.),  27 
Nucleins  (see  Appendix  I.),  27 
Nucleolus,  27 

Nucleo-proteins  {see  Appendix  II.), 
556 
action  of  gastric  juice  on,  312 
action  of  trypsin  on,  320 
of  urine,  566 
Nucleus,  26  et  seq. 
accessorius,  172 
chemistry  of,  27 
division  of,  28 
functions  of,  28 
membrane  of,  28 
of  Deiters',  126 
structure  of,  26 
Nutritive  ratio,  368 

Oats,  366 

Oblique  muscles  of  eye,  1 59 
Oculomotor  nerve,  161,  201 
Esophageal  groove,  295 
CEsophagus,  293 

peristaltic  action  of,  308 
(Estrous  cycle,  622 
Oleic  acid,  42 


668 


INDEX 


Olein,  42 
Olfactory  area,  134 

bulb  and  tracts,  134 

tubercle,  134 
Omasum,  294 
Oncometer,  452,  573 
Oocytes,  619 
Ophthalmoscope,  142 
Opsonins,  615 
Optic  axis,  159 

chiasma,  162 

disc,  141 

fibres,  decussation  of,  162 

nerve,  162 

thalamus,  112,  162 

tracts,  162 
Optimum  stimulus,  227 
Ora  serrata,  142 
Organ  of  Corti,  171 
Organic  acids  as  salts  in  food,  340 
Organs  of  Golgi,  105 
Osmosis,  21  (see  Appendix  III.) 
Osseous  labyrinth,  170 
Ossicles  of  ear,  167 
Ossification,  process  of,  45 

centre  of,  45 
Osteoblasts,  46 
Osteoclasts,  46 
Osteomalacia,  50 
Otoliths,  118 
Ovary,  607,  619 
Ovum,  616,  618 
Oxalates  on  blood,  477 
Oxalic  acid  in  urine,  566 
Oxidation  coefficient,  254 
Oxy-acids  (see  Appendix  I.) 
Oxybutyric  acid,  358 
Oxygen,  head  of,  545 

in  blood,  492 
Oxyhsemoglobin,  487 
Oxyntic  cells,  310 
Oxyphil  leucocytes,  483 

Pacemaker  of  heart,  430 
Pacinian  corpuscle,  97 
Pain,  100 

referred,  98 

sensation  of,  100 

spots,  100 
Palmitic  acid,  42 
Palmitin,  42 
Palpation  of  pulse,  442 
Pancreas,  300,  601 

islets  of   Langerhans  {see  Lan- 
gerhans) 

removal  of,  357,  601 


Pancreatic  secretion,  319 
action  of,  320 
character  of,  320 
enzymes  of,  320 
nervous,  control  of,  322 
physiology  of,  321 
Papillary  muscles,  387 
Paradoxical  contraction,  72 
Paralysis,  191 

crossed,  194 
Paramyosinogen,  208 
Parathyreoid,  603 
Parotid  gland,  305 
Parthenogenesis,  616 
Partial  pressure  of  oxygen  and  car- 
bon dioxide  in  blood,  541 
pressure  of  gases  in  air  vesicle, 
539 
Passage    of    carbon     dioxide    from 
tissues  to  blood,  546 
of  oxygen  from  blood  to  tissues, 
545 
Peas  as  food,  290 
Pectinate  muscle,  386 
Peduncles  of  cerebrum  (see  Cerebrum) 
Pelvis,  nerves  of,  198 
Penis,  621 

erection,  623 
Pentosans,  287 
Pepsin,  310 

source  of,  310 
Pepsinogen,  311 
Peptides,  di-,  tri-,  19 
Peptone  (see  Appendix  II.) 
Perfusion  method  of  studying  action 

of  drugs,  453 
Pericardium,  389 
Perichondrium,  46 
Perilymph,  170 
Perineurium,  57 
Peripheral  resistance,  451 
Peristalsis,  333 

nervous  mechanism  of,  333 
of  bladder  wall,  581 
of  intestine,  333  et  seq. 
of  wall  of  ureter,  581 
Peyer's  jaatches,  296 
Pfliiger's  law,  72 
Phacoscope,  147 
Phagocyte  action,  484 
Pharvnx,  307 
Phenol,  330,  563 
Phloridzin,  injection  of,  357 
Phosphatides,  19,  58 
Phospho-protein  (see  Appendix  II.) 
Phosphorus  in  food,  289 


INDEX 


669 


PhosphoriTs-containing     bodies     in 

the  urine,  565 
Phototaxis,  26 
Phrenic  nerves,  527 
Physiology,  growth  of,  1 

study  of,  3 
Pigment,  blood  {see  Hsemoglobiii) 

cells,  43 
Pigments  of  the  bile,  324 

of  the  urine,  566 
Pitch  of  sounds,  174,  553 
Pituitary,  599 
Placenta,  628 
Plantar  cushion,  233 
Plasma,  479 
Plasmodia,  11 
Plasmolysis,  21 
Platelets,  blood,  484 
Plethysmograph,  452 
Pleura,  complemental,  518 
Pleural  cavity,  515 
Plexus,  Auerbach's  (see  Auerbach) 

Meissner's  {see  Meissner) 

terminal,  55 
Pneuma,  1 

Polar  excitation,  law  of,  64 
Polycrotic  pulse  {see  Pulse) 
Polymorph o-nuclear  leucocytes,  483 
Polypeptides,  19 
Polysaccharids,  287 
Portal  circulation,  353 
Positive  accommodation,  147  et  seq. 

accommodation,  varying  power 
of,  149 
Posterior   columns   of    spinal   cord 
(see  Spinal  cord) 

ganglion  reflex,  93 

grey  commissure  (.s«e  Spinalcord) 

roots,  ganglia  of,  107 
Posture  on  circulation,  471 
Potassiiim  in  food-stuffs,  289 
Potatoes,  composition,  290 
Precipitins,  614 
Predicrotic  wave,  441 
Preformed  sulphate  in  urine,  563 
Pregnancy,  course  of,  608 

metabolism,  631 
Prepyloric  sphincter,  295 
Presbyopia,  149 
Presphygmic  period,  404 
Pressure,  arterial,  447  et  seq. 

arterial,  factors  controlling,  450 

blood,  432 

in  capillaries,  460 

in  lymphatics,  464 

intracardiac,  399 


Pressure,    variations     in,     due     to 
respiration,  446 

venous,  463 
Primitive  nerve  sheath,  56 
Private  path,  87 

Production  of  heat  (in  muscle),  268 
Propionic  acid  {see  Appendix  1.) 
Proprioceptive  receptors,  96 
Prostate  gland,  621 
Protamine,  17 

Proteates  {see  Appendix  II.) 
Protein,  absorption  of,  347 

chemistry  of,  15 

classification  of  {see  Appendix 
II.) 

conversion  into  fat,  352 

determination   of  amount   oxi- 
dised, 257 

disintegration  of,  16 

energv  value  of,  257 

in  food,  283 

metabolism  of,  257  et  seq.,  554 

native  {see  Appendix  II.) 

physical  characters,  14 

products  of,  15 

regulation  of  supply  359 
Protein,  requirements,  370 

storage  of,  352 

synthesis  of,  18 

vegetable,  284 
Proteins,  15  {see  Appendix  II.) 
Proteolytic  period  of  gastric  diges- 
tion, 311 
Proteoses,  311 
Prothrombin,  477 
Protoplasm,  5 

chemistry  of,  12 

liberation  of  energy  by,  7 

metabolism  of,  9 

structure,  11 
Protoplasmic  activity,  5,  21 
Proto-proteoses,  312 
Pseudoglobulin,  480 
Pseudopodia,  24 
Ptvalin,  302 
Puberty,  618 
Pulmonarv  area,  407 

valve^  392 
Pulmonic    heart    {see    Heart,    pul- 
monic), 383 
Pulse,  anacrotic,  442 

arterial,  434 

capillary,  444 

characters  of  wave,  436 

dicrotic,  440 

form  of  wave,  438 


670 


INDEX 


Pulse,  height  of  wave,  437 

high  tension,  443 

length  of  wave,  437 

palpation  of,  442 

polycrotic,  442 

rate  of,  442 

rhythm,  442 

tension,  443 

velocity  of  wave,  436 

venous,  444 

volume  of,  442 
Pulsus  celer,  443 

parvus,  442 

plenus,  442 

tardus,  443 
Pupil,  140 

dilatation,  148 

dilator  of,  140 

drugs,  action,  149 

sphincter  of,  140,  147 
Purin  bodies,  556,  561 
Purkinje's  cells,  125 

images,  152 
Pyloric  end  of  the  stomach,  295 

sphincter,  316 
Pyramidal  tracts  (see  Spinal  cord), 

194 
Pyriform  lobe,  134 
Pyrimidin  bases,  28 
Pyrocatechin,  564 
Pyrrol  derivatives  {see  Appendix  I.) 

Quality  of  sounds,  553 

Racemose  glands,  34 
Radiation  of  heat  from  skin,  267 
Radicles,  organic  (see  Appendix  I.) 
Ranvier,  nodes  of,  57 
Reaction  time,  cerebral,  186 

of  degeneration,  66 
Receptor  spots,  306 
Receptors,  96 

connection         with         central 
nervous  system,  106 

distance,  130 
Recti  muscles  (see  Eye) 
Rectum,  296 
Recurrent     laryngeal     nerve      (see 

Nerves  of  larynx) 
Red  cells  of  blood  (see  Erythrocytes) 

marrow  of  bone,  501 

nuclei  (see  Cerebrum) 
Reduced  alkaline  hteniatin,  490 
Referred  pain,  98 
Reflex  action,  82  et  seq. 

bladder,  93  I 


Reflex  action,  fatigue,  89 

jjosture,  88 
Reiiexes,  peripheral,  92 

visceral,  89 
Refraction  of  light,  144 
Refractive  indices  of  media  of  eye, 

144 
Refractory  period  of  muscle,  216 
Regeneration  of  nerve,  77 
Regulation  of  growth  and  function, 

586 
Rennet  (see  Rennin) 
Renuin,  312,  321 
Reproduction,  616  et  seq. 
Requirements,  food,  368  et  seq. 

proteins,  370 
Reserve  air,  523 
Residual  air,  523 
Respiration,  513  et  seq. 
artificial,  549 
at  high  altitudes,  543 
chemical  regulation,  527 
Cheyne-Stokes,  531 
eifect  of,  on  air  breathed,  537 
effect  on  the  blood,  538 
external,  513 
influence   of   heart's  action  on, 

536 
influence  on  circulation,  535 
intermediate,  545 
internal,  547 
mechanism  of,  513  et  seq. 
movement  of  ribs  in,  519  et  seq. 
movements  in,  516 
nervous  mechanism  of,  523 
number  per  minute.  525 
physiology  of,  515 
reflex  control  of  breathing,  532 
rhythm  of,  525 
vagus,  influence  on,  532 
Respiratory  calorimeter,  259 
centre,  526 
reflex  regulation,  532 
Respiratory  interchange,  extent  of, 
647  ;  causes  of,  539 
movements,  special,  522 
passages,  elimination  of  heat  by, 

268 
quotient,  258 
Rete  testis  (see  Testis) 
Reticulum,  294 
Retina,  140 

central  spot,  153 
corresponding  areas  on  the  two 

sides,  158 
fatigue  of,  154 


INDEX 


671 


Retina,  layers  of,  141 

modes  of  stimulation,  153 
nature  of  changes  in,  154 
rods  and  cones  of,  142 
stimulation  of,  153  et  seq. 
Rhinencephalou,  135,  181 
Rhodopsin,  154 
Ribs,  movements  of,  in  respiration 

{see  Respiration) 
Rickets,  50,  281 
Rigor  mortis,  214 
Riva  Rocci  blood  pressure  apparatus, 

438 
Roaring,  553 

Rolandic  area  (see  Motor  areas) 
Roof  nucleus  of  the  cerebellum  {see 

Cerebellum),  125 
Roots  as  food,  290 
Roots  of  the  spinal  nerves,  107 
Rubro-spinal  fibres,  195 
Rumen,  294 

digestion  in,  339 
Rumination,  337 

Saccharomyces  cerevisfe,  5 
Saccule,  118 
Saliva,  302  et  seq. 

character  of,  302 

chemistry  of,  302 

functions  of,  303 

influence  of  chorda  tympani  in 
secretion  of,  304 

physiology  of,  303 

reflex  stimulation  of  secretion, 
304 

stimulation      of      sympathetic 
effect  on,  305 
Salivarv  centre,  305 

glands,  293 

glands,  nerve  supply,  303 

secretion,  physiology  of,  303 
Salts  of  the  bile  acids,  323 
Sanson's  images,  147 
Sarcolactic  acid,  209,  250 
Sarcolemma,  204 
Sarcostyle,  204 
Sarcous  substance,  204 
Scala  media,  171 

tympani,  170 

vestibula,  170 
Scheiner's  experiment,  146 
Schwann,  2 

white  sheath  of  {see  Nerve) 
Sclero-proteins  (see  Appendix  II.) 
Sclerotic,  139 
Scratch  reflex,  87 


Sebaceous  glands,  585 
Secretin,  321,  601 
Secreting  epithelium,  34 
Secretions,  internal,  588  et  stq. 
Sectional  area  of  circulatory  system, 

383 
Semen,  621 

Semicircular  canals,  117 
Semilunar  valves,  391 
Semi-starvation,  metabolism  of,  274 
Seminiferous  tubules,  620 
Sensation,  colour,  155 
integration,  179 
of  hunger  (see  Hunger) 
of  pain  {see  Pain) 
of  smell  (see  Smell) 
of  taste  (see  Taste) 
of  thirst  (.see  Thirst) 
physiology  of  colour,  155 
Sensation,    production     of    colour, 

156 
Sense  of  acceleration   and   retarda- 
tion of  motion  (see  Labyrinth) 
of  hearing,  166  et  seq. 
joint  and  muscle,  U)5 
tactile,  101 
thermal,  103 
Sensibility,  common,  98 
Sertolis  cells,  606 
Serum,  479 

albumin,  476,  480 
globulin,  476,  480 
Sesamoid  ligaments,  233 
Seventh  cranial  nerve,  199 
Sex,  determination  of,  617 
Sexual  organs,  618 

organs,  effect  of  removal  of,  606 
Sheath  of  Schwann,  white,  56 
Shingles,  91 
Side-chain  theory,  614 
Sight,  136  et  seq. 
Sino-auricular  node,  385 
Sinus  of  Valsalva,  392 
Sixth  nerve  (see  Abducens),  201 
Skatol,  330 
Skatoxyl-sulphate     of      potassium, 

563 
Skin,  elimination  of  heat  by,  267 
excretion  by,  582 
receptors,  99 
Slaughter  test,  369 
Sleep,  187 

Small  intestine,  295,  332 
Smell,  133 

centre  for,  134 
mechanism  of,  133 


672 


INDEX 


Smell,  physiology  of,  136 

Snake  toxin,  612 

Sneezing,  522 

Sols,  13 

Soluble  nitrogen  in  feeding  stuffs, 

284 
Somatic  fibres,  54,  195 
Somatopleur,  625 
Sounds  of  the  heart,  407 
Specific  dynamic   action  of    foods, 
272 

nerve  energy,  97,  138 
Spermatids,  621 
Spermatocytes,  621 
Spermatogonia,  620 
Spermatozoa,  618 
Spherical  aberration,  147 
Sphincter  ani,  296 

centre,  148 

ileo-caecal,  334 

of  stomach,  316 

pupillee,  140,  147 
Sphygmograph,  439 
Sphygmomanometer(see  Riva  Rocci) 
Spinal  accessory  nerve,  200 
Spinal  cord,  106  et  seq. 

anterior  columns.  111 

arc,  78,  82 

ascending  degenerations,  108 

conducting  paths  in,  108,  194 

hemisection,  108 

lateral  columns.  111 

nerves,  106 

posterior  columns  of,  111 

reflexes,  82  et  seq. 

tracts  of,  107,  194 

upgoing  fibres,  107 
"  Spinal  dog,"  82 
Spino-tectal  synapsis,  1 1 3 
Spino-thalamic  synapsis,  113 
Splanchnopleur,  625 
Spleen,  504 

circulation  in,  469 

movements,  506 
Spot,  blind  (see  Blind  spot) 

cold  {see  Cold  spot) 

hot  (see  Hot  spot) 
Squamous  epithelium,  32 
Squint,  162 

Stannius'  experiment,  425 
Stapedius  muscle,  167,  169 
Stapes,  167 
Starch,  287 

equivalent,  377 

value,  377 
Stearic  acid,  42 


Stearin,  42 

Stenosis,  valvular,  410 
Stercobilin,  332 
Stereoscopic  vision,  158 
Stethograph,  525 
Stimulation,  unilateral,  25 

nocuous,  85,  100 

of  muscle  direct,  217 
Stimuli,  10 

successive,  on  muscle,  228 
Stomach,  295 

absorption  from,  318 

after  feeding,  310 

digestion  in,  308 
in  horse,  342 
in  ruminant,  34(i 

during  fasting,  309 

movements  of,  315,  318 

nervous  mechanism  of,  317 

rate  of  passage  of  food  from,  317 

regurgitation,  318 

secretion  of,  310 

structure  of,  295 
Storage  of  surplus  food,  351 
Storing  mechanism  (cerebral),  185 
Strain,  muscular,  253,  471 
Stroma   of  red   cells   of  blood   (see 

Erythrocytes) 
Strom uhr,  466 
Sublingual  gland,  293 

gland,  nerve  supply,  304 
Submaxillary  gland,  293 

gland,  nerve  supply,  304 
Subminimal  stimulus,  84 
Substrate,  8 
Succus  entericus,  328 
Sulcus  centralis,  190 
Sulphates,  ethereal,  563 

inorganic,  556,  563 

preformed    (see    Inorganic  sul- 
phates) 
Sulphur-containing   bodies    in    the 

urine,  563 
Sulphur,  neutral,  556,  564 
Superior  cardiac  branch   of  vagus, 
418 

oblique  muscle,  159 
Suprarenal  bodies,  590 

cortex,  609 

medulla,  590 

metabolism  after   injection   of, 
593 
Surface  tension,  13  (Appendix  III.) 
Suspensory  ligament  of  eye,  141 
Swallowing,  306 

time  of,  308 


INDEX 


073 


Sweat,  chemistry  of,  585 

evaporation,  267 

glands,  583 

secretion  of,  584 
Sympathetic  nerves,  true,  198 

nerves,  para,  199 
Synapsis,  53 

Systemic  heart  {see  Heart),  382 
Systole  of  heart,  394 
Systolic  blood-pressure,  449 

Tactile  sense,  101 

corpuscles,  102 
Tapetum  nigrum,  142 
Taste,  131 

Taurocholate  of  soda,  323 
Taurocholic  acid,  323 
Tecto-spinal  fibres,  195 
Tectum,  112,  127 
Teeth  of  horse,  292 

of  ruminant,  292 
Temperature,  264  et  scq. 

chemical  regulation,  269 

physical  regulation,  269 
Tendon  sheaths,  40 
Tendons,  40 

Tensor  tympani  muscle,  167,  169 
Tenth  nerve  (see  Vagus) 
Terminal  ganglia,  55 
Testis,  606,  618 

interstitial  cells,  606 
Tetanus,  complete,  230 

incomplete,  229 
Tetany,  65 
Thermal  sense,  103 
Thermo-electric  method,  247 
Thermopiles,  247  (Appendix  III.) 
Thermotaxis,  26 
Third  nerve,  199 
Thirst,  99 

Thoracic  breathing,  520 
Thoracico-abdominal     sympathetic, 

198 
Thorax,  in  respiration,  518 
Thrombin,  477 
Thromboplastin,  478 
Thymus,  602 

effect  of  castration  on,  603 

removal  of,  603 
Thyreoid,  595 

gland,  administration  of  extracts 
of,  598 

gland    removal    of   (thyroidec- 
tomy), 597 
Thyreo-oxy-indol,  596 
Tidal  air,  522 
43 


Tissues,  31 

adipose,  41 

areolar,  40 

cancellous,  46 

connective,  37 

fibrous,  38 

lymph,  499 

master,  51 

mucoid,  37 

vegetative,  31 
Tone,  plastic,  212 
Tongue,  291 
Tonometer,  541 
Tonus,  muscle,  211 
Touch,  centre  for,  115,  116 

spots,  100 
Tract,  crossed  jayramidal,  195 

direct  cerebellar,  112 
Tracts  of  cord,  112,  194 
Transitional  epithelium,  33 
Tricuspid  valve,  390 
Trigeminal  nerve,  201 
Triple-phosphate,  559 
Trochlearis  nerve,  201 
Trophoblast,  626 
Trypsin,  320 
Trypsinogen,  321 
Trytophan,  17,  320,  555 
Tuberculum  acusticum,  172 
Tubers  as  food,  290 
Tubules,  secretion  in  kidney,  575 
Tunica  albuginea,  620 
Turnips  as  food,  290 
Twelfth  nerve,  220 
Tympanic  cavity,  167 
Tyrosin,  17,  33,  555 

Umbilical  veins,  631 
Unconsciousness,  185 
Unilateral  stimulation,  25 
Urates,  561 
Urea,  359,  559 

sources  of,  360 
Ureter,  581 
Urethra,  582 
Uric  acid,  556,  561 
Uricoclastase,  556 
Urinary  bladder,  581 
Urine,  554 

composition,  558 

excretion  of,  581 

formation  of,  567 

method  of  estimating  solids  in, 
557 

micro-organisms  in,  559 

nitrogenous  substances  of,  559 


674 


INDEX 


Urine,  non-urea  nitrogen  in,  559 

pigments  of,  566 

reaction,  557 
Urobilin,  566 
Urochrome,  566 
Uroerythrin,  566 
Uterus,  621 

nervous  control,  633 
Utricle,  118 

Vaccines,  615 
Vagus,  200 

Valsalva  sinus  ct'  {see  Sinus  of  Val- 
salva) 
Valve  action  m  reflexes,  84 

ileo-csecal,  334 
Valves  of  the  heart,  390,  402 

action  of,  402 
Valvular  incompetence,  410 
Vas  deferens,  620 
Vasa  efferentia,  620 
Vaso-constrictor  centre,  457 

mechanism,  454 
Vaso-dilator  centres,  460 

mechanism,  458 
Vegetable  proteins,  284 
Veins,  432 

pressure  in,  463 

umbilical,  631 
Velocity  of  blood,  465 
Vena  cava,  inferior  (foetal),  632 

cava,  superior  (foetal),  632 
Venous  pulse,  444 
Ventilation,  548 

positive  and  negative,  533 
Ventricles  of  the  heart,  386 

left,  386 

muscular  structure  of,  387 

pressure  in,  402 

right,  388 
Vermis,  superior,  of  cerebellum,  125 
Vesicular  sound,  525 
Vestibular  root  of  the  eighth  nerve, 

119 
Vestibule  of  the  ear,  117,  170 
Villi,  chorionic,  626 

of  intestine,  295 

primitive,  624 
Visceral  muscle,  203,  215 

receptor  mechanism,  97 
Vision,  136 

binocular,  158 

colour,  155 

distant,  144 

double,  159 


Vision,  field  of,  152,  158 

glance,  160 

monocular,  144 
Visual  centre,  164 

sensation,  duration  of,  165 
strength  of,  165 

purple,  154 
Vital  capacity  of  the  thorax,  523 
Vitamines,  2S2 
Vitreous  humour,  140 
Vocal  cords,  550 
Voice,  552 

physiology,  552 
Volar  aspect  of  hoof,  233 
Voluntary    contraction,   nature    of, 

230 
Vomiting,  318 

in  horse,  346 
von  Mohl,  3 

Wallerian  degeneration,  76 
Waste,    rate    of    (during    fasting), 

274 
Water  in  feeding  stuffs,  289 

soluble  B,  281 
Watering,  367 

Wave,  characters  of  pulse  (see  Pulse) 
Weigert's  method  of  nerve-staining, 

76 
Weight  estimation,  106 
White  cells  of  blood,  483 

fate  of,  503 

source  of,  499 
White  fibro-cartilage,  45 

line  (hoof),  235 
Wolff-Lehmann  standards,  376 
Work  collector,  245 

done  by  muscle,  242 

muscular,     effect    on     excreta, 
262 

X-CHROMOSOMES,  617 

X-rays  in  the  examination   of  the 
stomach,  315 
of  the  intestine,  333 

Yawning,  522 
Yeast,  5 

Zein,  277 

Zona  granulosa,  619 

pellucida,  619 
Zymin,  7  (see  also  Enzymes) 

secreting  epithelium,  36 
Zymogen,  36 


Lorimer  &  Chalmers,  Printers,  Edinburgh. 


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